May 23, 2012

Cholesterol

The straight dope on cholesterol – Part V

In this post we’ll address the following concept: Does the size of an LDL particle matter?

Read Time 10 minutes

In Part I, Part II, Part III and Part IV of this series, we addressed these 6 concepts:

     #1What is cholesterol?

     #2What is the relationship between the cholesterol we eat and the cholesterol in our body?

     #3Is cholesterol bad?

     #4 How does cholesterol move around our body?

     #5 How do we measure cholesterol?

     #6How does cholesterol actually cause problems?

 

 

Quick refresher on take-away points from previous posts, should you need it:

  1. Cholesterol is “just” another fancy organic molecule in our body but with an interesting distinction: we eat it, we make it, we store it, and we excrete it – all in different amounts.
  2. The pool of cholesterol in our body is essential for life.  No cholesterol = no life.
  3. Cholesterol exists in 2 formsunesterified or “free” (UC) and esterified (CE) – and the form determines if we can absorb it or not, or store it or not (among other things).
  4. Much of the cholesterol we eat is in the form of CE. It is not absorbed and is excreted by our gut (i.e., leaves our body in stool). The reason this occurs is that CE not only has to be de-esterified, but it competes for absorption with the vastly larger amounts of UC supplied by the biliary route.
  5. Re-absorption of the cholesterol we synthesize in our body (i.e., endogenous produced cholesterol) is the dominant source of the cholesterol in our body. That is, most of the cholesterol in our body was made by our body.
  6. The process of regulating cholesterol is very complex and multifaceted with multiple layers of control.  I’ve only touched on the absorption side, but the synthesis side is also complex and highly regulated. You will discover that synthesis and absorption are very interrelated.
  7. Eating cholesterol has very little impact on the cholesterol levels in your body. This is a fact, not my opinion.  Anyone who tells you different is, at best, ignorant of this topic.  At worst, they are a deliberate charlatan. Years ago the Canadian Guidelines removed the limitation of dietary cholesterol. The rest of the world, especially the United States, needs to catch up.  To see an important reference on this topic, please look here.
  8. Cholesterol and triglycerides are not soluble in plasma (i.e., they can’t dissolve in water) and are therefore said to be hydrophobic.
  9. To be carried anywhere in our body, say from your liver to your coronary artery, they need to be carried by a special protein-wrapped transport vessel called a lipoprotein.
  10. As these “ships” called lipoproteins leave the liver they undergo a process of maturation where they shed much of their triglyceride “cargo” in the form of free fatty acid, and doing so makes them smaller and richer in cholesterol.
  11. Special proteins, apoproteins, play an important role in moving lipoproteins around the body and facilitating their interactions with other cells.  The most important of these are the apoB class, residing on VLDL, IDL, and LDL particles, and the apoA-I class, residing for the most part on the HDL particles.
  12. Cholesterol transport in plasma occurs in both directions, from the liver and small intestine towards the periphery and back to the liver and small intestine (the “gut”).
  13. The major function of the apoB-containing particles is to traffic energy (triglycerides) to muscles and phospholipids to all cells. Their cholesterol is trafficked back to the liver. The apoA-I containing particles traffic cholesterol to steroidogenic tissues, adipocytes (a storage organ for cholesterol ester) and ultimately back to the liver, gut, or steroidogenic tissue.
  14. All lipoproteins are part of the human lipid transportation system and work harmoniously together to efficiently traffic lipids. As you are probably starting to appreciate, the trafficking pattern is highly complex and the lipoproteins constantly exchange their core and surface lipids.
  15. The measurement of cholesterol has undergone a dramatic evolution over the past 70 years with technology at the heart of the advance.
  16. Currently, most people in the United States (and the world for that matter) undergo a “standard” lipid panel, which only directly measures TC, TG, and HDL-C.  LDL-C is measured or most often estimated.
  17. More advanced cholesterol measuring tests do exist to directly measure LDL-C (though none are standardized), along with the cholesterol content of other lipoproteins (e.g., VLDL, IDL) or lipoprotein subparticles.
  18. The most frequently used and guideline-recommended test that can count the number of LDL particles is either apolipoprotein B or LDL-P NMR, which is part of the NMR LipoProfile.  NMR can also measure the size of LDL and other lipoprotein particles, which is valuable for predicting insulin resistance in drug naïve patients, before changes are noted in glucose or insulin levels.
  19. The progression from a completely normal artery to a “clogged” or atherosclerotic one follows a very clear path: an apoB containing particle gets past the endothelial layer into the subendothelial space, the particle and its cholesterol content is retained, immune cells arrive, an inflammatory response ensues “fixing” the apoB containing particles in place AND making more space for more of them.
  20. While inflammation plays a key role in this process, it’s the penetration of the endothelium and retention within the endothelium that drive the process.
  21. The most common apoB containing lipoprotein in this process is certainly the LDL particle. However, Lp(a) and apoB containing lipoproteins play a role also, especially in the insulin resistant person.
  22. If you want to stop atherosclerosis, you must lower the LDL particle number.

 

Concept #7 – Does the size of an LDL particle matter?

There are few, if any, topics in lipidology that generate more confusion and argument that this one.  I’ve been leading up to it all month, so I think the time is here to address this issue head on.  I’ve read many papers and seen many lectures on this topic, but the one that stole my heart was a lecture given by Jim Otvos at the ADA 66th Scientific Sessions in Washington, DC.   Some of the figures I am using in this post are taken directly or modified from his talk or subsequent discussions.

At the outset of this discussion I want to point out two clinical scenarios to keep in mind:

  1. The most lethal lipoprotein disorder is familial hypercholesterolemia, which I have discussed in previous posts.  Such patients all have large LDL particles, but most of these patients die in childhood or early adulthood if not treated with medications to reduce particle number.
  2. Conversely, diabetic patients and other patients with advanced metabolic syndrome have small LDL particles, yet often live well into their 50s and 60s before succumbing to atherosclerotic diseases.

The common denominator is that both sets of patients in (1) and (2) have high LDL-P.  What I’m going to attempt to show you today is that once adjusted for particle number, particle size has no statistically significant relationship to cardiovascular risk.  But first, some geometry.

 

“Pattern A” versus “Pattern B” LDL

The introduction of gradient gel electrophoresis about 30 years ago is what really got people interested in the size of LDL particles.  There is no shortage of studies of the past 25 years demonstrating that of the following 2 scenarios, one has higher risk, all other things equal.  [This is a big disclaimer and I went back and forth for a while before deciding to include this point.  It is an uncharacteristic oversimplification. If you’ve been reading this blog for a while, you’ll know I’m rarely accused of that sin – but I’m about to be].

Here’s the example: Consider 2 patients, both with the same total content of cholesterol in their LDL particles, say, 130 mg/dL.  Furthermore, assume each has the “ideal” ratio of core cholesterol ester-to-triglyceride (recall from Part I and III of this series, this ratio is 4:1).  I’m going to explain in a subsequent post why this assumption is probably wrong as often as it’s right, but for the purpose of simplicity I want to make a geometric point.

  1. LDL-C = 130 mg/dL, Pattern A (large particles) – person on the left in the figure below
  2. LDL-C = 130 mg/dL, Pattern B (small particles) – person on the right in the figure below

Under the set of assumptions I’ve laid out, case #2 is the higher risk case.  In other words, at the same concentration of cholesterol within LDL particles, assuming the same ratio of CE:TG, it is mathematically necessary the person on the right, case #2, has more particles, and therefore has greater risk.

Bonus concept: What one really must know is how many cholesterol molecules there are per LDL particle.  It always requires more cholesterol-depleted LDL particles than cholesterol-rich LDL particles to traffic cholesterol in plasma, and the number of cholesterol molecules depends on both size and core TG content.  The more TG in the particle, the less the cholesterol in the particle.

So why does the person on the right have greater risk?  Is it because they have more particles?  Or is it because they have smaller particles?

This is the jugular question I want to address today.

 

Small vs. large particles

If you understand that the person on the right, under the very careful and admittedly overly simplified assumptions I’ve given, is at higher risk than the person on the left, there are only 4 possible reasons:

  1. Small LDL particles are more atherogenic than large ones, independent of number.
  2. The number of particles is what increases atherogenic risk, independent of size.
  3. Both size and number matter, and so the person on the right is “doubly” at risk.
  4. Neither feature matters and these attributes (i.e., size and number) are markers for something else that does matter.

Anyone who knows me well knows I love to think in MECE terms whenever possible.  This is a good place to do so.

I’m going to rule out Reason #4 right now because if I have not yet convinced you that LDL particles are the causative agent for atherosclerosis, nothing else I say matters.  The trial data are unimpeachable and there are now 7 guidelines around the world advocating particle number measurement for risk assessment. The more LDL particles you have, the greater your risk of atherosclerosis.

 

But how do we know if Reason #1, #2, or #3 is correct?

This figure (one of the most famous in this debate) is from the Quebec Cardiovascular Study, published in 1997, in Circulation.  You can find this study here.

 

Relative risks

This is kind of a complex graph if you’re not used to looking at these.  It shows relative risk – but in 2 dimensions.  It’s looking at the role of LDL size and apoB (a proxy for LDL-P, you’ll recall from previous posts).  What seems clear is that in patients with low LDL-P (i.e., apoB < 120 mg/dl), size does not matter.  The relative risk is 1.0 in both cases, regardless of peak LDL size.  However, in patients with lots of LDL particles (i.e., apoB > 120 mg/dl), smaller peak LDL size seems to carry a much greater risk – 6.2X.

If you just looked at this figure, you might end up drawing the conclusion that both size and number are independently predictive of risk (i.e., Reason #3, above).  Not an illogical conclusion…

What is not often mentioned, however, is what is in the text of the article:

“Among lipid, lipoprotein,and apolipoprotein variables, apo B [LDL-P] came out as the best and only significant predictor of ischemic heart disease (IHD) risk in multivariate stepwiselogistic analyses (P=.002).”

“LDL-PPD [peak LDL particle diameter] — as a continuous variable did not contribute to the risk of IHD after the contribution of apo B levels to IHD risk had been considered.”

What’s a continuous variable?  Something like height or weight, where the possible values are infinite between a range.  Contrast this with discrete variables like “tall” or “short,” where there are only two categories. For example, if I define “tall” as greater than or equal to 6 feet, the entire population of the world could be placed in two buckets: Those who are “short” (i.e., less than 6 feet tall) and those who are “tall” (i.e., those who are 6 feet tall and taller). This figure shows LDL size like it’s a discrete variable – “large” or “small” – but obviously it is not. It’s continuous, meaning it can take on any value, not just “large” or “small.”  When this same analysis is done using LDL size as the continuous variable it is, the influence of size goes away and only apoB (i.e., LDL-P) matters.

This effect has been observed subsequently, including the famous Multi-Ethnic Study of Atherosclerosis (MESA) trial, which you can read here.  The MESA trial looked at the association between LDL-P, LDL-C, LDL size, IMT (intima-media thickness – the best non-invasive marker we have for atherosclerosis), and many other parameters in about 5,500 men and women over a several year period.

This study used the same sort of statistical analysis as the study above to parse out the real role of LDL-P versus particle size, as summarized in the table below.

 

unadjusted-vs.-adjusted-table

This table shows us that when LDL-P is NOT taken into account (i.e., “unadjusted” analysis), an increase of one standard deviation in particle size is associated with 20.9 microns of LESS atherosclerosis, what one might expect if one believes particle size matters.  Bigger particles, less atherosclerosis.

However, and this is the important part, when the authors adjusted for the number of LDL particles (in yellow), the same phenomenon was not observed.  Now an increase in LDL particle size by 1 standard deviation was associated with an ADDITIONAL 14.5 microns of atherosclerosis, albeit of barely any significance (p=0.05).

Let me repeat this point: Once you account for LDL-P, the relationship of atherosclerosis to particle size is abolished (and even trends towards moving in the “wrong” direction – i.e., bigger particles, more atherosclerosis).

Let me use another analysis to illustrate this point again.  If you adjust for age and sex, but not LDL-P [left graph, below], changes in the number of LDL particles (shown in quintiles, so each group shows changes by 20% fractions) seem to have no relationship with IMT (i.e., atherosclerosis).

However, when you adjust for small LDL-P [right graph, below], it becomes clear that increased numbers of large LDL particles significantly increase risk.

 

Adjustment-for-large-LDL

I’ve only covered a small amount of the work addressing this question, but this issue is now quite clear.  A small LDL particle is no more atherogenic than a large one, but only by removing confounding factors is this clear.   So, if you look back at the figure I used to address this question, it should now be clear that Reason #2 is the correct one.

This does not imply that the “average” person walking around with small particles is not at risk.  It only implies the following:

  1. The small size of their particles is probably a marker for something else (e.g., metabolic derangement due to higher trafficking of triglycerides within LDL particles);
  2. Unless you know their particle number (i.e., LDL-P or apoB), you actually don’t know their risk.

Let’s wrap it up here for this week.  Next week we’ll address another question that’s probably been on your mind: Why do we need to measure LDL-P or apoB?  Isn’t the LDL-C test my doctor orders enough to predict my risk?

 

Summary

  • At first glance it would seem that patients with smaller LDL particles are at greater risk for atherosclerosis than patients with large LDL particles, all things equal.  Hence, this idea that Pattern A is “good” and Pattern “B” is bad has become quite popular.
  • To address this question, however, one must look at changes in cardiovascular events or direct markers of atherosclerosis (e.g., IMT) while holding LDL-P constant and then again holding LDL size constant.  Only when you do this can you see that the relationship between size and event vanishes.  The only thing that matters is the number of LDL particles – large, small, or mixed.
  • “A particle is a particle is a particle.”  If you don’t know the number, you don’t know the risk.

By NASA Mercury image: JHUAPLVenus image: JPLMars image: HST [Public domain], via Wikimedia Commons

Disclaimer: This blog is for general informational purposes only and does not constitute the practice of medicine, nursing or other professional health care services, including the giving of medical advice, and no doctor/patient relationship is formed. The use of information on this blog or materials linked from this blog is at the user's own risk. The content of this blog is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Users should not disregard, or delay in obtaining, medical advice for any medical condition they may have, and should seek the assistance of their health care professionals for any such conditions.

267 Comments

  1. This is a nice presentation.

    Thank you for the work.

    Have I understood correctly that the biological mechanism proposed as a result of these is:

    That current coronary risk is measured by IMT
    and that FUTURE IMT increases because present Particle Number is what mathematians describe as a first derivative?

    Slainte

    • Is an IMT ultrasound test a better choice than calcium scan as a test for current atherosclerosis status? I plan to get an NMR in 6 months and am considering something like the IMT or Calcium scan as well.

      • The look at really different things. IMT is looking at carotid artery plaque. Calcium scan looks at calcium build up in coronary arteries. Each has a benefit and detriment.

    • There is so much to discover about this disease;

      http://www.sciencedaily.com/releases/2012/06/120606132302.htm

      The guilty party is not the smooth muscle cells within blood vessel walls, which for decades was thought to combine with cholesterol and fat that can clog arteries. Blocked vessels can eventually lead to heart attacks and strokes, which account for one in three deaths in the United States.

      (Snip)

      For the first time, we are showing evidence that vascular diseases are actually a kind of stem cell disease,” said principal investigator Song Li, professor of bioengineering and a researcher at the Berkeley Stem Cell Center. “This work should revolutionize therapies for vascular diseases because we now know that stem cells rather than smooth muscle cells are the correct therapeutic target.”

      The finding that a stem cell population contributes to artery-hardening diseases, such as atherosclerosis, provides a promising new direction for future research, the study authors said.

      “This is groundbreaking and provocative work, as it challenges existing dogma,” said Dr. Deepak Srivastava, who directs cardiovascular and stem cell research at the Gladstone Institutes in San Francisco, and who provided some of the mouse vascular tissues used by the researchers. “Targeting the vascular stem cells rather than the existing smooth muscle in the vessel wall might be much more effective in treating vascular disease.”

      (Snip)

      “If your target is wrong, then your treatment can’t be very effective,” said Dr. Shu Chien, director of the Institute of Engineering in Medicine at UC San Diego, and Li’s former adviser. “These new findings give us the right target and should speed up the discovery of novel treatments for vascular diseases.”

    • Hello Peter,
      Apologies in advance for the length of this post.
      Thanks very much to yourself and Dr Dayspring for exponentially increasing my knowledge of cholesterol metabolism. Like yourself but to a lesser degree, I have been immersed in the references related to this topic for the last 9 months or so. I am in agreement with most points of your arguments.Particularly with regard to LDL-C as a poor marker for risk as well as lipid responses to carbohydrate restriction. However, the Apo-B and LDL-P story has left me with persistent questions with respect to risk applications in my own patients.
      In this section V you refer to 4 possible assumptions dismissing option 4 ( size and number indicate something else) as well as minimizing the role of systemic inflammation. While I agree that APO-B and LDL-P seem more precise as a marker for hard outcomes (events, deaths, etc.), I remain unconvinced about the inherent atherogenicity of the the APO-B containing lipo-proteins themselves with respect to either their number or the greater reactivity of certain subtypes. I derive this doubt mainly from Dr Daysprings intricate descriptions of the highly regulated trafficking of these particles. I am sorry if I am being pedantic with language, but you both refer to LDL particles as being able to “penetrate” the arterial endothelium( gaining access to the subendothelial space)once particle number reaches a certain concentration. Am I to believe that arterial endothelium simply succumbs to passive diffusion once that arbitrary threshold is reached? If this is the case please can you help clarify? As I understand it, APO-B particles are thought to enter the artery by way of either trancytosis or intracellular transport.
      I would assume that like most organs and tissues in the body, arterial cells would have receptors to transport various insoluble metabolites across cell membranes. Indeed this is the case. Arterial endothelial cells express the LDL receptor(LDL-r) that binds to Apo-B particles. As you have described, every cell has the ability to synthesise its own cholesterol. One could assume that like most other cells, arteries would up-regulate LDL-r when more cholesterol was required and down-regulate when to much is available. What role or mechanism can particle number have in this scenario?
      This brings me back to my initial mention of inflammation. My particular interest lies in the initiating event in the atherosclerotic process. This is a chicken and egg argument that has never been solved convincingly. So, does the presence of “atherogenic” lipoproteins within the arterial cell spark the attraction of the 1st monocyte/macrophage and thereby initiate the the typical inflammatory cascade that leads to the production of a foam cell (known as the retention hypothesis)? Alternatively, is it that inflammation was already present and cholesterol and lipoproteins merely offer themselves as “building blocks” to the dubious motivations the that macrophage/foam cell?
      If the later were the case, one could generate an alternate hypothesis where by the inflammatory signalling might up-regulate LDL-r in order to gather more “building blocks”. As fanciful as this may sound, its plausibility has been confirmed in a 2012 study:
      (Inflammation Disrupts the LDL Receptor Pathway and Accelerates the Progression of Vascular Calcification in ESRD Patients)
      What then to make of LDL particles in the ” inflammation comes 1st hypothesis”. Taken at a localised level, this atherogenic intra-arterial inflammatory process might, in its effort to obtain more cholesterol, deplete the various LDL particles of there cholesterol and spit them back out into circulation (small dense LDL and oxidised LDL for instance). Could this be a substantial factor in the plasma levels of small LDL? Perhaps on a system wide basis?
      What about particle number in this hypothesis? This same process might also lead to some additional particles in circulation but I am doubtful that arterial ” recycling” would shift measured LDL-P numbers significantly. However, “systemic” inflammation could mediate a similar process in other body tissues, say visceral fat for instance. So we might use metabolic syndrome as an example. LDL-P might increase as peripheral tissues deplete LDL particles of their contents to aid in an adaptive response to an inflammatory stimulus (hyperglycaemia/insulin resistance). Thus the liver might increase LDL particle output in response to cholesterol depleted Apo-B lipoproteins circulating in the plasma. So in effect this would not be a problem of decreased clearance but more of increased production of LDL particles.
      What then to make of familial hypercholesterolaemia. The lack of LDL-r function is mainly described as it relates to the liver. Perhaps you can help answer whether there is a difference in LDL-r between the liver and the arterial endothelium. Maybe peripheral tissues in FH are just doing their part to aid in cholesterol clearance with unfortunate consequences within vascular tissue. In any case, I am yet to be convinced that FH is a good proxy for arguments for or against LDL-P risk. Particle number is increased by default not by design in this situation.
      So what clinical relevance can all my nitpicking have? I want to believe the LDL-P/APO-B story and hopefully apply it as a powerful risk assessment guide in the treatment of my patients. However, is it anymore powerful than HDL/trig ratios or CRP? If the LDL-P is discordant with other favourable patterns do I still start the patient on a statin? If already on a statin do I start a use a bile acid resin if LDL-P remains high? Probably not if the statin is mainly operating as an anti-inflammatory agent irrespective of its cholesterol effects. Is LDL-P more relevant to established CAD than primary prevention? Can guidelines and targets really be formed if LDL-P is really just the smoke and not the fire?
      To believe the LDL-P risk story I need a better mechanism than a simple concentration gradient, especially in a highly regulated receptor mediated environment. Otherwise, I fear we may fall into the same logical traps as the saturated fat and total cholesterol debates.

      • Tim, I appreciate your rigor and in a rare act of violating my rule of how much time I allow myself to address questions… here goes (PLEASE note, I won’t be able to respond again on this thread, but I will re-visit some of these themes in part X of the cholesterol series one day):

        I am sorry if I am being pedantic with language, but you both refer to LDL particles as being able to “penetrate” the arterial endothelium( gaining access to the subendothelial space)once particle number reaches a certain concentration. Am I to believe that arterial endothelium simply succumbs to passive diffusion once that arbitrary threshold is reached? If this is the case please can you help clarify? As I understand it, APO-B particles are thought to enter the artery by way of either trancytosis or intracellular transport.

        A: There are both endothelial gaps (spaces between “irritated” or damaged endothelial cells) and there a number of endothelial receptors (mostly types of scavenger receptors) that internalize lipoproteins – both LDL particles and remnant particles carrying apoB. Normal LDL receptors do not play a major role in endothelia.

        I would assume that like most organs and tissues in the body, arterial cells would have receptors to transport various insoluble metabolites across cell membranes.

        A: Insoluble (hydrophobic) or amphipathic lipids can also enter via diffusion. Much cholesterol gets in to black via microscopic plaque hemorrhages (which introduce cholesterol laden RBC). Many are unaware that RBCs are a significant pool of cholesterol (on their membranes).

        Indeed this is the case. Arterial endothelial cells express the LDL receptor(LDL-r) that binds to Apo-B particles. As you have described, every cell has the ability to synthesise its own cholesterol. One could assume that like most other cells, arteries would up-regulate LDL-r when more cholesterol was required and down-regulate when to much is available. What role or mechanism can particle number have in this scenario?

        A: Unlike the liver and intestine there is no LDLr-modifying regulatory pool of cholesterol in enndothelial cells – they like most cells synthesize all the cholesterol they need. But for sure the more LDL-P the easier it is for any expressed LDLr to “find their prey” – meaning ligands. A fishing net will capture a lot more fishes if lots of fish are present.

        This brings me back to my initial mention of inflammation. My particular interest lies in the initiating event in the atherosclerotic process. This is a chicken and egg argument that has never been solved convincingly. So, does the presence of “atherogenic” lipoproteins within the arterial cell spark the attraction of the 1st monocyte/macrophage and thereby initiate the the typical inflammatory cascade that leads to the production of a foam cell (known as the retention hypothesis)?

        A: Yes, see all of the Ira Tabas review articles on this topic: atherogenesis requires apoB-entry and then the maladaptive inflammatory process that begins. He has shown that the initial event in atherosclerosis is apoB particle entry.

        Alternatively, is it that inflammation was already present and cholesterol and lipoproteins merely offer themselves as “building blocks” to the dubious motivations the that macrophage/foam cell?

        A: If there is an underlying vasculitis of other etiology, then the process is facilitated and lipoprotein entry occurs at lesser concentrations.

        If the later were the case, one could generate an alternate hypothesis where by the inflammatory signalling might up-regulate LDL-r in order to gather more “building blocks”. As fanciful as this may sound, its plausibility has been confirmed in a 2012 study:
        (Inflammation Disrupts the LDL Receptor Pathway and Accelerates the Progression of Vascular Calcification in ESRD Patients)
        What then to make of LDL particles in the ” inflammation comes 1st hypothesis”. Taken at a localised level, this atherogenic intra-arterial inflammatory process might, in its effort to obtain more cholesterol, deplete the various LDL particles of there cholesterol and spit them back out into circulation (small dense LDL and oxidised LDL for instance). Could this be a substantial factor in the plasma levels of small LDL?

        A: No that does not seem to occur to any appreciable degree. Small LDLs are generated during lipolysis in plasma due to many factors, mostly core TG, CEPT activity and lipase activity.

        Perhaps on a system wide basis?
        What about particle number in this hypothesis? This same process might also lead to some additional particles in circulation but I am doubtful that arterial ” recycling” would shift measured LDL-P numbers significantly. However, “systemic” inflammation could mediate a similar process in other body tissues, say visceral fat for instance. So we might use metabolic syndrome as an example. LDL-P might increase as peripheral tissues deplete LDL particles of their contents to aid in an adaptive response to an inflammatory stimulus (hyperglycaemia/insulin resistance).

        A: Other tissues outside of steroidogenic tissues do not rely on LDL particles for sterols. They have no need for a cholesterol delivery — that is why there is no LDL-C required for life, cholesterol outside of LDL particles is sufficient.

        Thus the liver might increase LDL particle output in response to cholesterol depleted Apo-B lipoproteins circulating in the plasma. So in effect this would not be a problem of decreased clearance but more of increased production of LDL particles.

        A: The liver does not make LDL particles.

        What then to make of familial hypercholesterolaemia. The lack of LDL-r function is mainly described as it relates to the liver.

        A: There are numerous causes of FH and many have nothing to do with a lack of LDL function (whatever that is).

        Perhaps you can help answer whether there is a difference in LDL-r between the liver and the arterial endothelium.

        A: Liver and likely the proximal gut have much more LDLr expression.

        Maybe peripheral tissues in FH are just doing their part to aid in cholesterol clearance with unfortunate consequences within vascular tissue.

        A: That is true with fibroblasts expressing scavenger receptors and internalizing LDL particles in areas where xanthomas occur – but not in other tissues.

        In any case, I am yet to be convinced that FH is a good proxy for arguments for or against LDL-P risk. Particle number is increased by default not by design in this situation.

        A: Particle number is increased due to a multitude of pathological processes in FH.

        So what clinical relevance can all my nitpicking have? I want to believe the LDL-P/APO-B story and hopefully apply it as a powerful risk assessment guide in the treatment of my patients. However, is it anymore powerful than HDL/trig ratios or CRP?

        A: Actually it is, although all of those have high correlation, they are incredibly discordant in many folks, especially IR folks. That has been shown in multiple studies. So in places where LDL-P can’t be measured easily (e.g., Canada, Europe), apoB is probably best bet. Thereafter, sure, look at these markers, but understand the further one goes from particles, the more likely there is a chance of missing something. Is this chance large? Probably not, especially in non-IR patients.

        If the LDL-P is discordant with other favourable patterns do I still start the patient on a statin?

        A: Tough to say. First, you’d want to know other factors (e.g., apoE status, dietary pattern, sterol pattern). If you are evidenced based, though, you probably ought to act if LDL-P crosses a high risk threshold. In my personal practice, I do take a pretty nuanced approach and don’t necessary turn to meds in this setting if everything is great (e.g., no IR, very low CRP, very low MPO/LpPLA2), but I may look at and track other studies, such as CIMT.

        If already on a statin do I start a use a bile acid resin if LDL-P remains high?

        A: Depends on synthesis and absorption status and a host of other abnormalities as to which is the better second line drug – in most ezetimibe (zetia) would be the choice.

        Probably not if the statin is mainly operating as an anti-inflammatory agent irrespective of its cholesterol effects.

        A: I know of no clinical trial evidence supporting that the benefit of statins is anti-inflammatory – that is just a plausible hypothesis at this time.

        Is LDL-P more relevant to established CAD than primary prevention?

        A: Both are LDL-P diseases AFCAPS TexCAPS was primary prevention and apoB was the best marker of risk and establish a goal of treatment.

        Can guidelines and targets really be formed if LDL-P is really just the smoke and not the fire?

        Guidelines are based on evidence that exists and many now utilize apoB/LDL-P in a variety of functions.

        To believe the LDL-P risk story I need a better mechanism than a simple concentration gradient, especially in a highly regulated receptor mediated environment. Otherwise, I fear we may fall into the same logical traps as the saturated fat and total cholesterol debates.

        A: Perhaps, but that will take years of studies to “prove” anything in medicine. In my humble opinion – which you can freely choose to disregard without hurting my feelings — in 2013 ignoring LDL-P, at least in most people, may not be the best strategy for mitigating heart disease.

    • Thanks very much for your gracious response. I am humbled, as I am only a “weekend white belt armchair lipidologist”. Your explanation regarding endothelial permiability does alleviate many of my concerns. More homework for me I guess. Here in Australia, APO-B ( LDL-P unavailable ) is not even on the radar . Naturally stepping outside established guidelines with few colleagues doing the same is bound to produce some professional anxiety. I only desire to cover as many bases as possible in order to do so. I certainly will not disregard any of your opinions which are extremely reasonable and well thought out.
      Cheers

  2. CommentDear sir,’. Very enlightening.must admit that quite Difficult to understand initially.I would like to know hOw prof .Dean Ornish was successful with low fat diet

    • I think the Ornish results showing improvement were not from diet alone, at least the initial results were. The program included very low fat PLUS a stress reduction program, emotional support from group meetings, and probably other healthy things like stopping smoking.

      Yet people cite his results and attribute them to diet alone.

  3. Peter is right on: If you have too many small LDLs or too many big LDLs you are at risk for CHD. If you have all small LDLs but total LDL-P is normal, there is no risk. If you have normal numbers of very large LDLs your LDL-C might be high but LDL-P will be fine and thus no riskexists. Peter nicely illustrates that if you have small LDLs, it will take 40-70% more LDLs to traffic a given mass of cholesterol, hence untreated folks with small LDLs always have a high LDL-P and thatis what drives risk. Likewise if one’s LDLs are pathologically carying TG instead of cholesterol (i.e. a cholesterol depleted LDL)it will take many more such cholesterol-depleted LDLs to carry a given acholesterol mass. Risk is related to particle number not to the amount of cholesterol within the LDL particles, because it is particle number that drives the LDLs into the artery

    • I’m still having trouble with the concentration gradient aspect (particle number “driving” LDLs into the artery wall). A table in part III states that a good LDL-P is given as <1000 nmol/L and a risky LDL-P is 1300 nmol/L. This is way less than 2-fold concentration difference. When you are talking about this kind of mass-action mechanism, that's not very much.
      Seems to imply that the "design" of the endothelium really has us on the knifes-edge of keeping LDL out of the wall and rather catastrophically letting way too much in.

    • Thanks to you Dr.Dayspring and Dr.Attia, the nonsense that pervades the internet in believing that so long as you large LDL, no matter the number, all is ok.

    • Dr Dayspring,
      I have read you talk about the delipidation of VLDL causing high LDL-p which are small and CE poor,TG rich. Is that the mechanism by which fish oil can increase LDL?would statin plus or minus zetia still be the appropriate treatment?

    • Another great post, Peter. Thank you.

      My NMR results for LDL-P were 1130. The LabCorp report shows anything over 1000 as “high.” LP-IR score was 5. How worried should I be about about coming in at 1130?

    • To Jim> Fish oils containing DHA and EPA at high doses reduce TG and increase LDL-C and are neutral of LDL-P. Thus the LDL-C rise has no clinical meaning. Those with high dose EPA with no DHA do decrease LDL-P ~8%. Check out Vascepa a soon to be approved n3 FA product

  4. Great series, I have learned so much.

    One question that I have had that may relate to Reason #4 above is the role of oxidation.

    Specifically I am referring to Chris Masterjohn’s statements I have heard on different podcasts that seem to imply that oxidation of a LDL particle was a pre-requisite for that particle to be part of the oxidation process.

    In Part IV you stated “not long after an LDL particle gets into the sub-endothelial space … it is subject to oxidative forces”.

    Could you please clarify this point for me?

    Thanks!

    • Once in the S-E space, assuming the LDL particle is retained, the immune cells react to the presence of the particle and it’s sterol cargo, which results in an inflammatory cascade, including oxidation.

  5. Peter, do you have a model that fits this situation? Off the top of my head I could guess that the more particles, the greater chance that they could end up where don’t want them to go. In this realm I assume that it dosn’t matter how high your good HDL is, as it gets overwhelmed by the sheer volume of particles to clear. I found out this week that my LDL-P was close to 1500, while LDL-C was 116. Will you be commenting in future posts as to the best modulators of LDL-P? I have a few guesses…. 🙂 Thanks, Dave

  6. So, in essence, you are maintaining that the presence of small LDL is a surrogate for something else that is contributing to atherosclerosis?

    And a small sticking point. As a 50 year old with hzFH (I have the corneal arcus and tendon xanthomas to prove it!), I can emphatically assure you that I did not die in my 30’s or 40’s from untreated hypercholesterolemia. We don’t ‘all’ die an early death. 😉

  7. a side: Attia is a character from my favorite hbo series all time: Rome. Anyone not seen it is deprived and possibly and unrelatedly depraved.

    a propo: assuming a man age 67 with type 2 diabetes and just completed successful radiation therapy for stage 1 prostate cancer, could you lay out what you expect the levels of pertinent apo b and ldl p would likely be on his nmr if you had to guess?

    More simply, this profile describes a relative of mine and i suggested he get an nmr at his next doc appt. What would you look for in his levels given the intensely high insulin levels be undoubtably has due to injecting it daily? What realistic goals should he have towards the results, eg, what should he shoot for as a diabetic, visa-viz his nmr results.

    Writing on a kindle so excuse the crappiness of my phrasing of these questions.

  8. As a science wonk I love all this background we’re getting but I’m wondering if later in the series there will be a post that I can show to my friends and family that their slavish adherence to the belief that LDL-C and HDL numbers is a waste of time and that they must get an advanced lipid panel showing their LDL-P in order to truly gauge their cardiovascular health. Don’t get me wrong, I love all this info but I’m already a believer that the conventional wisdom is dead wrong. Getting someone like my dad to read the series just isn’t going to happen but I’m sure I could get him to read a single post. Thanks!

    • Absolutely, Bob. You will write the 3 page summary of this 9 part series and share with your family, friends, and the rest of us to do the same. Thank you. 🙂

    • Better yet a special message to physicians. Something the doc would listen to when not inclined to listen to my own sketchy description.

    • But then there is the issue of what to when the advanced lipid profile shows risk, as in high apoB. This is what I’m dealing with now. My neighbor – a retired radiologist – says to absolutely not take a statin. Other doctors say absolutely take a statin.

    • Hi Peter! I’d love to read the 20 page summary of Gary’s book WWGF that you wrote for your family. That would be a great addition to the Media section 🙂

    • I’m currently trying to work on a “friends and family” version. I’ve taken a lot of info from these posts as well as a bit from other reading I’ve done. I have family and friends who cut fat and cholesterol and are on statins in hopes of lowering cholesterol. I’m anxious to share some of this info with them, but I know many won’t be able to really understand all of the science in these posts (I love it though!). If anyone would like to see a copy and/or proof it to make sure I am accurately representing ideas I’d be delighted. I’m not going into all of the science. My aspiration is to get into enough of the science for the non-science minded to understand some of the background without overwhelming.

  9. Peter, thanks again. Had my annual with my GP yesterday and discussed my now 3-months of VLC-HF diet, my progress on weight loss (from 185 >> 166, so far), and my newly normal blood pressure (was high enough to medicate before). He was all sorts of skeptical about my diet, and advised me to “moderate”. I rather suggested he use me as his n=1 observation and see how things go. I asked for the NMR lipid panel, and he didn’t know anything about it. Thinks the standard panel is fine (and my cholesterol numbers have always been good on this panel) anyway. I didn’t press, but wish I had a way. Want to know my particle count, thanks to your teaching!!

    • John, as an N = 1, I have been LCHF for about 4 months. I tested my LDL-P and it was close to 1500 with an LDL-C of 116. I had to educate my GP but she’s on board now. You own your own health and not your GP so you need to go back and insist on the NMR lipid test. I am working actively now to lower LDL-P with my GP onboard. I hoped eating LCHF would resolve any LDL-P issues, but it’s going to take more than that. See Tara Dall’s video series on Lecturepad.org for various case studies and why and when intervention is needed.

    • None of my physicians would prescribe an NMR test, so I paid for it myself. You don’t need a prescription. Then none of my physicians understood it.

      • Hopefully, in time, this will not be the case. Remember (everyone), if you want your docs to understand this, you have two choices:

        1. Give them resources to learn it (e.g., Lecturepad.org).
        2. If they refuse to do so, you have the right to find a new doctor.

    • Second the suggestion to listen to the Tara Dall lecture on lecturepad.org. It’s surprisingly accessible (well, after you’ve read all these posts here at least). Then you’ll be able to compare yourself to the case studies she gives and know when you possibly might ask your doctor for 1 – metformin; 2 – a statin; 3 – both.

      Note that she repeats several times that diet & lifestyle changes are her preferred treatment. (Hint.) One of her slides even lists “low carb diet” as an intervention.

  10. Very interesting and informative article. What conclusions can one derive as it relates to diet? The “Fat Head” documentary brings forth the theory of dense and non-dense LDL particles, further stating that carbs create dense LDL particles. Is there any truth to that? How does one lower his LDL number?

    Thank you!

    • Carb reduction is certainly associated with a shift towards more Pattern A, but as far as what is lowering risk, it’s not the size of the particle. Presumably it’s that each particles is more importantly carrying more cholesterol and less TG, which means fewer particles.

    • so are you saying that TG and particles are tightly interlinked? VLDL production is primarily a result of the need to export TG from the liver and hence high TG correlates to high LDL-P?

      • That is a large driver of discordant LDL-P and LDL-C, as you’ll see this week. But there are many reasons, including genetic ones, that lend to higher than ideal LDL-P.

  11. this series has been VERY educational! among other things, it’s always nice to know what kinds of tests we SHOULD be getting, as opposed to what is standard.

  12. Peter,

    One of your charts shows the “correlation between LDL size and LDL-P = -0.64”, which seems to be a pretty strong inverse correlation. So, “all else being equal” (ahem), is it fair to say that those with smaller LDL size will have higher LDL-P, and those with higher LDL size will have lower LDL-P?

    I’m guessing that neither LDL size nor LDL-P is causative in this correlation, but rather a different cause is causing this correlation — i.e. something else is causing LDL size and LDL-P to move in an inverse relationship. Is this guess on track?

    Thanks for the great posts.

    • Yes, there is a correlation, but would you bet your life on a 0.64? Maybe if it were 0.99 we could have the discussion. This correlation is largely a result of the epidemic of metabolic syndrome.

    • The take home for me from this and some related references is that it’s the folks with discordant LDL-C and LDL-P who won’t see their risk without particle number measured. These are the folks that don’t fit the correlation.

      Interesting to me from Otvos’ review of some of the population studies is that even with factoring for particle, waist size still held up as a risk for hard CV outcome.

  13. Maybe this is starting to come together…

    How about this:
    LDL-C = LDL-P X (cholesterol/particle). So if the amount of cholesterol/particle decreases, the particle number will go up. The amount of cholesterol/particle decreases if cholesterol is displaced by something else, namely too much triglyceride.

    So you can (in theory) reduce LDL-P by increasing the amount of cholesterol/particle (presumably by decreasing triglycerides) or by reducing LDL-C. I’m guessing that reducing triglycerides is “easier” and has other benefits as well.

    I think the piece I was missing is that high triglyceride displaces cholesterol from LDL – time to go back to re-read Part I-III and sort this out. Right now I’m not real clear on the relationship of cholesterol and triglyceride in the various types of particles (LDL and HDL).

    • You’re on the right track, Bruce. When the liver is exporting TG at a geometric rate (such as in the case of someone eating a lot of sugar, simple carbs, or alcohol), the ratio of CE:TG decreases in the lipoprotein particle. Ero, you need more particles to traffic the same amount of cholesterol.

    • Peter,

      In your reply you mention the liver’s response to alcohol: do you mean “a lot” of alcohol? Or does alcohol consumption, however much, always cause the the “liver [to export] TG at a geometric rate”?

      Do you drink alcohol, and if so, how does it affect ketosis?

      Thanks,

      Patrick

      • I’ll cover this in subsequent posts. Too much for a quick answer. Search through old comments, though, as I’ve answered this question 8 or 9 times over the last few months in one form or another.

    • Alcohol kicks most people outta ketosis – your body will stop burning fat to burn off the liquor first, Dr. Phinney has said. So at best, it reduces your fat-burning time. It’s a pain to sit around thinking – “darn, I’m metabolizing this wine when I could be chewing through 250 calories of ugly junk on my trunk instead!” Basically, I’ve stopped drinking. Burn fat, burn!

  14. Is there a fluid dynamics explanation for why LDL-P is so much more important? Is it that the tendency for particles to spread themselves uniformly within the blood makes a much bigger difference than the relative difference in particle size?

    • I did some research. And there have been quite a few papers on fluid dynamics models for solutes in the blood. Most of the models that are used for particle-artery wall equations use a diffusion equation with a generic term “concentration.” <One paper explains that "concentration" in the context of diffusion is linearly correlated with partial pressure. SIAM J. NUMER. ANAL. c 2001 Vol. 39, No. 5, pp. 1488–1511

      Partial Pressure! – That is only affected by moles, not mass.

      Another paper Med Eng Phys. 2010 Oct;32(8):867-77. Epub 2010 Jul 2. proposes that LDL accumulation occurs when the right combination of LDL Concentration (read Molarity) combine in the artery.

      So I guess, in summary, if we assume that the process by which LDL particles are driven into the S-E space is a gradient driven process, then only an increase in LDL-P would increase the gradient and drive more particles into the S-E space.

  15. Many thanks for this important work Dr Attia

    I get (and have done so for some time) that LDL-C is effectively a useless test — despite the fact that it is used as the basis for a great many (the vast majority?) statin prescriptions. Here in Nova Scotia, Canada, LDL particle count is rarely done and my Endo. is not even authorized to order an ApoB test – despite it being recognised in the lipid guidelines.

    I do think that the work carried out looking into LDL particle size was an important step along the way to establishing that LDL-C is not useful. I accept that size is not just large and buoyant or small and dense but on a continuum.

    I also get the fact that the chief predictor of risk is LDL particle count (LDL-P) rather than size.

    But I am still not sure where or if you show that: increased LDL particle count is a cause of CVD rather than a marker for some other process?

    You say >>The trial data are unimpeachable and there are now 7 guidelines around the world advocating particle number measurement for risk assessment. The more LDL particles you have, the greater your risk of atherosclerosis.<< so I will have to go back and reread the earlier posts to look for the trial data that I must have skipped over but surely particle count measurement for risk assessment does not preclude the possibility that LDL-P is a marker rather than the cause?

    It may be a moot point as I’m hoping a future post will explain what influences LDL-P… and I suspect that diet (rather than medication) will play a crucial role.

    • It’s not a moot point. It is THE point. The mechanism of atherosclerosis very clear and it is entirely initiated by the LDL-P (you may consider re-reading part IV of this series). Next week I’ll address the ability of LDL-C to predict risk. It’s certainly not useless, but it’s a gamble. At least one third of patient have a low-normal LDL-C and yet high-risk LDL-P. This is called discordance, and it will be the focus of Part VI.

  16. Hi John M. I sympathize. I’ve been trying to get the heart post transplant group at Cleveland Clinic into a dialogue on adding LDL(P) to my testing, but so far they are blowing me off. I have no doubt there are doctors at the Clinic that are conversant with LDL(P), I just am not dealing with them (yet). Interestingly, my home area GP is supportive. Go figure.

  17. The question that has sat unanswered in my mind since early in this series, and now burns more fiercely than ever: why does the body make unhealthy numbers of lipoprotein particles, and what can we do to correct the problem? Lifestyle? diet? exercise? Is there some imbalance or other aberration in the body that the body is trying to correct with the copious production of lipoprotein particles? If so, THAT is what needs to be addressed. I hope this knowledge has been developed and that you will address it. I can’t find the answers.

  18. This is very interesting. I changed my diet about 6 months ago. At that point, my LDL-P was 930, with 45% being small LDL. At that point, my diet was low carb and grain-free, and centered around fish and vegetables. After reading Gary Taubes, I then added saturate fat — meat and coconut oil, fairly liberally — only to find my LDL-P go up to 1500 after four months on this diet. (My HDL went from 59 to 71, and all the small LDL-P disappeared.)

    This doesn’t comport with some of your earlier posts. Any thoughts on what might be going on? I think I need to go back to avoiding saturated fat, unless there is another plausible explanation. (FWIW, I’m ApoE 2/3.)

    • Hard to know without a great deal more information. Make sure when you do a repeat NMR you have them look at absorption/synthesis markers. HDL, Inc. does this very detailed assay.

    • David, try having them do a test for ApoE genotype. From what I’ve read, different genotypes (2/2,3/3,4/4) will influence whether or not saturated fat will impact your LDL levels.

    • @David,

      Good question. I’ve been mulling this too. My Apo-B almost doubled since going VLC, while every other health marker improved dramatically (weight, blood pressure, TG, HDL). I’m focused on trying to understand the LDL-R activity in all this.

      Theory 1… prolonged carbohydrate restriction may result(evidence in rat and human models) a quasi-starvation response and reduced metabolic rate. This metabolic rate reduction is mediated by thyroid hormones, which turn out to be an important requirement for the expression of LDL-receptor. Thus, low T3 or high rT3 will lower your clearance of LDL particles.

      Theory 2: Increased saturated fat might also downregulate LDL-R activity. I’ve been trying to understand the relationship between LDL-R and various fatty acids and I came across several references that saturated fat would increase LDL via downregulation in LDL-R activity. This would directly lead to an increase in LDL-P and LDL-C.

    • What do you mean by “liberal” use of meat & coconut oil David? Taubes is suggesting 4-6 oz max protein each meal and maybe 2 tablespoons of coconut oil a day. If anyone is eating more than that, they’re overdoing it. No one is saying you can eat 2 lbs of meat a day and 6 tablespoons coconut. That’s crazy unless you’re Michael Phelps. 🙂 How much are you really eating?

    • Did you lose weight? Phinney and Volek’s “Art and Science of Low Carbohydrate Living” says there’s “a transient rise in serum total and LDL cholesterol that can occur with major weight loss.”

      “It turns out that along with the triglyceride stored in adipose tissue, our fat cells also contains a small amount of dissolved cholesterol. After about 30 pounds of weight loss, the shrinkage of these cellular fat droplets proceeds to the point that some of this cholesterol has to be released into the serum.”

      They also say it should return to baseline after weight loss stops.

    • Thanks for all the comments. To answer: This is my second NMR after starting to eat meat again. The first one came out similarly, with an LDL-P of about 1300, and the second one was LDL_P of 1500. Corey, My ApoE is 2/3. Ed, very interesting; my TSH has gone from 1.5 to 2.2, though the T3 and T4 are middle range. Maybe the low carb has a negative effect on thyroid. An interesting hypothesis! Grass Fed, no, I don’t eat that much meat; maybe four ounces of grass-fed beef 3-4 times per week, and 1-2 tablespoons of coconut oil per day. Victoria, I went from a BMI of 22.5 to 21.5 — not much of a weight loss at all, so that’s not the culprit.

    • @David-

      If you are interested in the thyroid connection, there are a few great blogpost discussions on Jaminets website discussing LDL, Thyroid and the Carbohydrate connection. It’s a great read and there is too much anecdotal as well as published evidence to ignore this effect, especially on a VLC diet.

      I’ve been working on this assumption and had my thyroid tested. Turns out you can drive a semi truck through the reference ranges for TSH, FT3 and FT4. I also got my rT3 checked. All my nums were all within reference range (Peter- who the F??? decides on a reference range), but when you look at thyroid blogs they will tell you that the ratio of FT3/rT3 should definitely be greater than 20, and mine was 14… So depending on how you interpret the numbers, you come to different conclusions.

      • Folks, I cannot address this point adequately in a few sentences. So let me ask of you all one favor: Relax on this topic until I’ve had a chance to shed some slight on it. There is a lot of confusion out there, and some outright incorrect information, but I need to really explain it in the same way (don’t worry, it won’t take nearly as long) I’m doing with cholesterol.

Leave a Reply

Facebook icon Twitter icon Instagram icon Pinterest icon Google+ icon YouTube icon LinkedIn icon Contact icon