January 20, 2020


#89 – AMA #11: All things fasting

“Truthfully, in humans, we don't necessarily know exactly what the dose curve looks like for duration of fasting to physiologic output of desired consequence.” —Peter Attia

Read Time 17 minutes

In this “Ask Me Anything” (AMA) episode, Peter answers a wide range of fasting-specific questions from subscribers. Peter starts by defining the various fasting protocols, details his own personal fasting regimen, explains his revised plan for 2020, and provides a ton of value to anyone interested in fasting. Once again, Bob Kaplan, Peter’s head of research, will be asking the questions. If you’re not a subscriber and listening on a podcast player, you’ll only be able to hear a preview of the AMA. If you’re a subscriber, you can now listen to this full episode on your private RSS feed or on our website at the AMA #11 show notes page. If you are not a subscriber, you can learn more about the subscriber benefits here.

We discuss:

  • Defining the various fasting protocols [1:00];
  • Why Peter plans to switch to a 3-day fast once per month in 2020 [11:00];
  • How Peter uses his CGM to gain insights into the depth of fast [13:15];
  • Peter’s supplement protocol during fasting, and why he eats a ketogenic diet leading up to a prolonged fast [17:00];
  • Peter’s exercise regimen during a fast [23:30];
  • Peter’s hunger levels during a typical 7-day fast [26:45];
  • Fasting observations—Core body temperature and thyroid hormone [30:30];
  • Fasting observations—Glucose, BHB, and hunger levels [33:15];
  • Peter’s sleep protocol during a fast [40:15];
  • Does Peter observe any differences between men and women in their ability to fast? [47:00];
  • How Peter prefers to break a long fast [50:15];
  • Importance of community support while fasting, and is there a perfect fasting protocol? [52:00]; and
  • More.


Defining the various fasting protocols [1:00]

The overnight fast

  • The standard fast that almost by definition everybody does between dinner and breakfast
  • Example: Finish dinner at 7pm, eat breakfast at 7am the next morning = 12 hour overnight fast

Time restricted feeding (TRF)

  • The idea where you increase that window from just an overnight fast into a slightly longer window
  • To put the overnight fast example used above into time-restricted feeding nomenclature, you would call it a 12/12 (meaning 12 hours of fasting followed by a 12-hour window)
  • You can extend that and you can say a 14/10 window or a 16/8 window or an 18/6 window 
  • The first number refers to the period of time that has no nutrient exposure and then followed by the window in time in which one would be able to consume food (typically ad libitum aka without restriction)

Alternate day fasting (ADF)

  • Generally speaking, ADF means that every other day you do something other than eating ad libitum
  • Most strict ADF = Consume nothing everything other day (this would be challenging to maintain over the long haul)
  • More common versions of ADF = every other day you have a hypocaloric day
    • Example, 1,000 calories on the “fasting” day and then eat ad libitum (or they’re normal calories amount) on the non-fasting day
    • Could also refer to this as “alternate day calorie restriction” instead of ADF

Intermittent fasting (IF)

  • This term has become popular with the public
  • But many people mistake IF with TRF… in other words…

{end of show notes preview}

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  1. Based on your comments about T-3 and the thyroid’s importance in metabolism, I’m not recalling if you have spoken on whether those with thyroid conditions should or should not fast. Specifically wondering about whether there is a difference between the effect on those with hyper- vs. hypothyroid conditions. I’m sure it may be based on the individual’s overall health and goals for the fast and that you can’t generalize per se. Just wondering if you’ve seen trends or have general cautions. Thanks for any light you can shed, and thank you overall for your podcasts and posts and writing. They’ve made a difference in my life.

  2. I fully support paid content over advertising models. But I would like to ‘buy’ just this episode, is that possible? The issue with the subscription model is that you’re all in for all topics, which is not a great fit for me right now.

  3. Great episode! Interested in your experience with xylitol gum and ketone suppression – do you think this would be true also for other sugar alcohols like erythritol? Any idea about mechanism?

  4. Interesting about the thyroid during a fast. It makes me wonder if someone on replacement who frequently fasts would be better off with cytomel vs levothyroxine.

    Personally – I like to do 5-7 day water (&tea) only fasts. I do not measure ketones or use CGM, do not do any supplements other than my O.N.E. vitamin and have always found it very easy, ?too easy. I break my fasts usualy because my child becomes concerned.

  5. My comment is more of a question. When you evaluate your patients, so you take into account their blood type and their body types such as endomorph, etc. and then determine if the outcomes of these fasts are similar between blood types with similar body types, and vice versa. I think that analysis would be beneficial for people to know. That way they can customize their fast based on body type and blood type accordingly.

  6. 1) What is a small effective doze of L-threonate?
    2) If not buying Doc Parsley, what are small effective dozes of
    (a) melatonin and
    (b) 5-HTP?
    3) Life Extension labs has 300mcg and 500mcg 6-hour time-released melatonin. Is it a good replacement for Natrol? (On Amazon, Natrol’s time-released melatonin is at least 1mg.)

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