May 8, 2024

Body Composition

Body composition: impact on disease risk and how to assess and improve it

A refresher on why you should care about body composition and a practical guide to determining and interpreting your measurements

Peter Attia

Read Time 21 minutes

Countless different metrics are used to assess metabolic health. Body mass index (BMI) – the mass of the body divided by height squared – is among the simplest and most commonly used, but while BMI may provide a fair indication of overall trends in metabolic health across a given population, it falls short for assessing the metabolic health of any given individual. I often joke that at the individual level, it tells me little more about one of my patients’ health than their eye color! The reason for this failure is that BMI does not take into account how mass is distributed between fat mass and lean mass, which have largely opposing effects on metabolic health. In other words, BMI provides no information about body composition, a set of variables with far more relevance to health and mortality than body weight or BMI alone. 

The importance of body composition for healthspan and lifespan cannot be overstated, which is why it is a subject that repeatedly comes up in our content (e.g., see AMA 17, AMA 40, and AMA 44). So given its importance, how do we know where we stand with respect to body composition? What are the best ways of assessing body composition, and what are their respective limitations? Which metrics in particular are most predictive of healthspan and lifespan, and what are the ideal values we should be aiming for within each? We will devote the rest of this piece to exploring these key questions in depth.

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  1. Nicely written. Would be nice to draw a relationship between VAT and hepatic fat content.

    That metabolic deregulation which can be seen anatomically, it would be worthwhile noting how it correlates with patients lipidology. Why do we see the LDL figure start trending up more aggressively in the third decade of life. What is concurrently happening with VAT, and what is concurrently happening with hepatic fat content.

  2. My two cents regarding reference graph for FFMI. In the study, from which it is sourced, it’s a reference for Lean Mass Index (LMI) as opposed to FFMI. In the study they don’t use FFMI and I believe Lean Mass (LM) in their definition doesn’t include Bone Mineral Content (BMC) as they operate with three compartment model: Fat Mass (FM), LM and BMC. They define LMI as LM/Height^2. Therefore I believe they didn’t include BMC when creating the reference graph for LMI and thus it shouldn’t be used as a reference for FFMI because it will artificially improve stats – considering BMC ranging between ~2.0-3.5 kg it’s not an insignificant difference especially for people with lower muscle mass where the stats are most important I think.

    Anyway hope this helps. Thank you again for a great article.

  3. This is incredible material that all my physicians should be educated on.

  4. Just how important is a Dexa? I can look in the mirror and put myself in the overnourished undermuscled category. As I learned in school; Don’t to a test if it won’t change your plan.

  5. Can you comment on a method of measuring body composition called a “Bod-pod”?

  6. Really enjoyed this content (as always!). One question – what are the sources of the underlying data for Figures 3 and 4? I am trying to understand if this information relates to the US population only, and, if so, whether the data are an estimate for ideal states of health. For example, considering current rates of obesity I would have expected the 50 percentile of 40 year old males would be something higher than 20% body fat with 25% body fat considered the cutoff in determining obesity in males. Apologies in advance if this is an obvious question – I am not in medicine and trying to better understand how my dexa stacks up against this information.

  7. Note that not all DEXA scans provide body comp metrics. I work at a large teaching facility and NONE of their DEXAs do it. I kept insisting that I needed those numbers and no one knew what I was talking about because they didn’t exist. They finally put me in touch with one of the radiologists who said that it wasn’t measured but that there might be one in one of the sports facilities that could do it.

  8. Hi,
    Is there someplace you can point me to so that I can assess what category I am in?

    Category 1: Under Muscled and Overnourished
    Category 4: Adequately Muscled and Adequately Nourished

    Thank you.

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