Tracking body comp. (esp. avoiding too low body fat) without measuring body fat %

After listening to the Stronger By Science podcast segment on the issues with / inaccuracy of different types of body fat measurement for individuals, I’m wondering how to decide whether to cut, maintain or bulk - both now and over time.

I’d really appreciate your thoughts on these two questions: 1. The podcast recommended looking at more direct biomarkers for general health. Would you do this via regular blood-testing (every 6 months?) or something else? And which markers are most important?
2. The podcast also mentioned waist-to-height ratio being a better measure than body fat % for cardiometabolic risks. But is this useful as a lower bound, or just an upper bound not to exceed? If not, what measures would you recommend using regularly instead to avoid cutting too much? This suggests BMI might be better as a lower bound, but I’m a little sceptical of the reasoning given that BMI doesn’t take into account muscle mass. (If it’s unhealthy to have body fat below x%, you could have <x% body fat but normal BMI just by having a lot of muscle.) Context:

  1. 5’9.5", ~160lb male with ~31.5-inch waist (measured from the anterior superior iliac spine). Started regularly running ~5 months ago and weight training ~3 months ago.
  2. I was cutting for a while on a higher-than-usual protein diet and unexpectedly lost a lot of body fat and weight. BIA said 8-9% in October; three back-to-back Bod Pod measurements, taken fasted, said the same (weight at time: 150lb). I didn’t have a clearly visible 6-pack or anything, so I was surprised my body fat % was this low. (I suspect combination of lack of training and storing more fat than average around abs.)
  3. Mainly, I’m using exercise as a lever to increase general health & productivity; a nice side benefit would be aesthetics / looking good. Admittedly, I’m not concretely clear on exactly what I want.
  4. I previously used bioelectrical impedance and Bod Pod measurements as a guide to body comp, trying to keep body fat between ~9.5-11% to stay lean & healthy but allow some downward ‘wiggle room’ so that if the % got slightly lower I would not be at risk of hormone and mineral decline, reduced energy, and bad cardiometabolic outcomes. ​

PW,

Thanks for the post and welcome to the forum.

Regarding body fat measurement, there are a number of reliable ways to measure this over time that can be used to help guide management. In fact, I would not really argue that the error bars surrounding the different types of tests make body fat testing a poor use of resources. Rather, I’d argue that there are cheaper/faster ways to determine whether or not someone is carrying too much body fat for health purposes, e.g. BMI and waist circumference. Additionally, someone with a goal of a particular body fat likely has a specific aesthetic associated with that number, so they could just use their eyes over any test-generated number. The accuracy and precision of most available body fat tests are good enough to use for long-term trends in my opinion, but don’t add much information that’s actionable.

Regarding “direct biomarkers for general health”, there are age- and demographic-appropriate screening tests that we highly recommend. The time interval for re-testing purposes depends on the test, the result, the intervention(s) being used, and medical history of the individual. We talk about recommended screening tests and health priorities here. Body fat measurements (by proxy in the case of waist and/or BMI) are very important for general health.

Waist to height ratio is not better for predicting adiposity-related chronic disease risks or trajectory than BMI and waist circumference, no. I - and the relevant clinical practice guidelines- do not use this metric. BMI is actually a better proxy for muscle mass than fat mass in individuals, which is why it isn’t great as a screening tool for excess body fat. Instead of overidentifying jacked people as carrying too much body fat, it misses people who have too much body fat, but who don’t weigh enough to be > 30kg/m2. This is why using waist circumference can be useful in individuals with a BMI < 30.

-Jordan

Jordan,

Thank you very much for this.

To clarify:

  1. What would you use to establish a lower bound on healthy weight, e.g. to know when to stop cutting? Having been surprised by a 8-9% Bod Pod measure before (when otherwise I would have kept cutting), I don’t want to rely on a visual guess alone.

Waist circumference? BMI? BIA machine at a gym (despite error bars), perhaps taken under standardised conditions (same time, fasted)? Bod Pod, taken less regularly?

  1. For upper bounds, e.g. to know when to stop bulking, would you use just BMI and waist circumference? What if waist circumference were below 40" but BMI>25?

Basically, what I’d love to find is a simple rule of thumb that would be reasonably accurate at determining whether to cut, maintain, or bulk for a person who cares primarily about general health outcomes - avoiding either too much or too little body fat

BMI of ~19 -20 or symptoms associated with RED-S would be my lower bound. I would not be concerned over bod pod measurements of 8-9% at all. I wouldn’t recommend a bod pod for assessing body fat either, as they are neither accurate nor precise. DXA would be the go-to if you absolutely had to have a number.

BMI > 30 or waist circumference > 36.5" in men (unless some other ethnicity-related cut off) or development of adiposity-related chronic disease.

Thanks, Jordan! Very helpful.