When should bodyfat actually be used as a metric?

Nutrition Q&A [Moderated]
1

Hi there,

DXAs and similar are becoming much more accessible and tbh I’m not convinced in their value beyond bone mineral density. This seems to have lead to people dieting harder than maybe is necessary to reach an arbitrary bf%

In terms of bodyfat percentage, visceral adiposity seems to me to be the most meaningful metric to target (specifically reducing it) instead of “this machine says I’m 20% bodyfat but I’m in outstanding shape and have a healthy waistline”.

It can be really challenging (see bodybuilding comp prep) to reduce bodyfat to teens and lower levels, and keep muscle as well as maintaining high performance.

Is there any reasons pertaining solely to performance or health where in the absence of visceral fat accumulation; body fat as a metric should be considered? Naturally if people were to gain only muscle, their bodyfat would reduce, so I guess there is that aspect.

Keen to hear your thoughts!

Thanks

Winston

2

I think measuring/tracking body fat is most useful when it affects management. If knowing the number doesn’t really change behavior or strategy, then it’s probably not worth knowing. With respect to body fat and health, liver fat is probably the most meaningful type to target. VAT and intramuscular fat would be second, and subcutaneous fat third. This should not be interpreted as sub-Q fat being benign because it’s not.

With that in mind, a few health metrics used to either measure body fat directly, or predict risk from it, can be ranked from best to worst:

  1. CT/MRI - The gold standard for measuring body fat amount and distribution in VAT and liver. Problem is, radiation (CT) and cost (MRI).
  2. DXA- pretty good for VAT and body fat. Not good for liver fat.
  3. Body Roundness Index/ Waist to Height Ratio/ BMI + waist together- decent for VAT and body fat. Also not good for liver fat.
  4. BMI- > 30 is very specific for identifying those who are carrying too much body fat (VAT, liver, sub Q). Misses a substantial portion of those with normal BMI, but expanded VAT and/or liver fat. Likely half of people with a non-obese BMI (<30) have one or both. Adding a waist reduces how often excess VAT is missed, but will still leave a cohort of folks with excess liver fat.
  5. Weight- pretty bad all around

Anyway, to your exact question, excess body fat is a risk factor for a number of health conditions. This risk is not limited to only VAT, liver, or intramuscular fat, but sub-Q fat is also implicated. That said, we don’t have great data on body fat %s and health outcomes, so we use other tests to predict risk and guide management.

And, gaining muscle and/or losing fat can be done in ways that affect VAT and liver fat, and ways that don’t.