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BMI cannot distinguish between fat and lean mass. Nevertheless, it is a good (initial) tool for assessing overweight and health. That is the conclusion of a study published last month.
BMI
We’ve had some discussion here about the Body Mass Index (BMI).
When people start taking strength training somewhat seriously, apparently part of that is ridiculing the BMI afterward. I’ve been guilty of that myself when I gained almost 20 kilos of muscle mass nearly two decades ago.
“Pfff, BMI means nothing. According to my BMI, I was at the lower end of a healthy weight, and now I’m almost overweight. It’s just that muscle mass has been added, but now I’m supposedly fat.”
It’s also a way for bodybuilders to distinguish themselves from normal mortals who are only concerned with fat loss. A way for the ‘pro’ to distinguish himself from the ‘amateur’. For a ‘pro’, it’s not enough to step on a scale. A ‘pro’ would preferably want rocket scientists to map out his body. Something like a skinfold caliper and a five- or rather a seven-point measurement sounds much cooler.
BMI and Health
But BMI wasn’t developed for people who can dreamingly name all the amino acids for you. BMI isn’t intended for people who prioritize their body(shape) above all else. BMI is meant for people and their caregivers who want quick insight into health based on limited data.
Of course, there are better measurement methods to gauge the potential effects of weight on health. Yes, a body fat percentage says more because it distinguishes between lean mass and fat mass. We also described the various methods to measure this percentage as accurately as possible. Knowing exactly where the fat is located provides even more information about health. We know that especially fat in the abdominal region can be harmful to health.
Even if you do want to work with weight and height as data alone, there may be a better method for that, namely the TMI.
And yet BMI is a good first indication of weight and potential health risks. This was reaffirmed last month by the publication of research from the University of Bristol [1].
Reliability of Body Mass Index
The researchers used body scans of 2,840 young people aged 10 to 18 from Bristol. They compared the conclusions based on BMI with the conclusions from more detailed fat measurements.
They looked at the effects of total fat amount and fat in the abdominal region on 230 different properties relevant to metabolism and future health risks. Think, for example, of cholesterol and blood pressure. They compared these effects with the effects observed when BMI was used as the starting point.
A higher BMI, more total fat, and fat around the abdomen were all found to have similar effects. Thus, according to the researchers, these have a large overlap as a measurement tool.
Lean Mass Doesn’t Protect
The well-known associations were found between higher total fat amount and harmful high levels of blood pressure, high cholesterol, and inflammatory markers at age 18. Lean mass (everything except fat and bones) did not protect against this. Bad news for powerlifters and strongmen.
BMI is often criticized. Our study asked how useful it really is for detecting the health effects of obesity by pitching it against more objective body scan measures. We found that trunk fat is the most damaging to health, but that simple BMI gives very similar answers to more detailed measures. This is good news since BMI is widely measured and costs virtually nothing.
Dr. Joshua Bell, University of Bristol
This doesn’t mean that more lean mass doesn’t offer benefits. Think especially of muscle preservation in later life. But it does imply that you don’t compensate for the health effects of high fat mass by developing more muscle mass.
References
- Joshua A. Bell, David Carslake, Linda M. O’Keeffe, Monika Frysz, Laura D. Howe, Mark Hamer, Kaitlin H. Wade, Nicholas J. Timpson, George Davey Smith. Associations of Body Mass and Fat Indexes With Cardiometabolic Traits. Journal of the American College of Cardiology, 2018; 72 (24): 3142
- https://www.sciencedirect.com/science/article/pii/S0735109718388302?via%3Dihub
