BMI is bullshit: my weight is muscle

I think one of the biggest lies I used to tell myself was that although I was fat, at least I was (also) strong. My first few sessions at the gym were bad. Not because anyone was mean to me, but because I was forced to admit that I wasn’t particularly strong after all. True, I was able to leg press a relatively large amount (a little over 100 kg) — not bad for a woman who’d never done any training. But when you consider that it wasn’t even three-quarters of my own body weight, it actually wasn’t very much at all. For comparison, I am now able to press 130 kg — about twice my own body weight.

As an alternative to the BMI, there is now a newly developed method of determining how fit you are (as a fat person). It’s called the sitting-rising test (SRT). It involves standing barefoot and in comfortable clothing in a clear space, then sitting down and standing up again in one motion without using your hands to support yourself (by touching the floor or other parts of your body). You begin with a score of ten points, and a point is deducted for every time you use your hands for support, while half a point is taken off for every wobble due to loss of balance. A score of less than eight indicates a doubled risk of dying in the next few years; a score of less than two points indicates a ten-fold increase in the risk of dying soon.

When I still weighed 150 kg, I usually found it difficult to get up from a flat floor if there was nothing I could use to haul myself up with. For a long time, I couldn’t even imagine how I would get up without using my hands to support me. To be honest, I had to watch some videos online to learn the correct way of executing this test before I was eventually able to score ten points — by which time I weighed 65 kg.

As for criticism of BMI measurements: in my opinion it’s directed in the wrong place. It’s true that the Body Mass Index does not differentiate between fat and muscle mass, as it just expresses the ratio between body weight and height. But the BMI was devised in a time when people were considerably more active than they are today. A 2012 study by Shah and Braverman combined BMI with body-fat measurement and found that the BMI is actually extremely inaccurate — but only when it comes to diagnosing excessive fat. Of the test subjects whose BMI indicated that they were obese, only 1 per cent had a normal amount of body fat. This means that only 1 per cent of subjects were muscular enough to render their BMI wrong. When divided by sex, this figure rose to 3 per cent among males, and sank to zero among female subjects. What this means is that when a BMI figure predicts that a person is obese, there is a 99 per cent chance that they really are obese. And, among the 1 per cent whose BMI is skewed by their muscle mass, those muscles are clearly visible for everyone to see.

Far more interesting, however, is the fact that the BMI is actually wrong in two cases out of every five — when it predicts that a person is not overweight. In fact, 39 per cent of the subjects in the abovementioned study had excess body fat even though according to their BMI they were not overweight.

An analysis of several studies of the BMI (Okorodudu et al., 2010) came up with a similar result: it also showed that the BMI was an accurate predictor of adiposity (obesity) 97 per cent of the time that it showed someone to be overweight, but the analysis also found that it assigned normal weight to about half the people who actually had too much body fat.

A study carried out by Finnish researchers (Männistö et al., 2014) found that 34 per cent of male subjects and 45 per cent of females were obese according to body-fat measurements, despite having a healthy BMI. As it happens, there is a problem with BMI–body-fat comparisons in that body-fat charts usually only include the categories ‘normal’ and ‘obese’, with slightly overweight people being placed in the normal group. So the proportion of those who have ‘slightly increased body-fat levels’ despite their normal BMI may often be even higher.

A German study by Hauner et al. (2008) investigated the prevalence of obesity as measured by BMI compared to waist circumference and came to a similarly drastic conclusion: while ‘only’ about a quarter of the subjects were obese according to their BMI, some 40 per cent were classified as obese on the basis of their waist measurements. Women were more likely to be obese according to their waist circumference, men were more likely to be obese according to their BMI. Abdominal girth is a good method of determining body fat, insofar as it accounts for the fact that very fit people, whose weight is increased by their high proportion of muscle mass, tend to have small bellies. So the results of this study once again show that BMI measurements are more likely to under-represent the number of overweight and obese people in our society, rather than judge muscular people too severely.

To add to this, in their 2015 study, Pellegrinelli et al. found that fat causes inflammation and atrophy in muscle cells. In effect, this means that the presence of fat leads to loss of muscle mass. So, even though overall muscle mass will grow along with the rest of the body because of the increased strain put on it as body mass increases, that growth is then partly negated by the damage actively caused by fat. In relative terms, that means increasing body mass tends to result in decreasing muscle strength. This effect is even more pronounced in people with low body weight but excessive fat.

Unless we are talking about body builders, the rule is that if your BMI places you in the overweight or even obese category, we can be pretty sure that you have too much fatty tissue in your body. And a BMI within the healthy range can still mean a person is carrying too much fat on their haunches.