“The Body Mass Index is outdated”

Geschatte leestijd: 8 minuten Why BMI is outdated and not always a good indication of overweight and why ‘we’ still use it. Since this subject recently sparked quite a few reactions.

‘BMI bullshit’

Two weeks ago I was metaphorically put on the pillory. Partially I could have expected this; I referred to plus size model Ashley Graham (whose career owes to overweight) as fat. Some wondered why I equated ‘fat’ with ‘ugly’. However, I did not do that. I merely said that personally I did not find it “gorgeous,” “yummy,” “gorgeous” or “splendid.” Quite a difference it seems to me. By the way, preceded by the confession that my ideal image may be rather distorted by my work. I mainly wondered if it is useful for a model to depend on overweight for her career (as the criticism also applies to too thin models). And yes, I used the word ‘fat’. And that in a time when thanks to Silvana we all know that there are limits to freedom of speech. ‘Fat’ of course really can’t anymore in this day and age. Oh yes, sarcasm. That was also something that was not appreciated. I find that very regrettable, really. I understood the barrage of insults I once received as a young hip-hopper when I called three Surinamese ladies in the Bijlmer phat (pronounced: ‘fat’). I hadn’t really thought about the fact that this might not come across as a compliment. However, I was somewhat surprised by the anger over the word ‘fat’ for a plus size model. To remove it from the subjective atmosphere, I used BMI to make a somewhat more objective assessment of her weight. Ashley turned out to be a borderline case of ‘overweight’/’obesity’ according to BMI. Funny to see how subsequently many criticized the use of BMI as an indicator of overweight. I say ‘funny’ because over the past 8 years I have written in several articles about the shortcomings of BMI. BMI can only guess at the ratio of lean mass/fat mass based on statistics. I personally found this out when I gained 20 kilos in muscle mass. This did not seem to be the case in the case of the plus size model. I think that among those commentators there are few who actually believe that, for example, her body fat percentage would tell a different story. Yes, BMI can give a wrong estimate of the amount of body fat. I will therefore further discuss studies that address the accuracy of BMI. No, this does not make Ashley’s story any prettier.

Overweight more often underestimated than overestimated by BMI

Research from 2008 and 2010 shows that BMI in many cases underestimates rather than overestimates the degree of (severe) overweight [1,2]. A study from 2008 found that BMI identified 19.1% of men in a test group as ‘obese’. However, based on body fat percentage, this would be 43.9% [1]. For women, this was a difference between 24.7% (BMI) and 52.3% (body fat percentage). In a 2010 study among overweight women, BMI identified 205 women from a group as obese (BMI >30), whereas according to their body fat percentage, it would be 350 women from that same group (body fat percentage >35%). Similar conclusions resulted from a more recent study from 2014 [3]. In the BMI scales between 25 to 29 (according to BMI ‘overweight’, but not ‘obese’), a large number of women were found to be considered obese according to their body fat percentage. Conversely, in only 1% of cases with a BMI higher than 30 was there no obesity according to body fat percentage. A BMI above 30 proved to be quite accurate, while below 30 there was a significant chance of underestimation. Another study from 2014 showed the same trend but compared the assessment from BMI not only with body fat percentage but also with waist circumference [4]. This time the group consisted of firefighters. BMI identified 33% of men who were obese based on body fat percentage as non-obese. 15% of men who were obese according to their waist circumference were identified by BMI as non-obese. Conversely, 8% and 9% of men who were respectively identified as non-obese according to body fat and waist circumference were considered obese according to their BMI. Here too, underestimation of the assessment according to body fat percentage, and to a lesser extent of waist circumference, occurred more often than overestimation. In the case of the article about the plus size model, statistically speaking, there is a much greater chance that body fat percentage will tell a less beautiful (mind you, I don’t say ‘ugly’) story than BMI.

BMI outdated: rough measurement of body fat

Of course, BMI cannot tell where fat is located. It merely makes an estimate of the distribution of lean mass/fat mass based on weight and height. BMI does not indicate where that fat is located, while this is very relevant information for health [5 to 10]. But even only that amount of body fat is often not accurately represented. For example, a study from as early as 1990 showed a large difference in the amount of body fat at the same BMI [11]. As expected as shown above, but the differences were still remarkably large. With the same BMI of 27, the body fat percentages ranged from 10% (low body fat percentage) to 32% (obese). In the aforementioned study from 2008, large differences in body fat were also observed at the same BMI [1]. At a BMI of 25, body fat percentage varied in men between 14% and 35%. In women, it varied between 26% and 43%. It was also noted that BMI seemed to be able to misjudge the amount of body fat while the amount of lean mass was better estimated. In another study from 2012, the results of a survey (NHANES) between 2009 and 2010 were compared with BMI data from the period 1999-2008 [12,13].

In the NHANES, however, BMI has been found to be highly correlated with percentage body fat as measured by dual x-ray absorptiometry. For men, the correlation between BMI and percentage body fat ranges from 0.72 to 0.79 within age groups; for women the correlation ranges from 0.72 to 0.84. At a given BMI, black men and women tend to have higher lean mass and lower fat mass than white men and women [4]

They compared four indicators for overweight: BMI, body fat percentage, waist circumference, and the ratio between this circumference and height (waist-stature-ratio or waist-to-height-ratio). BMI, waist circumference, and the waist/height ratio were found to be more related to each other than to body fat. Body fat percentage was more in line with waist circumference than with BMI in men. In women, however, body fat percentage was more in line with BMI than with waist circumference, except for the oldest women. There are several studies pointing to necessary corrections of BMI based on specific target groups. Therefore, for each measurement from BMI, based on the height of the BMI in combination with the type of person (origin, age, gender), you should assess how accurate BMI is expected to be.

If BMI is outdated why use BMI?

For lack of something better. I used BMI in the article about Ashley Graham because I could only find her height and weight. In our app, we use body fat percentage to coach clients. After all, we want to make the best possible estimate of nutritional needs and not a rough estimate that we have to adjust several times over the next few weeks after disappointing results. For this reason, we also use formulas that are based on body fat percentage instead of formulas that, like BMI, make estimates based on weight and height, among other things. However, in many cases, you do not have data such as body fat percentage or waist circumference. Body fat percentage provides our coaches with more insight, but it does mean that it must be measured or estimated based on images when measurement is not possible. However, the latter is still our preference. Now you can make an estimate based on photos with accompanying body fat percentage instead of a formula that estimates this based on statistics of weight, height, gender, and age (such as Harris-Benedict). The latter would, of course, be easier. BMI is therefore outdated and especially simple. The formula is simple (weight / height squared) and the required data is easy to obtain. The BMI table with score is easy to interpret and thus usable for everyone.

Conclusion

Of course, BMI is not the best way to assess whether there is overweight and to what extent that overweight can pose a risk to health. However, in the absence of better data such as body fat percentage or waist circumference, BMI is still a good indication of possible overweight (or underweight). This must take into account the accuracy per BMI scale and possible deviations in specific target groups such as certain ethnicities, the elderly, and young people [8].

BMI is an important indicator of overweight and obesity in childhood and adolescence. When measurements are taken carefully and compared with appropriate growth charts and recommended cutoffs, BMI provides an excellent indicator of overweight and obesity that is sufficient for most clinical, screening, and surveillance purposes J.H. Himes, University of Minnesota, School of Public Health

Another approach is to use BMI only as a first indication to assess to what extent it is useful to collect additional data. BMI does not tell the whole story, but it is also not meaningless.

To be continued:

In a separate article, I will delve into an alternative formula called TMI. No, not the corrected method to measure your penis length (thanks Southpark), but a formula that proves to be more accurate with the same variables in some target groups.

References

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