By Dr Harry
Body mass index (BMI) is the current measure which is used by the World Health Organization to determine whether you are obese. BMI is used the world over and applied to everyone in the same way.
One reason for this is that it is easy to understand and easy to calculate using the formula:
Weight in kg ÷ [Height (in metres) squared].
This simplicity however has also led to the BMI being criticised for being imperfect.
Flaw #1: BMI is not an estimate of body fat
The assumption is that once you work out your BMI you know how much fat you are carrying. And fat is unhealthy. So a BMI of 30 means obesity, and thus you need to lose weight, right? Perhaps, but not necessarily.
BMI does not make any direct measure of fat. It merely relates your current weight to your height. There are other ways of finding out your body fat content but these methods are complicated – and thus costly.
It is therefore not practical to insist on going into that much detail (or spending that much money) if you want to roll out this measure in mainstream practice. Body mass index has caught on in health circles because of its simplicity. It is simple to calculate, quick and therefore cheap.
Flaw #2: BMI treats all folk the same
It might seem strange that we can use the simple BMI formula no matter who we are. That’s because it is strange, and fundamentally wrong.
It is not credible that a man or woman, young or old, black, white or Hispanic can use a universal formula without any kind of adjustment. However, that’s exactly what we do with BMI.
The most obvious difference between men and women is that women naturally carry more fat, especially around the waist and thighs. This is not necessarily an unhealthy fat distribution.
Men are generally taller as well as having a greater muscle mass. On the other hand, if both a man and a woman with a similar lifestyle and diet have a BMI of 35 it is likely that the man’s fat is distributed more on the upper half of his body (where the vital organs are) and could thus be a slightly more precarious health situation.
We could use a different calculation for men and women, to give a more accurate gender specific result. This would add to the complexity but would not be unworkable especially with modern technology making BMI calculation as easy as entering your data into a BMI calculator with the results available automatically.
Another obvious difference is age. Older people tend to workout less than youngsters, and their muscle mass decreases as a result.
As physical activity decreases, muscle mass decreases but the BMI calculation is the same whether you are aged 75 or 25.
This has an effect on BMI because as body weight drops, so will BMI. So even though the older person is becoming less active the BMI metric may see their classification move from overweight or obese into the “normal” range just because muscle mass has fallen.
Children are a special case. You cannot use the same tables to interpret a child’s BMI. You have to take into account the child’s age and sex to see where he or she rests in relation to the normal range.
If you look at the BMI reference charts for children you will see that it is not as simple as saying 18.5 – 24.99 is normal. This is because of the enormous growth potential that children have, and the medical community understands this BMI flaw and have attempted to adjust for it.
Click the link to read an entire blog on BMI in Children.
The BMI charts were developed for the whole population by using a sample of a few middle class white Americans in the early twentieth century.
The normal ranges are therefore population specific and may be fine for the majority, but ethnic minorities may find the normal ranges unrealistic because they were not designed with them in mind.
Modern cities – where most people now live – have changed a lot in the past century. The measurements made back then which have been the foundation for all subsequent BMI reference ranges have not sufficiently changed with the times.
Some country specific adaptations have already taken place, with Hong Kong and some counties in Southeast Asia having taken this into account for their particular populations.
Importantly, research (e.g. in the Journal of Endocrinology) suggests that black people have a tendency towards higher bone density and higher muscle mass than white people and this could affect their BMI upwards.
Click the link to read an entire blog on BMI and Race.
BMI does not know where your fat is hiding, but treats all fat the same.
However, some body locations are better places than others for your fat to be distributed and although BMI is not a good way to distinguish these, other measures such as the waist to hip ratios can be used to give you a better understanding of where your fat is (although looking in the mirror will usually help too).
The idea is that fat distributed around the waist (called “visceral fat”) is worse for your overall health than non-visceral fat (around your buttocks and thighs).
Different body shapes are a consequence of the distribution of fat on the body and it is clear that this is a significant factor when assessing health which BMI cannot accurately reflect.
Flaw #3: BMI ranges are arbitrary
The World Health Organization uses the normal range for BMI as 18.5 – 24.99 which is what Fat Creep™ has used.
However you will come across other resources which use slightly different ranges for the classifications of normal, overweight and underweight. The fact that there is no universal consensus does show how arbitrary the BMI ranges are.
If your BMI lies in the overlap zones, you may wonder what label you should give to your value, and indeed you may question the validity of the result.
BMI ranges are not universally agreed upon because it is not clear what values for "normal" are risk-free. The normal range may differ between populations, which may have to do with other factors such as ethnicity, body shape, fat distribution etc. which we have discussed already.
This is an area of current research and we should not be surprised if the goalposts shift again in the future.
Flaw #4: BMI can misclassify
If you hit the gym once, twice or even more times per week chances are you are physically healthy. All training and especially weight lifting leads to you gaining muscle – or ‘bulking up’.
Even if you don’t lift weights with the intention of bodybuilding, even modest increases in muscle mass, if they affect the major muscle groups in your legs, arms, back, shoulders, chest and abs will lead to noticeable weight gain.
Since your height does not change, you will increase your BMI as you train and therefore you may change your classification from normal to overweight or obese!
Flaw #5: BMI is a blunt instrument
BMI is used to label people as overweight and obese but the measure is too crude to be used to reach judgments on overall physical health.
Only by considering the whole individual – including family history, current medical conditions and vital signs such as blood pressure can a proper conclusion on health be drawn.
Conclusion on BMI: use it but know it’s limitations
BMI is established as an essential tool in public health medicine and its ease of use means it is here to stay. A high BMI may define you as overweight or obese but this does not necessarily equal unhealthy.
Do not be afraid to question a BMI value that you do not agree with and ask your health provider for a more complete explanation. If you don’t get the answers you are looking for, post them to our comments page and we will read every one.
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