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The BMI fallacy – simple does not mean right

April 02, 2014

BMI is a simple parameter to understand. You just need a person’s weight and height to calculate it. But is not just wrong science but terribly unhelpful to patients and their treatment.

This is a topic of great controversy especially because on most occasions NHS funding is denied for women who are “obese” which is generally referred to as BMI over 30. The reason given is having a BMI over 30 reduces the chance of the woman falling pregnant and there are higher chances of complications of pregnancy including miscarriage, should such a woman fall pregnant. This is thus not the best use of the funds and that the woman should lose weight first.

There are several aspects to consider when we use this argument.


1. Is BMI the best way to gauge if a woman is “obese”?

BMI is a very crude method of judging somebody’s health. It is simply weight of the person upon the square of height which represents the surface area. It is well known that the BMI of athletic persons with larger than average muscle mass is higher as compared to non athletic woman. This is because whether your arms are big due to muscles but to fat, the surface area  is increased to the same extent. Muscles on the other hand are not bad to have and having them instead of fat should surely make you a fitter person. But muscles weigh more than fat of the same volume. This means that athletes with higher muscle proportion and lower fat will, paradoxically, have higher BMI.

Women of Afro-Caribbean origin have much thicker bone density than the average Asian of Caucasian woman. The proof of this is that Africans women rarely need hormone replacement therapy after menopause – Caucasian and Asian women often do. This is because their bones are so dense, they do not lose enough bone to get osteoporosis at the age of 65 like the Asian or Caucasian women do. However denser bone is heavier and thus if the African woman wears the same size dress as Asian woman. Her BMI is likely to be more than that of the other two.

It is also well documented that Africans are naturally more muscular than other races. This however, doesn’t help keeping the BMI low. A muscular woman, though more fit, will have higher BMI as compared to someone who is not. This fallacy of the BMI should alert us to the fact that more complex methods determining body fat amount should be used if at all if you want to predict the success of an IVF outcome.

Even within Asian and Caucasian women it is well observed that some have literally larger frames (“big boned”)and generally look muscular than others in spite of no difference in their lifestyle and exercising habits. This genetic variation of having denser bones or more muscles is not exclusively linked with racial genetics. All in all to go by the BMI is a very simple process but not necessarily the cleverest.

Losing weight on doctors orders is easier said than done

Overweight women or those with higher BMI are commonly told by fertility experts to go home and come back only after they have lost a few kilograms. Whilst this suggestion is well intended, it doesn’t work, for many reason. The tremendous anxiety that the couples in general and woman in particular are is perhaps the factor that makes weight gain most easy to happen and most difficult to control. More anxiety and its associated depression will cause more comfort eating and lack of exercise. Besides every month and year that the woman tries to lose weight, as he is losing on time which in turn reduces the chance of getting pregnant, for various reasons discussed elsewhere.

So though well intended the whole scheme is not entirely scientific and surely almost discriminatory against not just the obese but those who tend to have higher BMI for no of theirs.

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