When it comes to official advice about body weight, the norm is still to base recommendations on the body mass index (weight in kg divided by the square of height in metres). We are traditionally encouraged to conform to a ‘normal’ or ‘healthy’ BMI of 18.5-24.9. It is amazing to me just how rarely (if at all) this ‘desirable’ BMI band is justified in any meaningful way. Any justification is usually rich in rhetoric but low in scientific facts.
And perhaps some of the reason for that is that the science shows the traditional BMI banding is, well, wide of the mark. One could argue that perhaps the best judge of the effect of any lifestyle on health is its impact on overall risk of death. And when BMI values have been judged by this most fundamental of outcomes, time and again it turns out that the BMI category associated with the lowest is the ‘overweight’ one (BMI 25-29.9).
See here and here for relatively recent blog posts which explore this finding.
I don’t think anyone knows for sure why being bigger than is traditionally advised appears to be better for survival. One theory that has been put forward, however, is that some surplus fat can come in handy during a critical illness, in that it offers an energy reservoir the body can draw on in times of need. Critical illnesses are more likely to occur in elderly, which raises the suggestion that being ‘overweight’ would be particularly beneficial to those of advanced years.
This concept was studied recently in a study published in the Journal of the American Geriatrics Society . In this study, more than 9000 Australian women aged 70-75 (at the start of the study) were monitored over a 10-year period. It turns out that individuals in the ‘overweight’ category, compared to those of ‘normal’ weight, were at a 13 per cent reduced risk of death. These findings are reminiscent of another quite-recent study in which higher body fat levels were found to be associated with advantages for health and survival in the elderly.
This research adds to an ever-lengthening series of studies which suggest that, on a population basis, ‘healthy’ BMI values are actually too low. And this might be worth bearing in mind should you find yourself labelled as ‘overweight’ and then urged to lose weight.
1 Flicker L, et al. Body Mass Index and Survival in Men and Women Aged 70 to 75. Journal of the American Geriatrics Society 2010;58(2): 234-241