Friday, October 30, 2009

BMI and Health Status

The flaws of using body mass index as a measure for overall health status are apparent. It is a 19th century technique which ignores the distribution of both muscle mass and bone in the body. In addition, the relationship between and health status is likely to be non-monotonic, most likely quadratic.

However, the use of BMI in research does have some advantages. It is very quick and inexpensive to measure. In addition, it is plausible that BMI is a strong indicator of individual's health preferences and behaviours. Also, BMI is a continuous metric. Therefore, if we choose to use BMI as a proxy for health status we do not have to constrict ourselves to discrete choice statistics when estimating the conditional distribution for 'health'.

The way in which BMI is used in estimation strategies needs to be redefined. BMI's definition of 'overweight' is outdated, and does not recognise that the population has become bigger, stronger and healthier in the last 150 years. Bone structures with greater density and increased muscle mass are not the same as body-fat increases. They are health promoting, not health deterring. Obesity is rising, and the negative health effects are undeniable. However, the shift from 'normal' BMI to 'over-weight' BMI and the negative health outcomes are dubious. For example, Romero-Corral et al. show how coronary deaths amongst 'over-weight' BMI cases are lower those individual's defined as having 'normal' BMI.

So how should the eager researcher approach this issue? In my opinion, we should accept that the bounds defined by the BMI scale are now invalid and have no basis acting as a proxy for overall health. The mean of health and BMI has shifted in over the last century, however I would argue that the new mean indicates improvements in health - strongly supported by life-expectancy increases, height increases, etc. - and that it is the deviations away from this mean which give a more precise measure of overall health status. One estimation strategy which maintains the continuous properties of this metric would be to measure BMI in z-scores (deviations from the mean controlling for the size of the standard deviation) or the z-score squared.


Peter Carney said...

Interesting post Alan.

Your point that people, in general, are getting bigger and stronger overtime is undoubted. It seems like a big leap, however, to suggest discarding cut-off categories used to guide and classify peoples' health. What it might suggest is that we need to adjust the cut-off points for what can reliably be reconsidered healthy, unhealthy, and hazardous. For the latter - BMI>30 - the bigger bones and muscles argument will be less relevant and is unlikely to need adjusting, same is true for the morbidity obese category.

More sophisticated measures exist and they are all objectively measured. I don't see the benefit in a relative measure that could compensate for the likelihood that such a classification system would become misleading in an environment where people are becoming increasing obese.

Kevin Denny said...

The advantage of BMI is that it is easily calculated and widely recognized. Lets say you found that instead of BMI a better predictor of health was to divide weight by Height^2.3 (instead of the square): you are not going to get people to adopt it. A z score also has the problem: normal folks won't be able to calculate it.
So my instinct would be to stick with BMI as a measure but be flexible in how its used >30 etc cut-offs might not be appropriate. For clinical purposes, presumably medics have access to more subtle measures that take account of muscle, fat etc.

Alan Fernihough said...

This is not about causality; it is about the use of BMI as a proxy measure for individual's health status.

We should recognise that BMI does not have a monotonic relationship to health status. It is the deviations away from the mean that matter. I suppose it really boils down to how much you believe the population's increase in weight is related to the massive improvements in public health since the early 19th century.

Peter Carney said...

Perhaps. Ultimately BHI is only a rough guide and most health professional take it with a pinch of salt, low-sodium salt.

On a slightly related topic that might be of interest in relation to concerns about aging and height I recently read a piece about daily shrinkage - apparently we're about 2cm taller in the morning than the evening. Gravity is the main suspect. Time of day matters for precise measurement of height.

Kevin Denny said...

Does anyone actually claim that BMI is monotonically related to health? Isn't <18.5 "under-weight"?

Is there really such a thing as low-sodium salt? I thought it was NaCl, not much substitutability there.

You could test the circadian pattern by looking at astronauts or the bed-bound. I feel like testing it this week-end.

Eve OCallaghan said...

The stuff sold as low sodium salt is actually KCl Kevin.