Is this the death of the body mass index (BMI) as a measure of body weight?

The fact that our collective mass has grown considerably of late has inevitably triggered warnings from health professionals about how this may swell our risk of weight-related conditions such as heart disease. Ideal weight recommendations traditionally come in the form of the body mass index (BMI) – calculated by dividing an individual’s weight in kilograms by the square of their height in metres. However, the usefulness of the BMI in adjudging heart disease risk has recently been called into question on the publication of a study in the Lancet. This research has found that, once other associated factors such as smoking and exercise habits were taken into consideration, BMI has no significant bearing on heart attack risk.

This study is not the only research which has cast some doubt on the relevance of the BMI as a predictor of our propensity to disease and death. Earlier this year, the journal Circulation published a study which found that in individuals having surgery for heart disease (coronary artery bypass), mortality after surgery was actually lowest in individuals whose weight was categorised as overweight (BMI 25 ” less than 30) or ‘high-normal’. These results and those of the recent Lancet study clearly cast some doubt on the appropriateness of standard medical recommendations that a ‘healthy’ BMI of between about 19 and 25 is ideal in terms of heart health.

While research which asks questions of the conventional wisdom regarding the relationship between BMI and heart health is interesting, it is perhaps not as telling as research which seeks to establish the relationship between this measure of body weight and overall risk of death. Earlier this year saw the publication of such a study in the Journal of the American Medical Association. In this research, underweight and obesity (a BMI 30 or more) were both associated with an increased risk of death. However, somewhat surprisingly, this study found that compared to those in the ‘healthy’ BMI category, those categorised as ‘overweight’ (a BMI of 25 ” less than 30) were actually at reduced risk of death.

One of the limitations of the BMI is that, while it tells us something about the weight of an individual, it does not inform us regarding how much of this weight comes from fat. Also, there is growing recognition that it is not fat per se, but fat which aggregates around the middle of the body (so-called ‘abdominal fat’) that is most strongly linked with health issues such as heart disease. The Lancet study, for instance, found that a higher ratio of waist circumference to hip circumference (waist-to-hip ratio) was very clearly associated with increased heart attack risk. Ideally, men and women should have waist-to-hip ratios no larger than about 0.90 and 0.83 respectively. From a health perspective, the evidence suggests that it is an individual’s waist-to-hip ratio, rather than their BMI, that represents their vital statistic.