Previously on this site I have drawn attention to the fact that the body mass index (weight in kg divided by the square of someone’s height in metres) is not a particularly good gauge of health status. For a start, the BMI tells us noting about the body’s composition, and it is entirely possible for individuals who are quite muscular, for instance, to be labelled as ‘overweight’ or even ‘obese’ despite the fact that they’re not carry much in the way of fat. Also, there is the other matter that studies have found that individuals labelled ‘overweight’ (BMIs of 25-29.9) appear to enjoy at least as good health, if not better, than those with BMIs traditionally labelled as ‘healthy’ (18.5-24.9).

Another major deficiency of the BMI is that it tells us nothing about how any excess fat might be distributed in the body. This is important because it’s well known that fat that congregates around the midriff (abdominal fat) is quite strongly associated with conditions such as heart disease and type 2 diabetes, while weight that accumulates below the waist (gluteofemoral fat) is not. Waist circumference (WC) and the waist-to-hip ratio (WHR) have therefore been offered up as more useful measures with regard to general health and disease risk.

Last week saw the publication of a study which assessed the relationship between BMI, WHR and WC and overall risk of death in almost 360,000 adults across 10 European countries. Individuals in this study were followed for an average of almost 10 years. When assessing the relationship between the various measures and mortality, researchers took into account other so-called confounding factors such as smoking, alcohol consumption and activity.

The relationship between BMI and overall risk of mortality revealed J-shaped curves, means that both low and high BMI is associated with an increased risk of death. What was most interesting is where the low-point in the curve (the BMI at which mortality was lowest) came. For women, this was a BMI of 24.3 (very much at the upper end of the ‘healthy’ range). For men, on the other hand, the lowest risk is in men with a BMI of 25.3 (in other words, men who would traditionally be labelled as ‘overweight’ have the lowest risk of death).

Using a BMI of 23.5 - < 25.0 as a reference point, the researchers assessed risk of death in individuals with BMIs higher and lower than this. In men, individuals with a BMI of 25.0 - < 26.5 were at a 9 per cent reduced risk of death. Risk of death only became clearly statistically significantly increased once the BMI was over 30. In women, clearly enhanced risk of death was only seen those with BMIs over 30 too.

This evidence, like quite a lot that has gone before, does call into question the wisdom of telling individuals who are ‘overweight’ that they should lose weight.

The researchers went on to assess the relationship between WHR and WC and risk of death. Here, the impact of a person’s BMI was also taken into consideration. Here, there was found to be strong relationships between both WHR and WC, and risk of death. Men with the highest WCs had a more than 2-fold increased risk of death compared to those with the lowest waist measurements. In women, higher WC was associated with a 78 per cent increased risk of death. Also, the highest WHRs were associated with a 68 and 51 per cent increased risk of death in men and women respectively.

What this all boils down to is that, yet again, the BMI has been found wanting as a marker for health, and again the validity of the appropriate of the traditional bands of ‘healthy’ and ‘overweight’ has been called into question. And again we see that assessment of the extent of fatty accumulation around the middle is a potentially valuable tool in assessing health status.

Those keen to maintain a ‘healthy’ weight might do well to focus far less on their actual weight or BMI, and more on their size of their belly. In this study, the 20 per cent of men and women with lowest risk of death had waist sizes measuring less than 86 cm (34 inches) and 70 cm (25.5 inches) respectively.

References:

Pischon T, et al. General and abdominal adiposity and risk of death in Europe. NEJM. 2008;358(20):2105-2120

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