Having a 'healthy' BMI is not necessarily as healthy as we think

The body mass index (weight in kg divided by the square of someone’s height in metres) is well-established as a marker for health. Generally speaking, we’re told that BMIs of 18.5 are “normal” and “healthy”, while those of 25.0-29.9 are “overweight”, and those 30 and above mark people out as “obese” There’s a number of problems with the BMI, though, not least of all that that they tell us nothing about the composition of the body. So, it’s possible to bit fit, healthy and well-muscled, and have a BMI that classifies you as overweight and even obese Also, you can be frail, have low muscle mass, and be classified as healthy. Using the BMI to judge health is therefore, largely, a nonsense.

Another issue with the body mass index is that it tells us nothing about the distribution of weight (specifically, fat) in the body. This is important because there has been growing recognition of the fact that weight around the middle (particularly weight packed in and around the inside of the abdomen) has strong links with diseases such as heart disease and type 2 diabetes. On the other hand, weight under the skin, say on the legs or arms, does not seem to be a particular risk factor for ill-health.

The fact that abdominal fat has been identified as a marker for ill-health has led to increasing popularity of measures of abdominal fat such as the “waist circumference” and the “waist-to-hip ratio”. This is all good, I think, but as a recent paper points out, such measurements are only generally used in individuals who are branded “overweight” or “obese” on the basis of their BMI [1].

The authors of this study set about seeing what value the waist circumference was in predicting risk of developing diabetes in those of “normal” BMI. And they also compared the apparent risk to those considered “overweight” and “obese”, but whose waist circumference was not particularly expanded. The researchers followed more than 25,000 men and women aged 35-65 over and average of 8 years.

In men, median waist circumference was 94 cm (37 inches). In women, it was 78.5 cm (31 inches). In both men and women, lowest risk for diabetes was seen in those with waist circumferences below the median who also had a BMI value of less than 25 (“healthy”).

In men, compared to the low risk group, those with lower waist circumference but “overweight” BMIs were, on average, 2.26 times more likely to develop type 2 diabetes.

Those with higher waist circumferences and “overweight” BMIs were almost 5 times more likely to develop type 2 diabetes.

But what about those with “healthy” BMIs and higher waist circumferences? These individuals were, on average, 3.62 times more likely to develop diabetes.

And it was a similar story in women: Women with lower waist circumferences and “overweight” BMIs were not found to be at a statistically significant increased risk of diabetes.

However, those with higher waist circumferences and “healthy” BMIs were 2.74 times more likely to develop diabetes than those with “healthy” BMIs and lower waist circumferences.

Here are a couple of quotes from the authors of this study:

“The relative risk (RR) of developing type 2 DM [diabetes mellitus] among persons of low or normal weight (BMI <25) who had a large waist circumference was at least as high as that among overweight persons (BMI 25.0-29.9) with a small waist circumference.”

and

“These findings imply that the waist circumference is an important additional piece of information for assessing the risk of type 2 DM, particularly among persons of low or normal weight.”

To my mind, this study highlights the importance of focusing on keeping the body free of excess fat around the middle (not weight or BMI). It’s one of the reasons I wrote my latest book Waist Disposal.

References:

1. Feller S, et al. Body mass index, waist circumference, and risk of type 2 diabetes mellitus. Dtsch Arztebl Int 2010;107(26):470-6