The body mass index or ‘BMI’ (calculated by dividing someone’s weight in kilograms by the square of their height in meters is the most commonly-used weight-related measure of health. The problem is, though, it’s virtually useless for this purpose. This is for two main reasons:
1. It tells us nothing about body composition. It is therefore possible for two individuals to have the same BMI, though one is strong, fit and heavily muscled, and the other is quite low on muscle and big in fat. According to the BMI these individuals have the same health status, but in reality they are wildly different.
2. If someone does happen to be carrying excess fat around the body (and therefore has an elevated BMI), it tells us nothing about the distribution of this fat. This is important because fat the accumulates in and around the internal organs (so-called ‘visceral fat’) is quite strongly associated with enhanced risk of chronic diseases such as heart disease and type 2 diabetes, while fat under the skin surface (subcutanenous fat) is, generally speaking, not.
I was interested to read a study published last week on-line in the journal Obesity which looked at the relationship between ‘obesity’ and other measures and markers for health. It’s focus was men and women with BMIs of 30 or above (who, technically, fulfil the criterion for being ‘obese’).
Study subjects were assessed for the presence of ‘metabolic syndrome’. This syndrome is characterised by excess weight around the middle, along with other potential features such as raised levels of blood fats called triglycerides, raised blood sugar levels, raised blood pressure and low levels of ‘healthy’ cholesterol (HDL-cholesterol).
It turned out that about a third of the study subjects, although ‘obese’, did not fulfil the criteria for metabolic syndrome. In broad terms, these people could be described as being ‘metabolically healthy’. In other words, for a significant proportion of the population, being ‘obese’ does not at first sight appear to be such a bad thing in health terms.
Why? Well, it turns out that ‘obese’ but ‘metabolically healthy’ individuals tended to have less visceral fat, and more subcutaneous fat. This is an expected finding, but also highlights again why the BMI is a quite-useless marker for health.
Why is visceral fat so dangerous? This study also found that those fulfilling the criteria for metabolic syndrome (and therefore more likely to have excess visceral fat) were found to have generally higher levels of the ‘inflammatory’ substances interleukin-6, tumor necrosis factor-a, and plasminogen activator inhibitor-1. There is emerging evidence that, for whatever reason, visceral fat is associated with or encourages a ‘pro-inflammatory’ state, and is now quite well established that inflammation is a key (perhaps the key) underlying process in the development of chronic diseases including heart disease and type 2 diabetes.
What does all of this boil down to? To my mind, it means the BMI is not to be relied upon. However, if you’ve got a bit of a gut, it will probably pay to get rid of it.
1. Koste A, et al. Body Fat Distribution and Inflammation Among Obese Older Adults With and Without Metabolic Syndrome Obesity 15 April 2010 [epub ahead of print]
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