Fat people are unhealthy. Or at least that is what we are told. The ‘fact’ that fatness and mortality show a linear association is a very widely held belief amongst the medical community and the general population alike. However, a belief is only a personal acceptance that something is true, but it does not actually make it so. Therefore, when we look a little closer at the association, we find that there are some glaringly obvious problems with the idea that fat people die prematurely compared to thin people. Firstly, the commonly accepted measure of fatness, the body mass index (BMI), does not take into account those with large amounts of skeletal muscle as it is a simple a measure of the weight of the individual multiplied by the height squared. In this way it does not actually measure fatness at all, but only total body mass. As high amounts of skeletal muscle are actually associated with reduced mortality, particularly in the elderly, the use of this measure of weight should be questioned as being problematic.
The waist to hip ratio and waist circumference are better measures of fatness, but are not commonly used in a clinical setting. This is generally because these measures are more intrusive and take more skill and time and so doctors are less willing to employ them. However, if we accept that the BMI in some way predicts fatness, which it does, there still exists problems with the assumption that fatness is associated with mortality. For example, in a long term study performed in the West of Scotland1, researchers investigated the association between fatness and mortality in 2381 males between the ages of 45 and 75. Surprisingly the data showed that the lean, and not the obese had the highest risk of mortality. When the data was analysed further, it was shown that the lean individuals had the highest mortality rates for cancer while the obese had the highest mortality rates from cardiovascular disease. However, in those that smoked, mortality was higher than for non-smokers no matter what the body fatness level.
So what do these results really tell us? Well firstly it suggests that there is more to longevity than being lean as adiposity can be clearly be benign in some cases. The data in this study can be said to be valid because over 80 % of the population were respondents in the study and levels of fatness were assessed with measures that give accurate representations of the actual adiposity present. Some might point out that the risk of cancer was greater in the lean individuals because of the wasting of body tissue that is often seen in late stage cases of cancer. However, as the study was performed over 16 years this is unlikely to be the case as those who died of cancer are unlikely to have been in a morbid state for this entire time. The authors could not explain the association between leanness and cancer expect that it is known that certain anti-cancer vitamins such as vitamin A, carotenoids and vitamin D are sequestered in fat tissue and this may provide a reservoir of vitamins for metabolic needs in those with more adipose tissue.
Dr Robert Barrington’s Comments: Not all fat is the same. Some fat is present under the skin, and this is the fat we tend to see. However, we also have fat stores around our organs, and this visceral or abdominal fat is what produces the rounded apple-like appearance of the obese individual. Subcutaneous fat is not associated with negative health outcomes, but abdominal or visceral fat is. Evidence suggests that abdominal obesity is caused by the metabolic syndrome, a cluster of metabolic aberrations that lead to insulin resistance and non-alcoholic fatty liver. The nutritional literature supports a role for a poor quality in the development of insulin resistance. Some individuals can have high levels of subcutaneous adiposity and have no detrimental health outcomes.
RdB
