Monday, January 7, 2008

Old body mass, fat mass, and fitness

The problem of obesity in old age may be due to a lack of fitness, but it remains unclear which is the cause and which is effect. Body mass index in elderly people has a serious drawback because, as people get older, muscles and bones (the fat-free mass) are being reduced and body fat mass is increasing. Thus, in spite of being weight stable, seniors may become "fat". They may be better off adding weight and not losing muscle mass.

A new study has addressed this problem, analyzing fat mass and fat-free mass in seniors and their effects on fitness. More than nine hundred Canadian men and women between 67 and 84 years of age have been studied. Fat mass and fat-free mass have been measured by a special dual x-ray method. Fitness has been assessed by measuring walking speed and performance in the one leg stand test. It came out that fat mass is negatively linked to fitness.

Fat-free mass, on the other hand, is not linked to fitness - which is quite a surprise to me. Obviously, the fitness levels of all the study subjects have been high enough so that deficits did not (yet) emerge.

The old obesity paradox

Old obese often have been reported to live longer than old thin people. One aspect is easy to understand: a slow death is often preceded by weight loss. A second aspect is the diminishing accuracy of body mass index in old age given the shifts in fat and fat-free masses that may vary a lot between different people.

These two effects may offset possible bad effects of obesity in old age. It is not even possible to speak of an "effect" because we just have seen a link between more body fat and less fitness. Which one is the cause? A fat, heavy body with weak muscles is most likely a handicap to physical activity. But, on the other hand, a sedentary lifestyle may be the real cause that body fat is being accumulated and muscle mass is diminishing.

Key message: The role of obesity in old age remains unclear in many aspects, but this study shows a clear link between high body fat and low fitness. Given that fit old people live better and longer, fitness should be the main concern.

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