The physics of human body weight changes

Body Mass

If you put on a few pounds over the holiday season, this may interest you.
Weight changes are the result of an imbalance between the energy you derive from the food you’ve eaten and the energy you’ve been expending to maintain life and perform physical work. At first glance, you’d think this would be a pretty easy relationship to quanitfy.

Not so. If the question is how does an imbalance affect the the distribution of fat and lean tissues within your body, the answer turns out to be surprisingly hard to get a handle on.

Tha thasn’t stopped Carson Chow and Kevin Hall at the Laboratory of Biological Modeling in Bethesda, Maryland, from having a go. They’ve have created a mathematical model that describes what is going on when this energy balance changes.

They say there are essentially two answers to the problem.

In the first case, the composition of fat and lean tissue within your body and its overall mass are uniquely determined. So whatever you do, your body heads to a uniquely determined mass.

In the second case, the body composition can vary over an infinite number of states. So for example, when your overall mass increases, the amount of lean tissue may go up while the amount of fat goes down. Or the other way around is. Or the amount of both types of tissue may increase. So your body could respond to a particular change in eating habits in an infinite number of ways.

Curiously, it’s hard to distinguish between these two cases by simply changing diet and monitoring body weight. Chow and Hall say there simply isn’t enough evidence to determine whether the human body operates in the first regime or the second.

But they think they know how they can find out. The trick, they say, will be to create diets that change either the amount of lean tissue or the amount of fat tissue but not both. And then see what happens. In this way, you can hold one of the variables steady to see how the mechanism works. Just what this diet might be, they don’t say.

The whole area is of more than academic interest because it could have a direct bearing on the efficacy of treatments such as lioposuction and the way in which serious eating disorders are handled. Not to mention us ordinary snackers who want to control our weights more effectively.

So you might want to hang fire if you are hoping to get the holiday love handles removed surgically.

Ref: arxiv.org/abs/0802.3234: The Dynamics of Human Body Weight Change

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