Thinness Causes Exercise …

… not the other way around.

Check out yesterday’s post on Mark’s Daily Apple.  Researchers in Canada have discovered that thin rats are more motivated to exercise than fat rats.  (I haven’t had an opportunity to dig up the study yet, so I don’t know exactly how “motivation” was measured.)

I am unsurprised by this.  Just because exercise and thinness are correlated does not mean that exercise causes thinness, any more than flooding in your basement causes it to rain!

You know who else is probably unsurprised by this?  Gary Taubes.  Gary Taubes is the person who first got me thinking that thinness causes exercise.

In Good Calories, Bad Calories, Taubes examines a giant body of evidence and makes the following hypothesis:  obesity is starvation at a cellular level.  In an obese person (or rat), most energy consumed is funneled directly to storage rather than metabolism.  In other words, most calories are stored as fat, leaving the rest of the tissues without enough energy to function properly. This causes both excessive fatigue (and therefore reluctance to exercise) and excessive hunger (and therefore overeating).  According to the Taubes hypothesis, the cause of obesity is consuming the wrong type of calories (fuel like carbohydrates that are more prone to storage than other fuels), and the cure is cutting back on the storage form of calories.  Once that cure is undertaken, cells have more energy, exercise becomes possible, and the urge to overeat is tamed.  This hypothesis is consistent with what we know about insulin (and I really mean “know” - the next few sentences are so well established that you could find them in any biochem textbook).  Insulin levels rise in response to carbohydrate consumption; high insulin levels cause the storage of both carbohydrate and fat in the adipose tissue; and high insulin levels prevent the release of fat for use as energy.

This is a better explanation for the unmotivated fat rat than the hypothesis of the study’s authors, who speculate that we evolved this way because it’s advantageous for a thin person (or animal) to get off their butts and — go look for food so they don’t starve.  The first problem I have with this is that is that the researchers seem to have observed that the obese rats ate more!  But my biggest problem is that, like a lot of evolutionary explanations, this one doesn’t present a solution.  Saying that men are more likely to be unfaithful than women because it allows men more opportunities to pass on their genes may be true, but it is not helpful to someone who is attempting to deal with infidelity in their marriage.  Likewise, saying that our species evolved so that thin people want to exercise and fat people don’t may be true, but it doesn’t help a fat person who wants to become thin.  Please don’t get me wrong - I’m not one of those crazy fundamentalist evolution-deniers.  I respect and even revere the work of evolutionary scientists.  But evolution can only tell us how a phenomenon came to be.  It can’t tell us anything about what to do about it.  And in fact, a lot of evolutionary thinkers seem to fall into fatalism - men are programmed to cheat, humans are programmed to be fat, and therefore, all we can do is throw up our hands in defeat.

This is nonsense - plenty of men are faithful to their spouses and partners; and fat people do get thin, and some of them even stay thin.  Evolution is not destiny.  And even if it were, don’t forget, we also evolved with big, problem-solving brains.  Our big brains gave us culture and religion and economies, and these phenomena support marital fidelity.  And our big brains have figured out biochemistry, and biochemistry can help us tease out how fat people can be thin.