Preventive Medicine Column

Dr. David L. Katz

Our weight, of course, is substantially influenced by our behaviors and choices.  I have argued for years, against a backdrop of exotic theories, genetic discoveries, and snake-oil hucksterism, that weight represents energy balance, and is thus overwhelmingly influenced by calories in, and calories out.  I have made the case along the way that yes, a calorie is a calorie.

But I immediately append something I know abundantly from my clinical experience: two people can eat about the same, and exercise about the same, and one gets fat and the other stays thin.  This absolutely does happen; it is the truth.  It’s not fair, but who ever told us life would be?  Bad stuff does at times happen to thin people, too, and that isn’t necessarily fair either.

We don’t know all the reasons for accelerated weight gain, or weight loss resistance, but we do know some, and they are the usual suspects.  There are important gene variants, to some extent associated with ethnic variation, that influence metabolic rate.  There are marked differences in resting energy expenditure, with some individuals, and some whole populations such as the Pima Indians, and the Samoans, noteworthy for extreme “fuel efficiency.”  Fuel efficiency fostered survival when calories were scarce and physical activity unavoidable; in modern context, it fosters obesity and its sequelae.

We are also learning more, almost every day it seems, about the important influence of our resident bacteria on our weight, as well as our overall health.  Variations in gut microbes can substantially alter the fate of the calories we take in, and our propensity for weight gain.

None of this alters the fundamental importance of energy balance.  If person A has a lower resting energy expenditure than person B, it means that, all things being equal, person A will gain weight more readily and from fewer calories.  But that simply indicates that all things shouldn’t be equal; person A needs fewer calories.  There is inter-individual variation in the equilibrium point for energy balance, where calories in match calories out; but there is such an equilibrium point for us all, and when we find it and honor it, our weight is stable.  We need to accept this and deal with it, just as we deal with all other inter-individual variation.  Variation in the size of our feet requires corresponding variation in our selection of shoes.

Weight is powerfully influenced, but not directly determined, by our behavioral choices.  Some people, making all the right choices, will be heavier than others making the same- or even less good-choices.  And people making good lifestyle choices, including routine exercise, are apt to be fit even if they remain somewhat fat, and will be far better off than those who are either fat or thin, but unfit.

As we foray ever further into an age of incentives and disincentives to nudge people toward better health and lower disease-care costs, it becomes urgently timely to acknowledge that weight is not a behavior.   Body mass index is often included among the metrics considered for the application of such nudges in health promotion programming.  I think that’s wrong.  What if two people commit just the same efforts to improving their diets and physical activity, but one loses weight more readily than the other- is there any rationale for a different application of rewards or penalties?  In my view, if someone is relatively resistant to weight loss and is then penalized financially for this “handicap,” it is the literal addition of insult to injury (or vice versa).

The causes of epidemic obesity are all around us.  We don’t want to get carried away with invoking genes or resting metabolic rate to explain obesity at the population level.  A half century ago, human beings were the same as they are now; our genes, and metabolisms, were all but identical then as now.  Yet rates of obesity are staggeringly different.   You can’t account for a change in Y with an X that has been constant; if X explains a change in Y, it must be because X has also changed.  The changes that account for obesity in the modern world reside in the modern world, not in the relative constancy of human physiology.  But human physiology absolutely does account for variations among us.

My new book, Disease-Proof, emphasizes the transformational power of our behavioral choices over health and weight.  That power is remarkable, and with the right skill set, everybody can exercise it.  The book was written to advance that very goal.  But still, nobody wakes up and decides what to weigh.

We do, indeed, have stunning potential control over our health, and weight, and should make better use of it.  Our feet and forks are, or should be, the master levers of medical destiny.  But even the best application of skill-power will not turn weight into a behavior. Weight is not a choice, and we should all choose to recognize that.




Dr. David L. Katz