I wanted to take note of something said by Dariush Mozaffarian in his recent Wall Street Journal op-ed:
Two people with the same age, sex, size, weight and activity level can burn substantially different numbers of calories depending on what they eat. Recent global studies show that differences in obesity rates between populations are explained less by varying physical activity than varying resting metabolic rate. Analyses from the U.S. and U.K. find that the average energy we burn at rest has declined significantly over the past several decades.
Together, these discoveries help explain why decades of calorie-focused advice have failed to reverse the obesity epidemic: Calories represent arithmetic detached from biology.
None of this violates the laws of thermodynamics. Weight gain still reflects an imbalance between energy intake and energy expenditure. The mistake is assuming that calorie counts alone explain a process that is shaped by the microbiome, hormonal signaling, fat storage, muscle maintenance, thermogenesis and metabolism itself.
The emphasis is mine. How much of the differences among those factors is governed by behavior, how much by environment, and how much by genetics?
I think the honest answer is that we know these influences matter but we cannot reliably say how much each contributes to the differences he describes. Naming the biological mechanisms does not resolve that uncertainty.
There are several additional complexities. “The population” of the United States today is considerably different from what it was in 1980 and even more different from what it was in 1940. It also differs from the populations of Brazil, Germany, and China. How much of the reported variation reflects changes in diet and how much reflects differences in the populations being compared? I don’t think we can confidently say. Furthermore, does Dr. Mozaffarian think those differences matter only when it comes to diet? Why wouldn’t they matter in other aspects of health including responses to medications and other treatments? If they do, that raises questions about much of modern medicine as practiced in the United States. Population averages may be useful but applying them to an individual requires assumptions. If those assumptions warrant questioning in dietary advice, they warrant questioning elsewhere as well.







