Resting Metabolic Rate Predicts Human Mortality

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Allow me to point you to the results of a long-running study on metabolic rate and mortality:

Higher metabolic rates increase free radical formation, which may accelerate aging and lead to early mortality. ... Our objective was to determine whether higher metabolic rates measured by two different methods predict early natural mortality in humans. ... Twenty-four-hour energy expenditure (24EE) was measured in 508 individuals, resting metabolic rate (RMR) was measured in 384 individuals.

The study ran with hundreds of participants over more than twenty years and concluded that there is a good correlation between these measures of metabolic rate and risk of death:

For each 100-kcal/24 h increase in EE, the risk of natural mortality increased by 1.29 in the 24EE group and by 1.25 in the RMR group, after adjustment for age, sex, and body weight in proportional hazard analyses.

The higher your resting metabolic rate, the greater your expected chance of death by aging or disease sometime soon - a cheerful prospect. My first thought was that these measurements should reflect levels of physical fitness achieved through exercise, which we know has a strong effect on mortality, but apparently not:

Studies published in 1992 and 1997 indicate that the level of aerobic fitness of an individual does not have any correlation with the level of resting metabolism. Both studies find that aerobic fitness levels do not improve the predictive power of fat free mass for resting metabolic rate.

There's a lesson there concerning the practice of quickly leaping to what might seem to be sensible conclusions. My slower second thought involved calorie intake: even mild levels of calorie restriction have measurable impacts on health in humans and on longevity in lower animals. Possibly also on longevity in humans, though that study will likely never be undertaken - if started tomorrow, by the time it was even half-way complete we'd be well into the era of rejuvenation biotechnology, making the whole exercise rather pointless.

In any case, the practice of calorie restriction does lower resting metabolic rate, and does so across a range of species: stick insects, rhesus monkeys, and humans, to pick a few. So it seems reasonable to theorize that differences in mortality seen in the study quoted above are reflections of the natural variance of calorie intake amongst the participants, and the biochemical - and existential - consequences of lower versus higher calorie diets.