Does a rapid metabolism make you die earlier?

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In many ways (particularly around dessert time), having a rapid metabolism is a blessing. But is there a downside? No one really believes that we have a “set number of heartbeats,” and we die when we use them up. On the other hand, it seems reasonable to view aging as the consequence of a series of cellular events, and “metabolism” as basically a measure of how fast those (and other) cellular events are proceeding in your body. That would certainly fit with all the research about caloric restriction extending lifespan. But frankly, it would kind of suck to think that boosting your metabolism was going to rob you of a few of your twilight years.

A new study in the Journal of Clinical Endocrinology & Metabolism looks into this question; the study will be published in June, and unfortunately I don’t have access to the advance online version, so the only details I have come from this press release. The researchers looked at 652 healthy Pima Indian volunteers between 1982 and 2006, measuring their resting metabolic rate and/or 24-hour energy expenditure (using a metabolic chamber). During the study, 27 of the participants died, and higher metabolism was one of the risk factors:

“We found that higher endogenous metabolic rate, that is how much energy the body uses for normal body functions, is a risk factor for earlier mortality,” said Reiner Jumpertz, MD, of the National Institute of Diabetes and Digestive and Kidney Diseases in Phoenix, Ariz., and lead author of the study. “This increased metabolic rate may lead to earlier organ damage (in effect accelerated aging) possibly by accumulation of toxic substances produced with the increase in energy turnover.”

“It is important to note that these data do not apply to exercise-related energy expenditure,” added Jumpertz. “This activity clearly has beneficial effects on human health.”

So that last part is reassuring, though I’m not sure on what basis he makes the claim. (Isn’t it possible that exercise-related energy expenditure produces the same negative effects as “endogenous” energy expenditure, but the downsides are more than compensated for by other benefits provided by exercise?) And he only seems to be referring to energy burned during exercise. But what about changes in basal metabolism induced by regular exercise? I’m under the impression (though I’m just assuming here, and happy to be corrected) that regular exercise does boost your metabolism throughout the rest of the day. Is that a problem?

Anyway, nothing to get too worked up about here: it’s clear from epidemiological data that the net effect of exercise is to increase rather than decrease lifespan. Still, I’ll be curious to see the full study when it’s published, to get a fuller sense of the research in this area than the press release provides.

[As an aside, I’m now back in Toronto after a week of travelling from Sydney to San Francisco to Vancouver, so the blog should get back to full throttle after last week’s lull! Many thanks to all those who dropped by Forerunners and the Vancouver Marathon expo to say hello and pick up advance copies of the book — and sorry I ran out so quickly!]

UPDATE May 2: A friend sent me a pdf of the full paper (thanks, Joe!), so I now have a fuller sense of the argument. The authors are interpreting their results in terms of the “free radical theory of aging” (though they acknowledge that some recent experiments have cast doubt on this theory). In this theory, it’s clear that a chronic elevation of free radicals due to excess food consumption (i.e. a faster basal metabolism) is bad, while a transient spike in free radicals due to exercise (which in turn spurs the body to adapt by developing a more powerful antioxidant response) is good rather than bad. One other point: the average BMI of the volunteers was ~34, and many of the deaths were linked to alcohol. So overall, I’m not necessarily confident that the primary result (faster metabolism = earlier death) is really generalizable to, say, a group of committed runners.