That's the conundrum posed by an article over at the transhumanist H+ magazine on using calorie restriction (CR) to increase human lifespans. A lot of research finds that feeding some critters, like worms, flies, and lab mice, about two-thirds of what they would otherwise prefer to eat dramatically increases their lifespans. The idea is that this mechanism evolved as a way to keep individuals alive through lean times until they could reproduce once food is again available. So it's not too surprising that starving animals have less interest in sex.
The article in H+ suggests that many human practitioners of calorie restriction also suffer from shrunken libidos. Worse yet, perhaps permanent starvation will not actually increase the lifespans of practitioners:
So what about those glowing reports which purport to have demonstrated that caloric restriction doubles the lifespans of mice and rhesus monkeys, as well as giving them glossy pelts? Surely we can put up with a bit of mental confusion, even failing erections, in exchange for a longer life, as long as it’s of high quality –- otherwise we’ll end up like poor Tithonus, who was granted immortality but not youth and dwindled into a shriveled husk before the gods in their whimsical mercy turned him into a cicada. And it does seem that caloric restriction decreases such banes of extended human lifespan as diabetes and atherosclerosis. Well, there’s something interesting going on, all right, but not what people (like to) think.
In biology, details are crucial and mice are not humans. In Eldorado Desperadoes I: Of Mice and Men [See Resources], I explained at length why non-human studies are proof of principle at best, irrelevant at worst. Laboratory mice and monkeys are bred to reproduce early and rapidly. They’re fed rich diets and lead inactive lives –- the equivalent of couch potatoes. The caloric restriction studies have essentially returned the animals to the normal levels of nutrition that they would attain in the wild. Indeed, caloric restriction of wild mice does not extend their lives and when caloric levels fall below about 50%, both lab and wild mice promptly keel over,.... In the rhesus studies, lifespans appeared extended only when the investigators counted a subset of the deaths in the animal group they tested.
So what about calorie restriction and sex? Combing through the research literature, I came across an article in the journal of Manhattan studies, New York magazine, that addressed this topic:
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“Before CR, I was, if anything, hornier than most men,” says Michael [a calorie restrictor]. “But some people find that when they go on very severe CR, their classic male libido—that sort of aaaargh-there’s-a-pretty-woman-I-can’t-stop-my-neck-from-moving libido—goes down.” And Michael, it turns out, is one of those people.
“I’ve often thought that if you could explain to women that on CR, men will improve their sexual performance but decrease their skirt-chasing behavior so they only have eyes for you, who they’re in love with, women would be like, ‘I’m cutting your calories, honey. Half your dinner tomorrow,’” April [Michael's calorie restrictor wife] resumes.
Damn it! Why does everything have to be a trade-off?