A Make-Time-For-Sex Diet?
We’re slaves to our hormones. Teenagers and pregnant women are experts on that topic. Both ride an emotional roller coaster as their bodies produce vacillating amounts of sex hormones.
In fact, behind the scenes of all human biology–from conception to death–a delicate interplay of hormones drives everything from the expression of our gender to regulation of growth and maintenance of health.
In women, these hormonal cycles affect hunger and fullness, a surprising new analysis indicates.
Each month, a menstruating woman has a short window during her cycle when rising sex hormones signal an ovary to release an egg, making conception possible. During this ovulatory period, a California anthropologist now finds, women naturally and unwittingly eat some 5 to 25 percent less than at other times. Daniel M.T. Fessler of the Center for Behavior, Evolution, and Culture at the University of California, Los Angeles stumbled onto the phenomenon only after sifting through long-term dietary data from 16 published studies of women in North America, Europe, and Australia.
Ironically, he tells Science News Online, few of the authors of those earlier studies focused on or noted this trend in their published data. It emerged only after Fessler looked at the data collectively.
Upon reflection, Fessler says, the drop in caloric intake during ovulation makes sense. It would be a beneficial evolutionary adaptation–at least for our hunter-gatherer ancestors. For most of them, food was a scarce commodity, requiring much energy to find and prepare. Even scarcer, however, would be finding and hooking up with a mate at precisely the time–during ovulation–when conception was possible.
Women’s bodies must be telling them to give less attention to food and more attention to sex during the time each month when they could become pregnant. Fessler concludes in the latest Quarterly Review of Biology that in weighing the tradeoff between perpetuating the species and getting second helpings at meals, women’s bodies opt for making babies.
Declining impact
In the studies Fessler reviewed, the dip in calorie consumption during ovulation was generally small. However, Fessler says, the research “almost certainly underestimates the extent to which humans are prone to reduce food consumption.” The reason, he argues, is that, metaphorically, “these women are zoo-kept animals.”
Today, only a few tribes are truly independent in securing their food from nature. Most of us get plenty of what we need from farms and stores. In prehistory, however, populations depending on what forests and plains would yield typically oscillated between feast and famine.
“But in our world of supersized fast foods,” Fessler says, it doesn’t take extra work to get extra calories. The utility of a suppressed appetite for the sake of giving more attention to reproduction would seem a long-forgotten relic. “Seen in this light,” Fessler says, “the fact that Western women exhibit any [dietary consumption decline during ovulation] indicates that the phenomenon is probably quite robust.”
Support from animal data
For confirmation of his hunch, Fessler turned to studies of animals. After all, he notes, lab rats don’t succumb to media blitzes for burgers and fries. Most eat when they’re hungry and stop when they aren’t.
It turns out that reproductively mature female baboons, monkeys, dogs, pigs, goats, sheep, deer, cows, guinea pigs, and rats all experience a drop in food consumption at estrus–the period when these animals will accept a mate and are capable of conceiving. Indeed, Fessler notes, “rodent studies reveal that the estrus drop in food consumption occurs independently of changes in activity levels.” This and other findings suggest that females eat less not because they are too busy with other things, but rather because of some biological change–such as in satiety.
Moreover, there are some data from animal studies to suggest a biochemical change at play. Cholecystokinin (pronounced KOH-le-SIS-to-KY-nin) is the big name of a tiny molecule that the gut releases to tell the rest of the body that it’s had enough. It’s a clarion call of satiety.
With high estrogen concentrations in an animal’s blood, these data show, the amount of cholecystokinin needed to signal satiety drops. In other words, Fessler explains, “the impact of cholecystokinin on behavior is amplified by the presence of [such sex] hormones, which peak around the time of conception.”
With their hunger dampened during this period, females have more time to focus on mating.
“Now, skeptics might object that copulation does not take very long” so it shouldn’t really affect meal times, Fessler acknowledges. However, he counters, “copulation is merely the punctuation mark at the end of the mating process.” Indeed, he notes, for many female mammals–especially human ones–the most time-consuming facet of mating isn’t copulation: It’s finding a desirable male and then effectively signaling him to come hither.
