The idea that a good workout in bed could pile on the pounds rather than help to shed them seems bizarre, and yet this is exactly what a group of scientists are now proposing.
Writing this month in Medical Hypotheses, the researchers argue that the hormone prolactin may be to blame.
Prolactin stimulates milk production and fatherly love. Blood levels of the hormone rise after sex, especially following orgasm during intercourse. Increased prolactin has in turn been linked to weight gain in several species, including humans who suffer from hyperprolactinaemia (chronically high prolactin levels). And expectant dads are also thought to get chubbier due to a rise in prolactin.
Putting these observations together, Ritesh Menezes from the Department of Forensic Medicine and Toxicology at Kasturba Medical College in Mangalore, India, and colleagues suggest "that increased sexual activity may possibly be a causative factor in gaining body weight".
I couldn't reach Menezes to find out what inspired their idea: voluptuous nymphomaniacs, pot-bellied lechers? But Stuart Brody at the University of the West of Scotland, discoverer of the post-coital prolactin surge, thinks they are barking up the wrong tree.
"There is a relationship between frequency of penile-vaginal intercourse and slimness in humans, but in the opposite direction," he says.
Brody previously found that of 120 healthy men and women, those who had sex often were slimmer than those who did not. He argues that it is inappropriate to compare a medical condition like hyperprolactinaemia with a normal short-term hormonal surge.
"As an analogy, when you exercise, your heart rate increases to perhaps, say, 140 bpm. Good. If your resting heart rate is 140 bpm, that is not likely to be a good thing. Also, speaking of exercise, do not forget the exercise value of, especially, penile-vaginal intercourse."
So, is sex a good way to lose weight, after all, as some have claimed? Or could it still be that if you just do it often enough, prolactin levels stay so high that the scales are, so to speak, tipped in favour of weight gain? Personally, I imagine that having sex this often would either burn calories far in excess of any extra weight that might be put on - or leave no time to eat much in the first place.
Nora Schultz, New Scientist contributor
