Men with higher levels of vitamin D—the sunshine vitamin—also have higher levels of testosterone, the main male sex hormone responsible for libido (and sperm production), according to new research from Austria. The study, published online December 29, 2009, in the journal Clinical Endocrinology, tracked vitamin D and testosterone levels in more than 2,000 men over the course of a year. Stimulated by exposure to direct sun, 90 percent of vitamin D in the body is produced by the skin. And predictably, researchers found that men's vitamin D levels peaked in the month of August, and dropped off in the winter, hitting their lowest levels in March.
But researchers also found that fluctuations in testosterone levels followed the same pattern across study participants. While it is unclear why vitamin D matters so much, "men who ensure their bodies are ... sufficiently supplied with vitamin D are doing good for their testosterone levels and their libido...," says the Sunlight Research Forum, a Dutch research group commenting on the study.
It may not take much sun for your man to see results—the study backed up previous research that showed one hour of sunshine may temporarily boost testosterone levels by as much as 69 percent. Great news if you live below the 37-degree latitude line (roughly a line from San Francisco to Richmond, Virginia) where the sun is direct enough year round for skin vitamin D production. North of this line, skin is generally only able to produce vitamin D from April through October.
For guys in the northern latitudes, eating oily (and vitamin D-rich) fish such as salmon, sardines, and mackerel; taking a daily cod liver oil or 400 IU vitamin D supplement; and drinking vitamin D-fortified milk are good ways to maintain adequate levels of the vitamin when sunshine is short supply.