Up your vitamin D
Vitamin D deficiency is taking blame for a growing list of health problems.
"There has been increasing interest in the importance of optimal vitamin D concentrations in recent years, and low vitamin D has been associated with increased risk of several negative health outcomes," lead researcher Cecilia Host Ramlau-Hansen of Aarhus University Hospital.

Too little vitamin D has been tied to osteoporosis, autoimmune diseases such as multiple sclerosis and some common cancers. But, as Ramlau-Hansen notes, no one has yet looked for a link between vitamin D and the quality of a man's semen.

Prior studies have suggested vitamin-D deficiency lowered the fertility of male rodents. Further, human testes and sperm cells are known to carry vitamin D receptors, along with enzymes that metabolize the vitamin, suggesting that vitamin D might play a role in the production and maturation of sperm.

In the new study, Ramlau-Hansen and her colleagues compared blood levels of vitamin D and semen quality from more than 300 Danish men between the ages of 18 and 21.

The team found that about half of the men had healthy blood levels of vitamin D above 80 nanomoles per liter (nmol/L). Ninety-two men had levels below 50 nmol/L, which is considered suboptimal, and of those, just 19 men had levels of 25 nmol/L or less, which is considered deficient, report the researchers in the journal Fertility and Sterility.

When the team matched these numbers up with measures of semen quality and quantity, including the sperm cells' ability to swim, they were surprised to find that the sperm-filled fluid of men with low blood levels of vitamin D was of no less quality than the semen from men with normal or high vitamin D levels.

In fact, high levels of vitamin D appeared to slightly lower sperm counts, although the effect disappeared after the researchers took into account other factors that may affect semen such as season and smoking status.

Still, a relationship between vitamin D and semen quality can't yet be ruled out. Ramlau-Hansen pointed to some limitations of the study, including the small number of subjects who were actually vitamin-D deficient.

"We do not know whether the results apply to populations with vitamin D levels in the suboptimal range," she said.

Blood levels of vitamin D, which is manufactured in the skin in response to sunlight and can be consumed in some foods, can vary widely around the world.

The researchers also don't know how vitamin D supplementation might affect semen quality; they looked solely at actual blood levels in the men.

Until further studies strengthen and refine the findings, Ramlau-Hansen can only say that more vitamin D "may not be important" for semen quality in men with vitamin levels already in the optimal range.

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