The study, by researchers from the Medical University of Graz, Austria, involved 2,299 men. The researchers report that testosterone levels in men were directly linked to vitamin D levels in the body. Furthermore, men who had at least 30 nanograms of vitamin D per milliliter of blood had higher levels of testosterone as compared with men who had low levels of the vitamin.

Levels of testosterone appeared to peak in August when summer was at its best and dropped down in winter with the lowest levels being recorded in March.

"Men who ensure that their body is at least sufficiently supplied with vitamin D are doing good for their testosterone levels and their libido among other things," said a spokesman for Ad Brand of the Sunlight Research Forum in The Netherlands, which is a non-profit organization that updates the public on the latest news in sun and health.

Lead researcher Professor Winfried Marz and colleagues, who conducted this study, said that scientists would now have to look at vitamin D supplements and examine if they have the same effect on testosterone levels as sunbathing does.

The details of the study appear in the journal Clinical Endocrinology.

Low levels of testosterone not only decrease libido in males, but also reduce energy levels. Testosterone is also important in both males and females for ensuring adequate muscle strength and bone density. The findings of the above study also reinforce earlier findings, which said that an hour of sunbathing might increase testosterone levels by 69 percent in males.