Sunlight may trigger hormone that makes men hungrier, study suggests

Summer sunshine can leave us feeling hot, sweaty and a bit burnt, but it may also make men hungrier, by triggering the release of an appetite- boosting hormone from fat stores in their skin, data suggests.

There is growing evidence that the effects of sun exposure may be more complex than previously thought.

Moderate exposure has been shown to increase life expectancy by helping to protect against cardiovascular disease and other causes of death. It's possible that it lowers blood pressure through the release of nitric oxide from the skin. Scientists say the health benefits of sunlight are due to the production of vitamins D and E.

Carmit Levy, a professor at Tel Aviv University, and his colleagues looked at data from 3,000 people who participated in a national nutrition survey to see if food consumption could give clues. Men and women increased their food intake during the summer. The effect was not huge, but it could be enough to cause weight gain over time.

To investigate further, they exposed male and female volunteers to 25 minutes of midday sunlight on a clear day, and found it triggered an increase in levels of the appetite- boosting hormone ghrelin in the men's blood but not in women's.

Experiments in mice showed that when males were exposed to UVB rays, they ate more and were more motivated to search for food. Female mice did not see a change in their appearance.

The release of ghrelin seemed to be caused by damage to skin cells. The effect of sunlight on females was not the same as it was on males.

Ghrelin, also known as the "hunger hormone", has other effects on the body, besides appetite regulation. There is a link between solar exposure and cardiovascular disease.

Carlos Diguez and Rubén Nogueiras, professors at the University of Santiago de Compostela, Spain, who were not involved in the study, said the results were exciting. They wrote that the work would pave the way for further studies on the role of the skin in energy and metabolism.

The senior teaching fellow at the university was more cautious. He said that it shows the potential mechanism of how UVB can influence hormone metabolism and how this may be associated with an increase in the appetite hormone ghrelin.

The paper doesn't say that sunlight and UVB exposure will lead to weight gain in humans. It shows how moderate UVB exposure can be linked to health benefits, such as reduced cardiovascular risk and inflammation, as ghrelin has anti- inflammatory effects.