Sunlight may trigger hormone that makes men hungrier, study suggests | Biology


Summer sun can make us feel hot, sweaty and a little burned, but it can also make men hungrier, by releasing an appetite-enhancing hormone from fat stores in their skin, data suggests.

The study, which was published in the journal Nature Metabolism, adds to growing evidence that the effects of sun exposure may be more complex than first thought.

Overexposure is known to increase the risk of skin cancer, but recent studies have suggested that moderate exposure may increase life expectancy on average by helping to protect against cardiovascular disease and other causes of death. One possibility is that it lowers blood pressure by releasing nitric oxide from the skin, a process that causes blood vessels to relax. Other scientists have attributed the health benefits of sunlight to vitamin D production.

Carmit Levy, a professor in Tel Aviv University’s department of human molecular genetics and biochemistry, and his colleagues, who wondered whether food consumption might also provide clues, analyzed data from 3,000 participants who took part in a national nutrition survey. The researchers found that men, but not women, increased their food intake during the summer months. The effect wasn’t huge — equivalent to eating an extra 300 calories a day — but over time, this could be enough to cause weight gain.

To investigate further, they exposed male and female volunteers to 25 minutes of midday sun on a clear day, and found that it increased levels of the appetite-stimulating hormone ghrelin in the men’s blood, but not in women.

Experiments on mice similarly showed that when men were exposed to UVB rays, they ate more, were more motivated to forage for food and had increased levels of ghrelin in their blood. No such change was seen in female mice.

The trigger for ghrelin release was found to be DNA damage in skin cells. Estrogen blocked this effect, which may be why sunlight didn’t affect women in the same way.

Levy explained that ghrelin, also known as the ‘hunger hormone’, had different effects on the body than appetite regulation: it also reduces inflammation and heart muscle loss, and decreases arterial function. [blood] pressure. “Ghrelin may be the mechanistic link between sun exposure and reduction in cardiovascular disease,” she said.

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Carlos Diéguez and Rubén Nogueiras, professors at the University of Santiago de Compostela, Spain who were not involved in the study, described the results as “exciting”, adding that future studies examining the influence of age and ethnicity in relation to UVB exposure and assessing food intake would “be anxiously awaited”. “This work will certainly pave the way for further studies on the role of the skin in energy and metabolic homeostasis [balance]a field that had been largely overlooked,” they wrote in an accompanying news and views in Nature Metabolism.

dr. Duane Mellor, a dietitian and senior lecturer at Aston University who was also not involved, was more cautious. “What it does show is the potential mechanism of how UVB may affect hormone metabolism, and how it may be associated with an increase in the appetite hormone ghrelin, at least in mice,” he said.

“It is important to recognize that this article does not claim that exposure to sunlight and UVB will lead to weight gain in human males. Instead, it offers some interesting insights into how moderate exposure to UVB may be linked to health benefits, including reduced cardiovascular risk and inflammation, as ghrelin has anti-inflammatory effects.”

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