The study, published in the journal Nature Metabolism, adds to growing evidence that the effects of sun exposure may be more complex than first thought. Excessive exposure is known to increase the risk of skin cancer, but recent studies have shown that moderate exposure can increase life expectancy, on average, helping to protect against cardiovascular disease and other causes of death. One possibility is that it lowers blood pressure through the release of 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 analyzed data from 3,000 participants enrolled in a national nutrition survey, wondering if food consumption might also provide some clues. The researchers found that men but not women increased their food intake during the summer months. The effect wasn’t huge – the equivalent of 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 sunlight on a clear day and found that it caused an increase in the levels of the appetite-stimulating hormone ghrelin in the men’s blood but not in the women’s blood. Experiments on mice similarly found that when males were exposed to UVB rays, they ate more, were more motivated to forage, and had increased levels of ghrelin in their blood. No such change was observed in female mice. The trigger for ghrelin release appeared to be DNA damage in skin cells. Estrogen blocked this effect, and perhaps that’s why sunlight didn’t affect females in the same way. Levy explained that ghrelin, sometimes called the “hunger hormone,” had other effects on the body besides regulating appetite: It also reduces inflammation and heart muscle wasting, and lowers blood pressure. [blood] pressure. “Ghrelin may be the mechanistic link between sun exposure and reduced cardiovascular disease,” he said. Subscribe to First Edition, our free daily newsletter – every morning at 7am. BST 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 assessing the effect of age and ethnicity in relation to exposure to UVB and food intake would be “looked forward to”. “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 opinion piece in Nature Metabolism. Dr Duane Mellor, a nutritionist and senior lecturer at Aston University who was also not involved, was more cautious. “What this shows is the potential mechanism of how UVB can affect hormone metabolism and how this can be related to an increase in the appetite hormone ghrelin, at least in mice,” he said. “It is important to recognize that this paper does not claim that sunlight and UVB exposure will lead to weight gain in men. Instead, it provides some interesting insights into how moderate UVB exposure could be linked to health benefits, including reduced cardiovascular risk and inflammation, as ghrelin has anti-inflammatory effects.”
