The summer sunshine can make us feel hot, sweaty and a little burnt – but it can also make men hungrier by triggering the release of an appetite-boosting hormone from fat stores in their skin, data suggests.
The study, which is published in the journal Nature Metabolism, adds to growing evidence that the effects of sun exposure may be more complex than first thought.
It is well known that excessive exposure increases the risk of skin cancer, but recent studies show 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 by releasing nitric oxide from the skin, a process that causes blood vessels to relax. Other scientists attribute the health benefits of sunlight to vitamin D production.
Wondering if food consumption might also provide some clues, Karmit Levy, a professor in Tel Aviv University’s Department of Human Molecular Genetics and Biochemistry, and his colleagues analyzed data from 3,000 participants who were enrolled 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 big—equivalent to eating an extra 300 calories a day—but over time it 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 blood levels of the appetite-enhancing hormone ghrelin in men, but not in women.
Experiments with 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 cause of ghrelin release appears to be DNA damage in skin cells. Estrogen blocks this effect, which may be why sunlight doesn’t affect women in the same way.
Levy explained that ghrelin, sometimes called the “hunger hormone,” has other effects on the body besides regulating appetite: It also reduces inflammation and heart muscle wasting, and lowers arterial [blood] pressure. “Ghrelin may be the mechanical link between sun exposure and reduced cardiovascular disease,” she said.
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Carlos Dieguez and Ruben 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 influence of age and ethnicity in relation to UVB exposure and the food reception will be “looked forward to”. “This work will certainly pave the way for further research into the role of the skin in energy and metabolic homeostasis [balance]an area that has been largely neglected,” they write in an accompanying News & Views in Nature Metabolism.
Dr Dwayne Mellor, a nutritionist and senior lecturer at Aston University, also not involved, was more cautious. “What this shows is the potential mechanism for how UVB can affect hormone metabolism and how that can be linked 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 exposure to sunlight and UVB will cause weight gain in men. Instead, it provides some interesting insights into how moderate UVB exposure may be associated with health benefits, including reduced cardiovascular risk and inflammation, because ghrelin has anti-inflammatory effects.
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