Men boost calorie intake in summer because sunlight makes them hungrier

Sunlight hitting the skin seems to boost secretion of an appetite-boosting hormone in men, causing them to eat more in summer – but women aren’t affected in the same way

By Clare Wilson

Winter may be seen as the time to fill up with comfort food, but in fact, the sunny summer months are when men eat more calories – unlike women.

The effect may occur because sunlight makes the skin release an appetite-stimulating hormone called ghrelin, says Carmit Levy at Tel Aviv University in Israel.

Levy and her team noticed the unexpected effect in experiments in mice that investigated mechanisms behind skin cancer, in which male animals exposed to UV light ate more food.

To see if humans do the same, the researchers used existing data on about 3000 people who had filled in detailed dietary questionnaires as part of the Israeli government’s regular national health and nutrition survey. Between March and September, men consumed about 17 per cent more calories per day than they did during the rest of the year, while women’s food intake stayed about the same.

Human appetite is influenced by many complex systems, but a substance called ghrelin seems to be the only hormone that directly stimulates eating. It was thought to be mainly secreted by the stomach when empty. “It tells the brain to eat more,” says Caroline Gorvin at the University of Birmingham, UK.

Further investigation of the effect in animals revealed that exposing male mice to UVB radiation, which is present in sunlight, raised levels of ghrelin secretion by fat cells in their skin. This was blocked by the female sex hormone oestrogen, which may explain why the effect wasn’t seen in the female mice or the women.

Boosted ghrelin secretion was also seen in skin samples that were taken from men and exposed to UV light in the lab.

Gorvin says the skin hadn’t previously been thought to play a role in appetite. “If it’s real, it’s quite exciting,” she says.

It is unclear why the effect happens, but it may be an adaptive response to fuel greater physical activity in summer, says Levy. “The skin is the largest organ of the body, so it makes sense that this huge organ can sense the environment, sensing that there is UV now, that now is the time to go out.”

The team hasn’t investigated whether the higher food intake makes men gain weight.

Journal reference: Nature Metabolism, DOI: 10.1038/s42255-022-00587-9

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