Exposure to sunlight seems to stimulate the release of the hunger hormone – at least in men. That is why they eat more in the summer than in the rest of the year. Women do not experience this effect.
Although winter is known as the season to feast on comfort food and Christmas dinners, men appear to consume more calories in the summer. This may be because summer sunlight triggers the release of the hunger hormone ghrelin. That discovered molecular biochemist Carmit Levy from Tel Aviv University in Israel. Women’s appetites don’t seem to change with sunlight.
Dieetdata
Levy and her colleagues stumbled upon the unexpected effect when they examined the mechanisms behind skin cancer in mice. They noticed that male mice started eating more after exposure to UV light.
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To find out whether this effect also occurs in humans, the scientists delved into dietary data from about 3,000 people. This information was collected as part of the National Health and Nutrition Survey of the Israeli government. The data showed that between March and September, men ate about 17 percent more calories per day than the rest of the year. Women’s food intake hardly changed.
Hungry Skin
Our appetite is the result of the interaction between many complex systems, but ghrelin is the only hormone that directly influences our eating behavior. It was previously thought that the substance is mainly excreted by the empty stomach. ‘It then signals our brain to eat more’, says biomedical scientist Caroline Gorvin from the University of Birmingham in the United Kingdom.
But further research showed that exposing male mice to UVB radiation – which is also present in sunlight – stimulates the fat cells in the skin to release ghrelin, not the stomach. The female sex hormone estrogen blocks this ghrelin release. That may be why this effect only occurs in men.
A control study in cultured skin cells from men showed that direct UV radiation indeed stimulates the release of ghrelin.
Adaptability
Gorvin states that the skin has not been linked to appetite and hunger before. ‘If this result is actually correct, then that’s pretty exciting,’ she says.
It is still unclear why this effect occurs. According to Levy, the higher calorie intake allows the male body to provide more physical activity in the summer. “The skin is the largest organ in the body, so it makes sense that it can sense its surroundings. In this way, the skin can determine that there is UV radiation and therefore sunlight, and signal the body that it is time to go outside.’
Levy’s team did not investigate whether increased food intake in men also leads to weight gain.
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