Scientifically Speaking | Does sunlight make men (but not women) eat more?
Researchers at Tel Aviv University found that if you’re a man and you spend time out in the sun, you’re going to feel hungrier. Also, quite amazingly, women seem to be resistant to food-seeking behaviour induced by sunlight.
Sometimes, I come across a scientific article with a conclusion that seems so remarkable that at first it is hard to believe. On digging deeper and finding that the experiments in it are all well designed and support the results, I wonder why such fundamental and significant discoveries had not been made earlier!
The authors hypothesise that the amount of skin exposed might have an effect on light-induced ghrelin production. (File Photo)
Both these thoughts were running through my mind a few days ago when I read a research article published in Nature Metabolism. Researchers led by Carmit Levy at Tel Aviv University in Israel found that being exposed to sunlight causes men (but not women) to seek out and eat more food. If you’re a man like I am and you spend time out in the sun, you’re going to feel hungrier and eat more than if you’re a couch potato in wintery weather. Also, quite amazingly, women seem to be resistant to food-seeking behaviour induced by sunlight.
There are differences in how males and females respond metabolically to the environment, but until now, differences caused by exposure to sunlight, or more specifically to ultraviolet radiation, had not been known. How hungry we feel and how much food we eat is decided by the brain based on signals received from organs of the digestive system. But the new research, which tantalising hints that eating cues can originate from skin as well, is worth breaking down.
The researchers first examined epidemiological data of the eating habits of over 3,000 people while they were exposed to varying amounts of sunlight throughout the year in Israel. The team found that males increased the amount of food they ate in the summer when normal exposure to sunlight would be expected to peak. However, a similar increase in food consumption was not found in females.
To see if there was an explanation and process for this difference, the researchers then turned to mice. Mice aren’t people of course, but they serve as useful lab animals to tease out aspects of human physiology because experiments can be designed more easily with them. Plus, mice can be genetically manipulated. Exposing lab mice to ultraviolet (UVB) light for several weeks caused male mice to eat more and gain more weight. A similar observation didn’t occur with female mice. The results were now holding up in mice as well.
The researchers then painstakingly worked out a plausible way sunlight might be increasing appetite in males. An appetite-controlling hormone known as ghrelin is secreted by the stomach and also by fat depots in layers of skin. Levels of this hormone go down after eating a meal. Notably, ghrelin is found in both mice and men.
Ghrelin secretion in fat tissues in the skin is controlled by a protein that acts as a “switch”. What the researchers then found is that exposure to ultraviolet light turns on the switch, and this causes more ghrelin to be secreted. It is this excess ghrelin that prompts the brain to seek out more food. And male mice and men seem more ready to comply with these sunlight-derived cues.
If the switch protein plays an important role in sunlight-induced eating, then we can expect mice that lack it not to be affected by sunlight. And indeed, mice that were genetically modified and didn’t make that functioning switch in fat tissues in the skin didn’t make extra ghrelin when exposed to sunlight. These mice didn't have the resulting food craving either.
Quite curiously, what the researchers also found is that estrogen (which is a sex hormone in females) has the ability to dial down the switch, causing no sunlight-induced increase in circulating ghrelin in women. Presumably, this is what makes women less likely to be as ravenous as men after prolonged exposure to sunlight.
Overall, this is a wonderful study that leads to many questions, not least of which is why there’s a biological difference between exposure to sunlight in men and women in the first place. How exposure to sunlight correlates to amounts of ghrelin will also need to be answered. It’s too early to answer either question based on this single study.
The authors hypothesise that the amount of skin exposed might have an effect on light-induced ghrelin production. They mention earlier studies that show that music and odours can also cause ghrelin to be released. Not much is known about how these environmental factors might affect appetite, the possibility of being able to one day control appetite through music, for example, is fascinating.
Excessive exposure to ultraviolet light is known to cause skin cancer. More recent work also points to the significant benefits of sunlight in preventing heart and metabolic diseases. There’s definitely an effect of sunlight that is mediated through vitamin D, which is made in the skin on exposure to sunlight. But the recent study points to added involvement of skin in metabolism independent of vitamin D. And it shouldn’t surprise us since the skin isn’t just a barrier that protects us, it is also the largest organ in the human body.
Broadly, there’s a possibility that there are other factors that we don’t know of in the skin that influence not only our metabolic state but other aspects of our health and well-being. If we continue to search further, we might discover that there are signals of health that are literally skin deep.
Anirban Mahapatra is a scientist by training and the author of a book on COVID-19. He’s writing a second popular-science book
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