Amazing effect: The increased UV radiation in summer increases appetite in men – but not in women, as a study on humans and mice reveals. Accordingly, the UV light causes certain fat cells in the skin to increasingly release the hunger hormone ghrelin. That stimulates the appetite. In women, the sex hormone estrogen inhibits this process, causing them to develop less light-dependent hunger, as the researchers report in the journal Nature Metabolism.
Whether we feel appetite and hunger depends on a variety of internal and external factors. A drop in blood sugar levels and other nutrients in the blood triggers the release of the hunger hormone ghrelin. This in turn activates the appetite center in our brain. The result: we get hungry. But our intestinal flora or external influences such as the smell or sight of food can also stimulate our appetite.
Increased appetite and calories under UV exposure
Now it turns out that sunlight can also affect our appetite – at least in men. Shivang Parikh of Tel Aviv University and his colleagues received the first indication of this when they evaluated the data from 3,000 participants in an Israeli nutrition study. It found that men consumed significantly more calories per day during the particularly sunny summer months than in winter. In women, on the other hand, the energy content of the food did not change.
This light-dependent effect was confirmed in tests in which a group of men and women were exposed to a UVB lamp for 25 minutes a day. “The men report a stronger feeling of hunger after this treatment,” report Parikh and his team. In blood samples, they also measured an intensification of the fat and steroid metabolism in the men. Again, there was no comparable reaction among the women.
Skin fat cells release hunger hormone
But what is behind it? To clarify this, the researchers carried out additional experiments with mice and human skin samples. It turned out that when male skin is exposed to UVB radiation, certain subcutaneous fat cells produce more of the hunger hormone ghrelin and release it into the blood. This increases the ghrelin content in the blood – and with it the appetite. “The increase in appetite was associated with increased levels of ghrelin in both men and mice,” reports the team.
Further analysis revealed that ghrelin production in subcutaneous adipose tissue is closely related to the p53 protein. This is increasingly produced in our cells whenever DNA damage occurs and the cell’s repair mechanisms have to take action. In the skin, this happens, among other things, as a result of UV-induced cell damage. The skin fat cells then produce more p53, which in turn stimulates the release of ghrelin, as Parikh and his colleagues discovered.
In women, estrogen inhibits the process
But why does this only happen to men? An obvious reason would be the influence of the sex hormones testosterone and estrogen. “Previous studies have already shown interactions between ghrelin and these steroid hormones,” the researchers explain. To investigate this, they investigated how cultures of skin fat cells react to UV radiation when testosterone or estrogen is added.
The result: the male sex hormone increased ghrelin release under UVB light, but the female sex hormone estrogen inhibited the production of the hunger hormone. As Parikh and his team found, the estrogen binds directly to the p53 protein and inhibits its activity. “This blocks the positive effect of solar radiation on appetite and food intake in women,” they report.
Our skin as a metabolic player
This study reveals for the first time that our skin – the largest organ in our body – also plays an unexpectedly important role in the regulation of appetite and food intake. This could also open up new possibilities for treating pathological loss of appetite in men – for example through targeted UVB radiation. “Our studies indicate that the possible uses of phototherapy will thus be expanded,” the researchers write.
In addition, UV radiation or other environmental influences on our skin could also cause other still unrecognized effects: “It will be exciting to investigate whether other hormones are released by DNA damage or other skin stimuli – and whether and how they affect human behavior can change,” says Parikh and his team.
Carlos Dieguez and Ruben Nogueiras from the University of Santiago de Compostela in Spain have a similar view. In a comment they write: “This work will certainly pave the way for further studies on the role of skin in energy management and metabolic balance – a field that has long been overlooked.” (Nature Metabolism, 2022; doi: 10.1038/s42255-022-00587-9)
Quelle: Nature Metabolism
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