The sun gets a lot done. Not only is it responsible for fueling life on Earth and keeping us alert and happy, but it turns out the sun perhaps influences how much some of us eat. A team of researchers at Tel Aviv University describe a new mechanism in a paper published yesterday (July 11) in Nature Metabolism in which sun exposure appears to stimulate hunger—though only in males.
The researchers analyzed data from Israel’s three-year National Health and Nutritional Survey (MABAT), which included 3,000 participants between the ages of 25 to 65. By looking at season, food intake, and self-reported sex, they found that men increased their consumption by 17 percent during the warmer months of March through September relative to the rest of the year, while women’s caloric consumption remained the same.
One possible explanation for that finding is that there are sex-based differences in how sun exposure affects appetite. To confirm this, the scientists asked a group of 13 men and 14 women between the ages of 18 to 55 to spend 25 minutes in the sun. Participants were then asked questions about their appetites; men reported feeling hungrier, while women experienced no significant differences in their hunger levels before and after sun exposure.
The researchers collected participants’ blood samples before and after the exposure and found that circulating levels of ghrelin, a hormone that stimulates appetite, were elevated in men after they spent time in the sun.
“When we saw differences in their blood proteomics, which was amazing, we said ‘it is real,’” says study coauthor Carmit Levy, a molecular biologist at Tel Aviv University. “Males are reacting to the UV [with increased appetite] and females are not.”
The group further delved into the mechanism with mouse experiments, and the results of these corroborate the findings in humans, according to Levy. The researchers found that ghrelin levels rose significantly in the plasma of male mice, but not in female mice, after exposure to ultraviolet B (UVB) radiation, a component of sunlight, that was equal to about 30 minutes of sun exposure on a Florida summer afternoon. The researchers continued to expose the mice to UVB each day for 10 weeks, and found that sun-exposed male mice ate more frequently over the course of the experiment than did controls without UVB exposure, resulting in weight gain. They were more likely to make the effort of taking trips down a staircase in order to retrieve food than were their female counterparts, suggesting food cravings.
Males are reacting to the UV [with increased appetite] and females are not.—Carmit Levy, Tel Aviv University
Previous research has shown that elevation in ghrelin levels of male mice is driven by a gene called p53, which is responsible for DNA repair. According to Levy, other research groups have found that when male mice are exposed to UVB radiation, changes in the expression of p53 trigger the release of ghrelin from fat tissue in the skin. This hormone circulates in the bloodstream and signals hunger to the hypothalamus, a region of the brain that controls feeding.
“So the question is, why not in females then? Because we have estrogen,” says Levy. “Estrogen inhibits p53 activity and prevents the gene from activating.”
From an evolutionary perspective, Levy and her team speculate that there may be evolutionary benefits to increasing food intake in sun-kissed males, such as potentially increasing sperm production.
While this mechanism of ghrelin production is well-established in mice, some experts say it may be a reach to claim it applies to people. “There’s a bit of a disconnect between the mouse studies and the human studies in this paper,” says Richard Lang, an ophthalmologist at Cincinnati Children’s Hospital Medical Center who has previously published research on how violet light–detecting neurons in the brain can influence metabolism. Lang was not involved in the new study.
Lang says that the study does not show convincingly that prolonged sun exposure increases circulating ghrelin levels in humans through the same mechanism as it does in mice, according to Lang. Susan Roberts, a nutritionist and metabolism researcher at Tufts University Medical Center, agrees. “The human studies [in this paper] are really weak,” she says, pointing out that the researchers did not conduct a randomized trial, and instead looked at a single group of people pre- and post-sun exposure. This, according to Roberts, does not take into consideration what other factors might have changed during the period when participants were exposed to sunlight.
Prior studies have shown that women are also more sensitive to environmental cues for food consumption than men, she adds. “If anything, these results go opposite to what I would expect if metabolic or psychological stresses are at the root of the mechanism.”
Levy agrees that the reliance on an animal model is a limitation of this study, but says the team tried to “make sure that every aspect of the paper had some human evidence.”
Obtaining more relevant human data would be difficult, Lang argues, pointing out strict regulations in place to protect human subjects in scientific studies. He says he’s excited to see in future studies whether there are other molecules that stimulate appetite in response to sun exposure, such as light sensing proteins found in different parts of the body.
Levy says searching for such molecules is a possible next step for the team. “We’re just opening the discussion of what we can learn from the environment’s affect on our body,” she says. “I would like to understand how other type of signals from the environment is changing our behavior.”