With temperatures topping 30 degrees Celsius (86 degrees Fahrenheit) in parts of the United Kingdom this week, many of us may be craving ice cream or a few cold beers in a beer garden.
Scientists assert, however, that it may not be our need to cool off that compels us to consume these extra delights when the weather is warm.
The results of a recent study indicate that exposure to sunshine promotes the production of a hunger hormone from the skin, but only in males.
Males are more prone to gain weight over the summer, according to researchers from Tel Aviv University in Israel.
In women, however, the sex hormone estrogen interferes with the release of this hunger hormone, preventing it from altering appetite.
“These results highlight the skin as a critical mediator of energy balance and may lead to the therapeutic potential for sex-based therapies of endocrine-related disorders,” the research group said.
Communication between peripheral tissues, such as the gut and liver, and the brain is primarily responsible for regulating the need for food.
The hypothalamus, which regulates temperature, sleep, and hunger, receives hormones from peripheral organs.
The study, which was published in Nature Metabolism today, examined the findings of a survey of 3,000 Israeli participants who reported their diet between 1999 and 2001.
Carmit Levy and her colleagues observed that, on average, men increased their caloric intake during the summer months, when solar radiation is at its peak.
Women, however, did not demonstrate a similar summer surge.
Ten weeks of daily exposure of mice to ultraviolet (UVB) light provided support for the investigation.
Researchers reported a “significant increase in food consumption” in male mice but not in female mice.
UVB radiation was discovered to trigger the production of the hunger hormone ghrelin from the fat cells in the skin tissue of male mice, known as adipocytes.
Upon reaching the hypothalamus, ghrelin stimulated the mice’s hunger, resulting in increased food intake and weight growth.
In addition, the researchers observed that the mice had greater food-seeking behavior, indicating that the increased food intake was not just due to the need to satisfy calorie requirements but was also influenced by the gratifying qualities of food.
However, this impact was diminished in female mice due to the interference of the sex hormone estrogen with the release of ghrelin from fat cells in the skin.
When exposed to UVB, female mice with low levels of circulating estrogen ate more.
The researchers conducted three more studies, two of which examined human cell cultures and their sensitivity to UVB light in the lab.
Five-day exposure of male human skin samples to UVB led to an increase in ghrelin expression, the researchers discovered.
Another investigation investigated the appetites of individuals with skin diseases who received UVB sun therapy.
One month after beginning treatment, males generally reported feeling hungrier, whereas females reported no change in appetite.
The researchers believe that their findings reveal skin fat as a potential regulator of feeding behavior, introducing a new form of fat tissue to the energy balance equation.
Professor Kevin McConway, Emeritus Professor of Applied Statistics at The Open University, argues that additional factors may have affected these results.
He stated, ‘The results in mice and cell cultures, as well as the other human studies, make it probable that solar radiation and sex differences could be involved in producing the patterns, but they cannot rule out other alternative causes.
‘It is also likely that men and women respond differently to sunny weather based on societal factors.
Because of social conventions and practices about how much skin men and women might expose to the sun, because of various jobs that they might do, or for a multitude of other reasons, this may all have something to do with disparities in calorie consumption.
Duane Mellor, Registered Dietitian and Senior Teaching Fellow at Aston Medical School stated, “This study provides five pieces of a very interesting scientific puzzle, but due to data gaps, these pieces do not fully fit together.”
‘It demonstrates the potential method by which UVB might alter hormone metabolism, which is comparable to how it helps vitamin D production in our skin, and how this may be related to an increase in the appetite hormone ghrelin in male mice.
“It is essential to note that this article does not imply that exposure to sunlight and UVB would cause male weight gain.”
As ghrelin has anti-inflammatory properties, the study provides new insights into how moderate UVB exposure may be associated with health advantages, such as reduced cardiovascular risk and inflammation.
‘It is also crucial to keep in mind that when most people have more UVB exposure from sunlight, they may also be more physically active; this could partially explain increased food consumption, although there are no data to corroborate this in the present study.’
