Can Sunlight Improve Human Metabolic Health?

A new study shows that sunlight may influence fat metabolism. There are some proteins present on fat cells that are light-sensitive and can detect sunlight. Sunlight can bring changes in fat cells and may increase the risk of metabolic syndrome. The study published in the journal Cell Reports.

The study presents the new ways in which sunlight affects the lives of mammals on earth. The life on the earth is not possible without the sunlight as it drives many important processes of life such as photosynthesis in plants. Like plants, animals also use sunlight in different ways to maintain their daily rhythms. There are photoreceptors present in our eyes that detect the sunlight and pass the information to the brain. It then generates the image of the things that are present in the surrounding of a person. Sunlight also maintains our days and nights through a 24-hour circadian sleep-wake cycle.

There are some proteins expressed in retinal cells of animals that detect the sunlight. These proteins are called opsins. In humans, two well-known types of opsin are neuropsin and melanopsin.

Scientists believed that sunlight can only be detected through eyes. But many studies showed there is some light-detecting compounds in animals that present outside the visual system such as in frog light can be detected through cells present in its skin known as chromatophores.

In 2019, a study showed that in rats lights can be detected through neuropsin present in skin, helping them to maintain their circadian rhythms. But evidence about the photoreception in the mammals is insufficient and more research is needed to validate this concept.

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The first time, a study was conducted on mice for investigating the presence of opsins on the fat cells under the skin that might detect light.

Richard Lang who is the senior author of the study from Cincinnati Children Hospital Medical Center in Ohio said that the concept of light penetration deep into the tissue is new for me and my colleagues. So, we have been trying to find the opsins that present in the tissues.

Opsin 3 is present throughout the body including testis, kidneys, liver, and brain. Researchers said that OPN3 is present in both mouse and human adipose or fat cells. Light penetrates deep into the skin and specific blue light with a wavelength of 480nm triggers the OPN3. The stimulation only occurs in the presence of natural light not in artificial light.

Scientists used genetically engineered mice in their experiment who don’t have gene coding for OPN3 in adipocytes.

Mammals including humans start to burn fat in the cold conditions to generate heat. There are two types of adipose tissue, white adipose tissue and brown adipose tissue. White adipose tissue is responsible for storing energy while brown adipose tissue is responsible for creating the heat in a cold environment that does not involve shivering.

During the process of lipolysis, white adipose tissue releases glycerol and free fatty acids into the blood circulation. Brown adipose tissue use these free fatty acids for generating heat.

Brown fat when activated might protect the body from metabolic syndrome. The idea was supported through the growing body evidence. A metabolic syndrome is a group of different conditions that involve high blood sugar, hypertension, excess abdominal fat, and abnormal blood lipid profile.

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In an experiment, the mice that lack Opn3 in their adipocytes when they were exposed to cold temperatures (4 Celsius) did not respond. Their core temperatures of the body were lower as compared to the mice which have Opn3 genes.

Modified brown fat in mice did not generate enough heat as required to maintain the body temperature. When these mice were not fed with food then they burn little fat as compared to normal mice.

When the mice that lack Opn3 genes were placed in the cold conditions under the light that lacks specific blue light, they did not generate heat and their core body temperatures did not change. The lighting condition is called “minus blue lighting condition”. So, this the evidence that blue light is necessary for generating heat. Researchers conclude the sunlight is important for life activities and for maintaining healthy fat metabolism.