Obesity and related metabolic disorders represent escalating global health challenges. Emerging research suggests a potential link between sunlight exposure and weight regulation. This article explores the current understanding of this connection, drawing upon recent studies and established knowledge to provide a comprehensive overview.
The Role of UV Radiation
Ultraviolet (UV) radiation, a component of sunlight, has multifaceted effects on the skin, the body's largest organ. Recent research has begun to uncover the complex ways in which UV exposure may influence appetite and weight.
A study published in the Journal of Investigative Dermatology investigated the effects of UV exposure on appetite and weight regulation in mice. The researchers found that consistent UV radiation exposure led to an increase in appetite in mice, regardless of whether they were fed a normal diet or a high-fat diet. This increased appetite was attributed to a decrease in leptin, a crucial hormone responsible for regulating appetite. However, despite the increased appetite, the mice did not experience weight gain.
Norepinephrine, Leptin, and Fat Browning
The study elucidated a potential mechanism by which UV exposure can increase appetite while simultaneously inhibiting weight gain. It was discovered that UV radiation lowers leptin levels and increases norepinephrine, a hormone and neurotransmitter involved in the body's "fight or flight" response. This increase in norepinephrine promotes the browning of subcutaneous fat, a process that increases energy expenditure.
Subcutaneous fat, located directly beneath the skin, plays a critical role in regulating energy homeostasis. The "browning" of this fat transforms it into a mitochondrial-rich form, allowing it to break down fat more effectively following UV exposure, thus increasing energy expenditure.
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Implications and Cautions
These findings suggest that UV exposure plays a significant role in the body's energy metabolism and homeostasis processes and may offer new avenues for exploring prevention and treatment strategies for obesity and metabolic disorders.
However, it's crucial to exercise caution regarding UV exposure. As noted by co-corresponding author Dong Hun Lee, MD, PhD, UV exposure can accelerate skin aging and increase the risk of skin cancer. Therefore, it's advisable to minimize UV exposure and protect the skin with sunscreen.
The Impact of Sunlight on Subcutaneous Fat Cells
Research from the University of Alberta in Edmonton, Canada, has revealed another intriguing aspect of the sunlight-weight connection: the effect of sunlight on subcutaneous fat cells. This study, published in the journal Scientific Reports, investigated the impact of sunlight on subcutaneous white adipose tissue (scWAT), the major fat depot in humans and a central player in regulating whole-body metabolism.
Blue Light and Lipid Reduction
The researchers discovered that scWAT cells tend to shrink under the effect of the sun's blue light, a visible type of light known to boost attention and mood during the day. When blue light wavelengths penetrate the skin and reach the fat cells beneath, lipid droplets reduce in size and are released out of the cell.
White fat is often referred to as "bad" fat because it stores calories that should ideally be burned for energy. Dysfunctional white fat can lead to cardiometabolic disorders such as obesity, diabetes, and heart disease.
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OPN3 Gene and Energy Metabolism
A study published in Cell Reports uncovered that fat cells deep under the skin can sense light. Scientists at Cincinnati Children’s Hospital were studying how mice control their body temperature and found that light exposure regulates how two kinds of fat cells work together to produce the raw materials that all other cells use for energy.
Richard Lang, PhD, a developmental biologist and senior author of the study, explained that our bodies evolved under the sun’s light, including developing light-sensing genes called opsins. The study showed that light exposure regulates how two kinds of fat cells work together to produce the raw materials that all other cells use for energy.
The researchers found that the internal heating process is compromised in the absence of the gene OPN3 and exposure specifically to a 480-nanometer wavelength of blue light. When light exposure occurs, OPN3 prompts white fat cells to release fatty acids into the bloodstream, which various types of cells can use as energy to fuel their activities. Mice bred to lack the OPN3 gene failed to warm up as much as other mice when placed in chilly conditions. This data led the team to conclude that sunlight is required for normal energy metabolism, at least in mice.
Balancing the Benefits and Risks of Sun Exposure
While the research discussed above suggests a potential link between sunlight and weight regulation, it's essential to consider the broader context of sun exposure and its impact on overall health.
Negative Effects of UVR Exposure
Exposure to UVR has well-documented negative health effects, including sunburn, increased risk of skin cancers (melanoma, lip cancer, and keratinocyte cancers), and ocular diseases (cataracts, pterygium, ultraviolet keratitis, and conjunctival neoplasm). Excessive skin exposure can cause skin erythema (reddening of the skin), with oedema and tissue inflammation.
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UVR directly damages DNA through the formation of pyrimidine dimers and can promote tumor growth through immunosuppression. UVA radiation also damages DNA by inducing reactive oxygen species that causes the oxidation of DNA bases and activating the mitogen-activated protein kinase-dependent pathway.
Positive Effects of UVR Exposure
Sunlight may be beneficial for the treatment of a number of medical conditions including some cancers and neurodegenerative diseases. Chronic but not acute exposure to UVR was associated with significantly reduced incidence of cancers of the breast, prostate, colon/rectum and non-Hodgkin’s lymphoma. Cancer survival is associated with better responses in patients that have higher cumulative sun exposure or who are diagnosed in summer/autumn, which was attributed to the activity of increased circulating 25-hydroxyvitamin D.
Increased sun or UVR exposure is associated with reduced development and/or severity of immune-driven diseases including autoimmune diseases such as arthritis, inflammatory bowel disease, multiple sclerosis, and allergic conditions such as asthma and anaphylaxis. There are many pathways by which exposure to UVR suppresses immunity, through multiple mediators and cellular networks, locally and systemically.
Vitamin D Production
Sunlight exposure is a primary driver of vitamin D production in the skin. Vitamin D plays a crucial role in various bodily functions, including bone health, immune function, and calcium absorption.
Finding the Right Balance
Given the potential benefits and risks of sun exposure, finding the right balance is crucial. Factors to consider include:
- Time of day: UV radiation is most intense during midday hours.
- Season: UV intensity varies depending on the season, with higher levels during summer months.
- Geographic location: Areas closer to the equator experience higher UV radiation levels.
- Skin type: Individuals with lighter skin are more susceptible to sun damage.
Obesity and Related Metabolic Disorders
Obesity is typically defined as a body mass index (BMI) above 30 kg/m2, although this definition is controversial as it may not accurately represent adiposity. Waist circumference is also commonly used to classify obesity (>102 cm for men, >88 cm for women) and may be a better predictor (than BMI) of the risk of developing the co-morbidities associated with obesity (including type-2 diabetes).
Obesity is often associated with metabolic disorders, including:
- Metabolic Syndrome (MetS): A cluster of metabolic disturbances, including obesity, insulin resistance, dyslipidaemia, and hypertension.
- Type-2 Diabetes: A common metabolic disorder characterized by chronic hyperglycaemia and is associated with obesity.
- Non-Alcoholic Fatty Liver Disease (NAFLD): A disease state in which lipid accumulates in the liver in the absence of excessive alcohol consumption.
Obesity and the chronic disorders of MetS, type-2 diabetes, and NAFLD are associated with chronic and systemic low-grade inflammation.
Potential Anti-Obesogenic Mechanisms of UVR
While the precise mechanisms by which UVR might influence weight and metabolic health are still under investigation, several potential pathways have been identified:
- Vitamin D Production: UVR stimulates vitamin D synthesis in the skin, which may play a role in regulating metabolism and reducing inflammation.
- Nitric Oxide Release: UVA radiation may induce the release of nitric oxide from stores in the skin, which can improve blood pressure and potentially enhance insulin sensitivity.
- Immune Modulation: UVR has been shown to suppress immunity and may prevent the development and limit the severity of immune-driven diseases.
- Regulation of Appetite Hormones: As demonstrated in the Journal of Investigative Dermatology study, UV exposure can influence the levels of appetite-regulating hormones like leptin and norepinephrine.
- Browning of White Adipose Tissue: UV exposure may promote the conversion of white adipose tissue (which stores energy) into brown adipose tissue (which burns energy), thereby increasing energy expenditure.