When summer comes, sunbathers flock to beaches, parks and poolsides to bask in the sun's rays. Sure, sunbathing tends to make you sweat, which most of us associate with burning calories. But you might be wondering: Does lying prone in the sunshine actually have any weight-loss benefits? The answer is more than skin deep.
Heat Regulation and Metabolism
Humans, like all mammals, are endothermic or warm-blooded, meaning that they regulate their own body temperature through internal mechanisms. When the body senses that it is in an environment that is too hot or too cold, it initiates measures to regulate its own temperature, such as shivering in cold environments or sweating in warm ones. Other physical responses to warm temperatures include relaxing the muscles, lowering production of adrenaline and lowering the rate at which metabolism occurs.
Metabolism is the process by which the body turns food into energy and supplies that energy to cells. This energy can either come from a meal or from reserves stored in the body as fat. The amount of energy needed depends on the body's level of physical activity. But a body at rest still consumes a minimum level of energy to maintain basic bodily functions; this is known as the basal metabolic rate, or BMR.
When the body is warm and at rest, the BMR drops to a low level, meaning that the body is consuming fewer calories to keep itself functioning. Since sunbathing involves lying motionless in the sun, it consumes very few calories. Sunbathing can only be said to burn calories in the general sense that doing anything at all burns at least some calories. For example, the body uses calories, although at a low rate, even while asleep.
A Potential Fat-Burning Benefit
While dosing yourself with sunshine doesn't up your calorie churn, there is some scientific evidence that sunlight helps you burn more fat. Research published in Scientific Reports in November 2017 concluded that the sun's blue light wavelengths help shrink white fat cells located just beneath the skin's surface.
The discovery was accidental. Researchers at the University of Alberta in Canada set out to help those with type 1 diabetes by attempting to genetically engineer these fat cells to produce insulin when exposed to light. Along the way, they were surprised to note that sun exposure actually shrunk the lipid droplets in the fat cells under the skin and caused these cells to store less fat.
However, researchers also noted that more studies need to be done in this area, and that they don't yet know the intensity or duration of sunlight needed to activate this process. The takeaway: Don't swap your gym membership for a beach badge just yet.
Sunbathing and Other Health Factors
Beyond weight loss, there are health benefits to sunbathing. Sunlight helps the body make vitamin D, an important nutrient that helps the body absorb calcium and supports healthy bones and joints.
Some research, including an October 2013 study in the International Journal of Epidemiology, has also linked vitamin D with better heart health. And there's some evidence that sunlight exposure may lower your risk for certain types of cancer: A February 2014 study published in Environmental Health Perspectives found that it lowered breast cancer risk, while research published in April 2015 in the Journal of Steroid Biochemistry and Molecular Biology linked weak sunlight to a higher risk for pancreatic cancer.
However, prolonged exposure to the sun's ultraviolet rays also carries health risks, including increasing the likelihood of skin cancer. To help lower this risk, the American Cancer Society recommends generously applying a broad spectrum sunscreen (which protects against both UVA and UVB rays) with SPF of 30 or higher whenever you plan to be in the sun, and reapplying at least every two hours.
- Palomar College: "Human Biological Adaptability: Adapting to Climate Extremes"
- The Skin Cancer Foundation: "The Dangers of Tanning"
- Mayo Clinic: "Metabolism and weight loss: How you burn calories"
- Scientific Reports: "Subcutaneous white adipocytes express a light sensitive signaling pathway mediated via a melanopsin/TRPC channel axis"
- Harvard Health: "6 Things You Should Know About Vitamin D"
- UC San Diego Health: "Pancreatic Cancer Risk Linked to Weak Sunlight"
- Environmental Health Perspectives: "Sun Exposure, Vitamin D Receptor Genetic Variants, and Risk of Breast Cancer in the Agricultural Health Study"
- American Cancer Society: "Don't Fry: Preventing Skin Cancer"