Stay slim with sun exposure? New, never considered research adds a reason to stay active in the sun: The blue-light spectrum of sunlight, a spectrum that can penetrate the skin, can cause subdermal fat tissue to decrease in size. In other words, it can cause fat loss. Thus, the action of sunlight may help one to stay slim or become slim. The researchers showed that daily exposure of differentiated adipocytes [fat cells] to blue light resulted in decreased lipid droplet size and increased basal lipolytic [fat breakdown] rate.
The researchers had been doing research on light and diabetes, and they serendipitously found that the light could be an asset in maintaining (or producing) a slim body. But there are many other studies that show sun exposure is capable of assisting the body in being slim. For example: here is another benefit of sun exposure—morning sun specifically: A recent study from Northwestern Medicine demonstrates that timing and intensity of light correlate with body mass index (BMI). BMI is a numerical computation comparing height and weight, and is a commonly used method to assess obesity or the lack thereof. A high BMI usually means a person is obese or at least approaching obesity, while optimal BMI is 18-25. Below 18 is considered underweight, above 25 is overweight, 30 is obese and 40 and above is morbidly obese. However, BMI does not work for heavily-muscled people, who may have minimal fat, but whose BMI puts them in an obese category—in reality, they are very slim.
This study showed that exposure to bright morning light was directly related to BMI. After adjusting for confounders such as diet, exercise and timing of sleep, it was determined that very early exposure to morning light correlated remarkably to lower BMI—they were slim, or at least slimmer. Even when light intensity was equal at different times of the day, those who received the earliest bright light had lower BMI. Most noteworthy was the fact that for each hour later in the day when light exposure occurred, BMI increased by 1.3 units. This fact is especially relevant, since a person who has a BMI of 25 (upper ideal range) could approach 30 (obesity), over time, due to the habit of receiving sun exposure later in the day, e.g. 10:00 AM rather than 6:00 AM.
The authors suggested that the mechanisms by which early light exposure could influence the “slim” mechanisms, could be the following: (1) resetting the circadian rhythm (internal clock), (2) the greater quantity of blue light in morning sun and (3) effects on melatonin production. Whatever the mechanisms, we now know that early-morning sun is important to being slim. In addition, it may also be important to other health issues. Rather than think of sun exposure as the cure-all for obesity, we must realize that poor nutritional habits and lack of exercise are much more important. Nevertheless, sun exposure can furnish one more arrow in the quiver.
Sun exposure is far superior to vitamin D supplements in preventing weight gain.
Another scientific paper “sheds more light” on the subject of being slim. This research was conducted on mice with shaved backs that were placed on a high-fat diet and then exposed to non-burning ultraviolet radiation (UVR) during a three-month experiment. The mice, without the benefit of UVR, would have been expected to gain weight rapidly, but when they were exposed to UVR, the weight gain was impressively reduced. Furthermore, the UVR treatment achieved 30-40% less weight gain, compared to the expected weight gain with the high-fat diet. So, not only can sun exposure produce slim humans, it can help to produce slim rats.
Other benefits for the rats included: significant reductions in glucose intolerance, insulin resistance and fasting insulin levels (all markers and predictors of diabetes), nonalcoholic fatty liver disease measures and cholesterol. All of these factors, including obesity, are part of a cluster of maladies known as the metabolic syndrome, or MetS, which is indicative of deteriorating health and susceptibility to heart disease and diabetes. Finally, almost all people who have the aforementioned indications of MetS have a large problem maintaining slim bodies. Not a surprise, eh?
Other interesting findings:
Supplementation with vitamin D actually reduced the aforementioned beneficial effects. Dr. Shelley Gorman, one of the authors, made the following three interesting observations:
- “These findings were independent of circulating vitamin D and could not be mimicked by vitamin D supplementation.”
- “It looked like the presence of vitamin D in mice on the high fat diet prevented the [beneficial] effect of UV radiation on weight gain.”
- She also mentioned the mechanism of weight loss may be dependent on nitric oxide (NO), which originates from diet and can be mobilized by UV radiation to become bioactive. This was due to the fact that in another part of the experiment, skin induction of nitric oxide (NO)—also a product of skin exposure to sun—reproduced many of the positive effects of UVR, something vitamin D supplements could not do.
The Authors conclusions:
The authors concluded their research thusly: These studies suggest UVR (sun exposure) may be an effective means of suppressing the development of obesity and MetS, through mechanisms independent of vitamin D but dependent on other UVR-induced mediators such as NO.”
It has been suggested that since low 25(OH)D levels correlate to obesity, those low levels are a possible cause of obesity. However, vitamin D is stored in fat tissue. Therefore, an increase in fat will lower the quantity of 25(OH)D circulating in the blood. Hence, low vitamin D is a consequence of obesity, and not the cause.
Research continues to mount about the positive effects of sun exposure, independent of vitamin D production. This should in no way be construed to diminish the vital importance of vitamin D. Rather, it is to make a point that sun exposure works in many ways, including stimulating the production of vitamin D, NO, serotonin and endorphins. Why should we be satisfied with any one of these marvelous health aids when the sun is available? Because, with sun exposure, we can enjoy the benefits of the entire package.
So, being slim is dependent on a series of choices: avoiding junk food, eating large quantities of vegetables and fruits, taking a daily walk or engaging in other aerobic exercise, weight training and, finally, soaking up some daily, non-burning sunlight.
In conclusion: Stay slim, my friends.
 Katarina Ondrusova, Mohammad Fatehi , Amy Barr, Zofa Czarnecka, Wentong Long, Kunimasa Suzuki, Scott Campbell, Koenraad Philippaert, Matthew Hubert, Edward Tredget, Peter Kwan, Nicolas Touret, Martin Wabitsch, Kevin Y. Lee & Peter E. Light. Subcutaneous white adipocytes express a light sensitive signaling pathway mediated via a melanopsin/TRPC channel axis. Scientific Reports November 27;7:16332
 Reid KJ, Santostasi G, Baron KG, Wilson J, Kang J, Zee PC. Timing and intensity of light correlate with body weight in adults. PLoS One 2014;2;9(4)
 Geldenhuys S, Hart PH, Endersby R, Jacoby P, Feelisch M, Weller RB, Matthews V, Gorman S. Ultraviolet radiation suppresses obesity and symptoms of metabolic syndrome independently of vitamin D in mice fed a high-fat diet. Diabetes. 2014 Nov;63(11):3759-69
 Dr. Shelly Gorman, quoted on Science Network Australia article: Sun shines light on obesity challenge. http://www.sciencewa.net.au/topics/health-a-medicine/item/3618-sun-shines-light-on-obesity-challenge (accessed February 4, 2016)
 Cândido FG, Bressan J. Vitamin D: link between osteoporosis, obesity, and diabetes? Int J Mol Sci. 2014 Apr 17;15(4):6569-9.