When we think about sunlight and health, we usually associate exposure to the sun with negative consequences like sunburn and skin cancer. And it’s true—ultraviolet A and B radiation exposure can cause sunburn, damage to collagen fibers, vitamin A destruction in the skin, accelerated skin aging, as well as increased risk of skin cancer. Too much sun exposure can also lead to cataracts, immunosuppression, and reactivation of latent viruses. But, for all the negatives, there are also positives when it comes to sunlight. In fact, a daily dose of sunshine can actually help fight against a wide range of chronic illnesses.
But, before we get to that, you might be wondering whether the health benefits of sunlight can be obtained through the convenience of vitamin D supplements. Sadly, according to current research, the answer is no. In a review article published in Lancet Diabetes Endocrinology, for instance, researchers noted that although prospective studies support an association between serum levels of 25-hydroxyvitamin D (25[OH]D)—the principal type of vitamin D circulating in the blood—and various types of chronic disease, interventional studies involving vitamin D supplements do not corroborate such findings.
On the bright side, a little fun in the sun can be a free, stress-relieving way to help defend against these ten common chronic conditions:
Type 1 diabetes
Studies have shown a link between adequate vitamin D status earlier on in life and reduced risk of developing type 1 diabetes. In one study, non-obese mice that were predisposed to develop type 1 diabetes demonstrated an 80% decreased risk of developing the disease when they received a daily dietary dose of 1,25(OH)D. This finding was supported by that of another study in children who received 2,000 IU vitamin D daily from 1 year of age onward. Researchers found that these children had an 80% decreased risk of developing type 1 diabetes later in life compared with vitamin D-deficient children, who showed a four-fold increased risk of the disease.
An association exists among metabolic syndrome, risk for type 2 diabetes, and heart disease. Research shows that serum 25(OH)D is inversely associated with blood glucose concentrations and insulin resistance, irrespective of age. Furthermore, other studies have shown that there’s a high prevalence of decreased vitamin D levels in the type 2 diabetes population. But, it’s unclear whether low vitamin D levels are a cause or consequence of type 2 diabetes, as those with the disease often undertake less outdoor exercise (and exercise in general).
Sun exposure may guard against hypertension. According to one investigation, adults with hypertension exposed to full-spectrum UVR (UVA plus UVB) akin to summer sunlight exhibited a 180% decrease in 25(OH)D levels and a mean 6 mmHg drop in systolic and diastolic blood pressure levels—thus returning to levels within normal limits. By contrast, those exposed to UVA akin to winter sunlight experienced no change in either variable. The mechanisms for such effect may involve the inflammatory component of atherosclerosis. Vascular smooth muscle cells harbor a vitamin D receptor and relax when exposed to 1,25(OH)D.
Poor bone health
Unlike diet-sourced essential vitamins, the human body creates vitamin D through dermal exposure to UVB radiation. However, the efficiency of this reaction may be mitigated by a number of factors, such as clothing, sunscreen, body fat, and melanin. Photosynthesis in the skin initially results in the production of vitamin D3, the majority of which is then converted to 25(OH)D, which serves as a biomarker for vitamin D status. This metabolite is converted to 1,25(OH)D in the liver and serves as a hormone similar to steroids.
In the nuclei of intestinal cells, 1,25(OH)D accumulates and facilitates calcium and phosphorus absorption, mediating the exchange of calcium in and out of bones to maintain the metabolism of bone calcium. Without enough vitamin D, bone does not form properly. In children, this deficiency manifests as rickets, with the hallmark finding of bowed legs. In adults, this deficiency manifests as osteoporosis and osteomalacia.
According to one review article, vitamin D produced via photosynthesis exhibits innate immunity to tuberculosis (TB). When activated, the photosynthetic pathway for vitamin D production yields the antimicrobial peptide cathelicidin, which binds to intracellular Mycobacterium tuberculosis, likely leading to bacterial death. In a separate review article, findings suggested that high levels of vitamin D may lead to a 32% reduction in risk of symptoms of active TB.
A wealth of evidence links either the consumption of high levels of vitamin D or UV exposure to decreased risk of multiple sclerosis (MS). Additionally, some researchers have shown that vitamin D may offer certain benefits for people with MS, including reduced frequency and severity of symptoms, improved quality of life, and longer time to progression from relapsing-remitting MS to the secondary-progressive phase. But, the overall evidence for a causal relation isn’t conclusive, and it’s still unclear what levels of vitamin D are optimal for preventing disease.
In a review article published in Dermatoendocrinology, researchers found an inverse relationship between vitamin D levels and breast cancer risk. Specifically, women in the highest quintile of 25(OH)D levels (> 32 ng/mL) exhibited nearly half the breast cancer mortality rate compared with those in the lowest quintile (< 14 ng/mL).
In the same Dermatoendocrinology review article, researchers cited findings from a high-power prospective study involving elderly Americans initially without dementia in support of dementia-vitamin D link. In the study, those with serum 25(OH)D levels < 10 ng/mL had double the risk of developing Alzheimer disease compared with those with serum 25(OH)D levels > 20 ng/mL. Furthermore, those with serum 25(OH)D levels between 10 ng/mL and 20 ng/mL had a 69% increased risk of developing Alzheimer disease compared with those with levels > 20 ng/mL. Associations were similar with all-cause dementia.
Many observational studies have shown that exposure to UVR mitigates the development of non-alcoholic fatty liver disease by both vitamin D dependent and independent mechanisms. However, vitamin D supplementation does not result in the same benefits. This suggests that a mediator produced from sunlight exposure other than vitamin D may be responsible for decreasing non-alcoholic fatty liver disease risk.
Age-related macular degeneration
Studies have shown reduced risk of age-related macular degeneration in those with high levels of 25(OH)D. For instance, researchers of one study cited a 6.7-fold increased risk of age-related macular degeneration (AMD) in women with serum 25(OH)D levels < 12 ng/mL and family history of AMD.
Vitamin D and chronic disease
Finally, although a myriad of associations between low circulating vitamin D levels and chronic disease have been observed, some experts have debated whether low vitamin D is a cause or consequence of disease.
“25OHD deficiency predominately affects the occurrence and development of many chronic diseases. With an aging population, the numbers of patients with osteoporosis, cardiovascular diseases, cancers, diabetes and other chronic diseases dramatically increase. Similarly, the incidence of neurological and psychiatric diseases is also increasing year by year. These outcomes result in the loss of life of the population, reduced quality of life and significant social and economic burdens. Since these chronic diseases lack specific treatment or the treatment effects are not curative, strategies for the control of chronic diseases should focus on the prevention. Vitamin D deficiency is associated with chronic disease. Hence, it is challenging to clarify whether vitamin D deficiency is the cause or only the consequence of various chronic diseases. We need to continue to study Vitamin D,” wrote the authors of a review published in Aging and Disease.