As spring takes hold and the sun beckons us outside, warnings about the dangers of sun exposure are sure to follow.
Over the past few decades, people have become increasingly aware of the risks associated with excessive exposure to ultraviolet (UV) radiation from sunlight, including skin cancer, photoaging, and cataracts.
However, appropriate sun exposure provides benefits that are crucial for human health. Growing evidence suggests not only that sun exposure helps prevent and treat a variety of diseases but also that insufficient exposure to sunlight may increase the risk of cancer.
Sun Exposure Increases Skin Cancer Risk But May Prevent Other Types of Cancer
Many people associate sun exposure with skin cancer, but research has found that insufficient exposure to UV radiation may also be linked to other types of cancer.Before delving into this topic, we first need to understand the relationship between sunlight, UV radiation, and vitamin D.
Energy from the sun reaches the Earth in the form of visible light, infrared radiation, ultraviolet radiation, radio waves, X-rays, and gamma rays. Ultraviolet radiation is further divided into three types: UVA, UVB, and UVC, of which only UVA and UVB reach the Earth’s surface. UVC is absorbed by the atmosphere.
UVB in sunlight is absorbed by cholesterol in the skin and converted to vitamin D3. Vitamin D3 is then metabolized in the liver to 25-hydroxyvitamin D (also known as calcidiol) and in the kidneys to its
biologically active form (calcitriol).
The only way to determine a person’s vitamin D level is to measure the concentration of 25-hydroxyvitamin D in the blood. This 25-hydroxyvitamin D has a half-life of approximately two weeks and is the major circulating form of vitamin D. This link between sun exposure and vitamin D production appears to be at the heart of sunlight’s anti-cancer effect.
In 2022, a study published in the journal Nutrients found that, based on ecological (community- and population-based) studies of cancer related to solar radiation, sun exposure may decrease the incidence and mortality risks of approximately
23 types of cancer. According to an article in Environmental Health Perspectives, compared with those who live in regions with longer sunlight exposure, people living at high latitudes with less sunlight exposure have an
increased mortality risk from Hodgkin’s lymphoma and colon, breast, ovarian, prostate, and pancreatic cancers, among other kinds.
According to the Nutrients study, meta-analyses of multiple observational studies have shown that there’s a significant association between higher levels of 25-hydroxyvitamin D in the blood and a lower incidence of cancer.
U.S. researchers conducted a meta-analysis of two randomized clinical trials and one prospective cohort and found that among 5,038 women, the group with the highest 25-hydroxyvitamin D concentration (greater than or equal to 60 nanograms/milliliter) had an
82 percent lower incidence of breast cancer than the group with the lowest concentration (less than 20 ng/ml). The same group had an 80 percent lower risk of developing breast cancer than those with the lowest concentration of 25-hydroxyvitamin D, after adjusting for age, body mass index, smoking status, and calcium supplement intake.
Another meta-analysis found that breast cancer patients with the highest concentration of 25-hydroxyvitamin D had a death rate of
approximately half that of those with the lowest concentration. In a
clinical trial, more than 2,300 women aged 55 and above residing in Nebraska consumed 2,000 IU of vitamin D3 (two to four times the daily suggestion) and 1,500 milligrams of calcium daily. The results show that in the subsequent two to four years, people who achieved a 25-hydroxyvitamin D blood concentration of 55 ng/ml had a 35 percent lower risk of developing cancer than those with a concentration of 30 ng/ml.
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meta-analysis of five studies found that individuals with a serum 25-hydroxyvitamin D concentration less than or equal to 12 ng/ml had a 50 percent higher risk of colorectal cancer than those with a concentration of 33 ng/ml or greater.
How Does Sun Exposure Prevent Cancer?
“We believe that vitamin D plays multiple roles in helping to reduce the risk of many deadly cancers,” Michael F. Holick, a professor of pharmacology, physiology, and biophysics at the Boston University Chobanian & Avedisian School of Medicine, told The Epoch Times.
“The active form of vitamin D can prevent cells from becoming cancer cells, and if they do, it can also hinder their ability to receive nutrients and ultimately lead to their death by shutting down angiogenesis,” Holick said.
Active vitamin D3 is a hormone that can
regulate the immune system by acting on various immune cells, according to a 2022 study review in Nutrients.
Low levels of vitamin D are often associated with low-grade
inflammation, as reflected by elevated C-reactive protein levels, which is a significant risk factor for cancer. Meta-analyses such as the 2022 Nutrients review have shown that the level of C-reactive protein is associated with several types of cancer. Cell experiments have demonstrated that active vitamin D3 can inhibit the production of pro-inflammatory cytokines, which may help reduce chronic inflammation.
Small Amounts of UV Radiation Are Beneficial in Fighting Cancer
In addition to the vitamin D generated by UV radiation, the radiation itself can be beneficial. According to a review in
Progress in Biophysics and Molecular Biology, the
protective effects of less-intense solar UV radiation on the human body outweigh the radiation’s potential mutagenic effect.
Animal studies have shown that vitamin D supplementation and exposure to UV radiation can lead to a reduction in the area of
colon tumors in mice.
Sunlight Exposure Regulates Circadian Rhythm and Generates Other Active Substances
Regular exposure to sunlight can regulate circadian rhythm and influence the secretion of various hormones in the human body.
Some studies suggest that melatonin can
inhibit tumor growth and that daytime light exposure enhances the inhibitory effect of nighttime melatonin on the growth of prostate, liver, and breast cancer. The precursor of melatonin, serotonin, is affected by exposure to daylight. It’s usually produced during the day and converted to melatonin only in darkness. Exposure to sunlight in the morning promotes the secretion of serotonin, which in turn
accelerates the production of melatonin at night, according to the Environmental Health Perspectives article.
The increased risk of cancer among night shift workers may be due to their disrupted circadian rhythm, which affects the
production of melatonin.
Exposure to sunlight not only produces vitamin D3 in the skin but also generates
other photoproducts of previtamin D3, which have additional unique biologic activities.
“It generates a whole host of other things,” Holick said.
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systematic review in Dermatoendocrinology specifically indicated that inadequate sun exposure carries many other risks, including increased all-cause mortality, hypertension, cardiovascular disease, metabolic syndrome, Type 2 diabetes, obesity, and Alzheimer’s disease. It can also lead to multiple sclerosis, Type 1 diabetes, psoriasis, rheumatoid arthritis, nonalcoholic fatty liver disease, macular degeneration, statin intolerance, and myopia.
How to Get Sun Exposure Safely and Efficiently
The medical community has always advocated for moderate sun exposure.
According to the Progress in Biophysics and Molecular Biology review, under normal circumstances, about 90 percent of the body’s essential vitamin D is obtained through sun exposure. However, people are now spending much less time in the sun, especially after the COVID-19 pandemic.
According to the National Institutes of Health, a person’s level of 25-hydroxyvitamin D in plasma shouldn’t be lower than 20 ng/ml. Research has found that 32 percent of people in the United States have vitamin D insufficiency, according to the Dermatoendocrinology review. If the Endocrine Society’s standard for vitamin D sufficiency (30 ng/ml) were used, the proportion of people with vitamin D deficiency would be even higher.
It’s estimated that for every 100 IU of vitamin D ingested, the blood level of 25-hydroxyvitamin D increases by only
1 ng/ml (2.5 nmol/l). For most adults, periodic and brief sun exposure can provide sufficient vitamin D, which is more effective than taking 1,000 IU of vitamin D3 daily. Moreover, getting vitamin D through sun exposure prevents the potential toxicity of excessive vitamin D supplementation.
Holick said the 25-hydroxyvitamin D level should reach 75 nmol/l (30 ng/ml), and the ideal range is between 40 and 60 ng/ml, “just like the Maasai herders.” The Maasai are traditionally nomadic people who live in East Africa.
Generally, it’s recommended to expose the arms, legs, and other body parts to the sun for
10 to 15 minutes a few times per week. However, this isn’t an absolute rule, as factors such as season, latitude, weather, time of day, skin pigmentation, clothing, age, sunscreen use, and whether the light is passing through glass can all influence the production of vitamin D3 in the skin.
For example, during winter, sunlight enters the atmosphere at a more oblique angle, and more UVB photons are absorbed by the ozone layer. In areas north of 37 degrees latitude in the Northern Hemisphere from November to February, the decrease in the number of UVB photons reaching the Earth’s surface can range from 80 to 100 percent.
In the morning or evening, the angle of sunlight is so oblique that even in summer, the rate at which vitamin D3 is produced in the skin of individuals living in these regions is very slow.
People with lighter skin may only need a short amount of time in the sun to get enough vitamin D, while those with darker skin may require more time to achieve the same effect. For most light-skinned people, according to the Environmental Health Perspectives article, sunbathing in swimwear for half an hour in the summer sun can initiate the release of 50,000 IU (1.25 mg) of vitamin D into the circulatory system within the following 24 hours; the same sun exposure yields 20,000 to 30,000 IU vitamin D for tanned-skin individuals and 8,000 to 10,000 IU vitamin D in dark-skinned individuals.
According to research from the
University of Geneva, exposing 22 percent of the skin for 10 to 15 minutes to the sun during spring and summer can synthesize 1,000 IU of vitamin D. However, it’s more challenging to obtain sufficient vitamin D from sun exposure alone during autumn and winter because people usually expose only 8 to 10 percent of their skin, which may require 6.5 hours of sun exposure to obtain the same amount of vitamin D.
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study of 2,360 U.S. adults published in Frontiers in 2022 shows that exposure to sunlight for about 35 minutes between 9 a.m. and 5 p.m. every day, without deliberately sunbathing at noon, can result in ideal serum vitamin D levels (less than 50 nmol/l).
A software that Holick helped develop, “D Minder,” instantly calculates how much sun exposure is needed in a given area to obtain sufficient vitamin D.
Holick also recommended protecting the face, which is the most susceptible to sunburn, while sunbathing; exposing the arms, legs, abdomen, and back to the sun for a reasonable and shorter duration without using sunblock can generate a significant amount of vitamin D.
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