For many years I have offered my opinion that sunlight provides benefits that are not gained just from taking vitamin D pills. Recent research is confirming that opinion, and many scientists now believe that low vitamin D blood levels are only a marker for not getting enough sunlight.
People who get little sunlight are at increased risk for heart attacks, strokes, several types of cancers, infections, bone fractures, diabetes, obesity, depression, auto-immune diseases and other diseases that can cause premature death (J Intern Med, Oct 2016;280(4):375-87), yet the increased risks for these diseases are not reduced by taking vitamin D pills:
• Dr. JoAnn Manson and her associates at Harvard Medical School showed that giving 2000 IU of vitamin D3/day and one gram/day of omega-3 fatty acids for five years to 25,871 U.S. men and women 50 years of age or older did nothing to reduce deaths from heart attacks, strokes, or cancers of the breast, prostate, colon or rectum (N Engl J Med, Jan 3, 2019;380:33-44).
• A study that followed 217,244 Danish people for up to 10 years found that vitamin D did not protect them from developing cancers. Higher blood levels of hydroxy vitamin D were not associated with protection from breast, colo-rectal, urinary, ovarian or uterine cancers, and were associated with increased risk for non-melanoma skin cancers, melanoma, prostate and blood cancers, but a reduced incidence of lung cancer (International Journal of Cancer, Jan 9, 2019).
• The most recognized function of vitamin D is to strengthen bones, yet taking up to 48,000 IU of vitamin D per month for a year did not cause any increase in bone density in 379 men and women over 70 (The American Journal of Clinical Nutrition, Jan 8, 2019).
• Daily supplementation with 4000 IU of vitamin D3 for three years did not improve heart attack risk markers for 161 patients in heart failure (Annals of Nutr and Metab, Dec 17, 2018).
• Many other major controlled studies have also shown lack of benefit from vitamin D pills. A review of 290 prospective studies and 172 randomized trials of the effects of vitamin D pills on disease and disease markers in people aged 18 years or older showed that higher blood levels of hydroxy vitamin D are not associated with a lowered risk for diseases including cancers and that giving higher doses of vitamin D (>20 mg/ or >800 IU/day) to people with lower blood levels of vitamin D did not significantly reduce their chances of developing these diseases (The Lancet Diabetes and Endocrinology, Jan 1, 2014;2(1):76-89). See my recent report, Do You Need Vitamin D Pills?
Other Functions of Sunlight
Certainly sunlight causes the skin to manufacture vitamin D, but it also causes the skin to make large amounts of nitric oxide (J Invest Dermatol, Jul 2014;134(7):1839-1846). This nitric oxide can lower or prevent high blood pressure (Curr Opin Nephrol Hypertens, Jan 2016;25(1):11-15). Exposing the skin to 30 minutes of summer sunlight caused nitric oxide levels to go up and high blood pressure to go down. High blood pressure leads to heart attacks, strokes, and arteriosclerosis, so it may be more important to worry about getting enough sunlight even if sun exposure increases the number of skin cancers. Heart attacks kill 100 times as many North Americans each year as skin cancer does.
People who get ample sunlight are less likely to suffer diabetes (Diabetes Res Clin Pract, Oct 2010;90(1):109-14). A study of sunlight exposure in mice showed that it prevented obesity and metabolic syndrome independent of vitamin D in mice fed a high-fat diet (Diabetes, Nov 2014;63(11):3759-69). Another study in mice showed that ultraviolet light suppressed weight gain, glucose intolerance and insulin resistance, and lowered blood levels of fasting insulin, glucose and cholesterol, and that vitamin D pills did not produce these benefits (Int J Environ Res Public Health, Oct 2016;13(10):999).
Exposure to sunlight also helps to prevent infections including tuberculosis (PLoS ONE, 2013;8:999), increases cancer survival (Int. J. Cancer, 2006;119:1530-1536), and lowers cholesterol (QJM, Aug 1996;89(8):579-89). Sunlight converts squalene in the skin to both cholesterol and vitamin D, so sunlight may lower cholesterol by shifting squalene conversion to vitamin D, rather than to cholesterol.
Avoiding Skin Cancers
It is firmly established that excessive sunlight exposure causes skin cancers, so any recommendation for increasing sunlight exposure must be accompanied by an extensive list of precautions.
• The people who are most susceptible to skin cancer are those with the lightest skin pigments. It is likely that African Americans are at increased risk for diabetes and heart attacks specifically because of their darker skins and they may need, and can safely tolerate, larger doses of sunlight exposure.
• People who have very light colored skin are at high risk for skin cancers from sunlight and should limit their exposure to sunlight. It takes only a few minutes of exposure of a small area of skin to reap the benefits of sunlight.
• Protect the areas of your skin that have received the most exposure over your lifetime. For most people this will be the face and lower arms. Wear a hat and long sleeves, or use sunscreens to protect these areas. Target your sun exposure to less-damaged areas such as your legs and expose them only for short periods of time. Use clothing and/or sunscreens for any activity that keeps you in the sun for more than about 15 minutes.
Cautions About Sunscreens
The most effective protection for your skin is a roof, next are clothes and the least effective are sunscreens. Anything that rubs your skin or causes heavy sweating can remove enough of your sunscreen to lose its protection. Apply enough sunscreen so you can see it on your skin, and re-apply often.
Several common sunscreen ingredients may be more harmful than beneficial. For example, oxybenzone is an endocrine disrupter (Environ Health Perspect, 2001;109(3):239-44) that may increase cancer risk (J Cosmet Dermatol, Feb 2018;17(1):15-19). At this time, micronized zinc oxide and titanium dioxide appear to be safe and effective. For more on sunscreens see: Sunscreens and Sun Protection