Forty-two percent of North Americans have vitamin D levels below 20 ng/mL, which makes them deficient by most standards (Nutr Res, 2011 Jan;31(1):48-54). Vitamin D deficiency is associated with increased risk for developing many diseases (Aging Dis, May 2017;8(3):346–353), including:
• osteoporosis, broken bones and falls
• kidney disease and kidney stones
• fatty liver and other chronic liver diseases
• heart disease and high blood pressure
• Type II diabetes
• severe infections such as COVID-19 and hepatitis C
• multiple sclerosis
• psoriasis
• osteoarthritis
• autoimmune diseases
• colon, prostate and breast cancers
• muscle pain and weakness
• ulcerative colitis or Crohn’s disease
• depression and other psychiatric disorders

Are You Deficient?
The only dependable blood test to check if you are deficient in vitamin D is hydroxy vitamin D. Measuring dihydroxy vitamin D, the active form of vitamin D, is not dependable because its levels do not drop until you have almost no vitamin D left in your body. Most experts feel that any level of hydroxy vitamin D above 20 ng/mL is normal (Osteoporosis Internat, February 2017;28(2):505–515). Several studies show that 30 ng/mL may offer better protection against disease (Dermatoendocrinol, Jan 1, 2012;4(1):72–80). Some experts recommend levels above 30 ng/mL but recent studies suggest that 30 ng/mL is more than adequate. There is little evidence that people with hydroxy vitamin D levels above 30 ng/mL are healthier than those between 20 and 30 ng/mL.

Sources of Vitamin D
The main source of vitamin D is usually sunlight because ultraviolet rays convert cholesterol in your skin to vitamin D. You cannot get an overdose of vitamin D from sunlight (Am J Clin Nutr, 2004; 79: 362–371) since the sunlight also breaks down vitamin D in your skin (Science, 1982; 216: 1001–1003). Foods are a very limited source of vitamin D, so you need to get more than 80 percent of your vitamin D from sunlight or pills (Indian J Endocrinol Metab, 2018;22:249). The best food sources of vitamin D include fatty fish (tuna, mackerel, salmon and so forth), liver, cheese, egg yolks, spinach, kale, okra, collards, soybeans, and various fortified foods, but it would be very difficult to meet your needs for vitamin D from foods. The people most likely to suffer from vitamin D deficiency in the winter are those with darker skin and those who do not get much sunlight.

Sunlight May Offer More Than Vitamin D Pills
Some of the conditions thought to be caused by lack of vitamin D may not be cured by taking vitamin D pills because they actually may be caused by lack of sunlight. For example, 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, and these increased risks have not been shown to be prevented or treated by taking vitamin D pills (J Intern Med, Oct 2016;280(4):375-387). Most of the studies on benefits of vitamin D pills on various diseases show association, not cause and effect (BMJ, Apr 1, 2014;348:g2035), and many of the studies show that risks for these diseases have not been reduced by taking vitamin D pills:
• Dr. JoAnn Manson and her associates at Harvard Medical School showed that giving 2000 IU of vitamin D/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 D for three years did not improve heart attack risk markers for 161 patients in heart failure (Annals of Nutr and Metab, Dec 17, 2018).

Other Benefits from Sunlight
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 become diabetic (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.

Who is Most Likely To Need Vitamin D Pills?
Most researchers and clinicians now feel that, with few exceptions, doses of vitamin D3 greater than 1000 IU/day are not beneficial and are potentially harmful, so you should not take doses higher than that unless your doctor diagnoses a specific reason to do so. People who may benefit from treating low blood levels of hydroxy vitamin D include those who:
• are inactive and do not go outdoors
• suffer from generalized muscle and joint pain
• have osteoporosis (weak bones)
• are diabetic, particularly if their LDL cholesterol is over 100
• have an auto-immune disease
• are very obese
• are critically ill or debilitated
• suffer from muscle pain from taking statin drugs
• have difficulty absorbing food because of diseases such as sprue or irritable bowel syndrome.

Vitamin D Pill Dosage
There is disagreement on how much vitamin D healthy adults should take, with recommendations running from 200 IU to 2000 IU per day. In the winter, many people have a reduced sunshine exposure, so 600-1000 IU per day of vitamin D from supplements or foods would help to maintain vitamin D status at summer levels. Very high doses of vitamin D can cause nausea, constipation, kidney stones, frequent urination, muscle weakness, irregular heartbeats and possibly arteriosclerosis, and a study from Denmark showed that very high blood levels of hydroxy vitamin D above 140 ng/mL are associated with increased risk of premature death (J Clin Endocrinol Metab, Aug, 2012;97(8):2644-52). High-dose vitamin D supplements (70,000 IU/week for 12 weeks) caused a significant increase of a toxic vitamin D metabolite called 24,25[OH] vitamin D and reduced parathyroid levels and decreased body responses to vitamin D itself (Med Sci Sports Exerc, Feb, 2017;49(2):349-356).

My Recommendations
If you are concerned about your vitamin D level, get a blood test for hydroxy vitamin D. All other vitamin D blood tests are not dependable. If your hydroxy vitamin D level is above 20 ng/mL, you are presumed to be normal, unless your doctor is concerned that you have a condition that possibly may benefit from higher levels. Most people can get all the vitamin D that they need by exposing a few inches of skin to sunlight for a few minutes three times a week during the warmer months. I believe that sunlight offers benefits that cannot be obtained just by taking vitamin D pills, but take proper precautions to avoid skin cancer. During the winter months, you may want to take up to 1000 IU/day of vitamin D pills.