Lack of vitamin D can cause weak bones that break easily, bone pain, and muscle weakness, and may increase risk for diabetes, high blood pressure, certain cancers, nerve damage and autoimmune conditions such as multiple sclerosis. However, taking very high doses (>3000 IU/day) of vitamin D can harm you (Laboratory Medicine, 2018;49(2):123-129). Some of the conditions thought to be caused by lack of vitamin D may not be cured by taking vitamin D 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). See my report on Sunlight: More than Vitamin D

How to Tell if You Are Low on Vitamin D
The only dependable blood test to check if you are deficient in vitamin D is a test for 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. The current consensus is to get your blood level of hydroxy vitamin D above 20 ng/ml. Previous recommendations for hydroxy vitamin D as high as 50-80 ng/ml are not supported by adequate research. Recent studies suggest that 30 ng/ml is more than adequate and most experts feel that any level above 20 ng/ml is normal (Osteoporosis International, February 2017;28(2):505-515). Most of the studies extolling the benefits of vitamin D pills for various diseases show association, not cause and effect, and thus are likely to be due to chance (BMJ, Apr 1, 2014;348:g2035). There is no evidence anywhere that people with hydroxy vitamin D levels above 30 are healthier than those between 20 and 30. If “normal” blood levels of hydroxy vitamin D were set higher than 20 ng/ml, more than 80 percent of North Americans would probably be misdiagnosed as having low blood vitamin D levels.

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). 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 is 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 avoid sunlight.

How Much Vitamin D Should You Take if You Are Deficient?
There is disagreement on how much vitamin D healthy adults should take. Recommendations run between 200 IU to 2000 IU per day. In the U.S., the Institutes of Medicine recommend 600-800 IU per day for adults, while the Endocrine Society states that optimal vitamin D status may require 1500-2000 IU per day. In the winter, people have a reduced ability to make vitamin D when they go outside, so amounts of at least 600 IU per day of vitamin D from supplements or foods would help to maintain vitamin D status at summer levels.

The major expected benefit of vitamin D is to help keep bones strong, but raising blood levels of hydroxy vitamin D from 20 to 30 ng/ml with high doses of vitamin D pills increased calcium absorption by only one percent and did not increase bone mineral density or physical function, when compared with placebo (Curr Opin Endocrinol Diabetes Obes, Dec 2016;23(6):440-444). Higher blood levels of vitamin D (greater than 20 ng/ml) do not make bones stronger than lower blood levels as they do not reduce levels of parathyroid hormone or bone resorption (Curr Rheumatol Rep, June 2011;13(3):257-64). Large doses (4000 IU/day) of vitamin D did not slow declining physical function in sedentary men over 70 (Journal of the American Geriatrics Society, 11/22/2016).

Too Much Vitamin D Can Harm You
High vitamin D levels have been reported to be associated with increased risk for skin, prostate and blood cancers (American Association for Cancer Research meeting in Chicago, April 16, 2018) and high blood calcium levels that increase risk for kidney stones (Lab Med, 2018;49(2):123-129). I have never seen vitamin D toxicity from too much sunlight or from foods that contain vitamin D. Most experts recommend no more than 4,000 IU per day of vitamin D. Calcium pills, which many people take with vitamin D supplements, can harm you by increasing risk for heart attacks and kidney stones (BMJ, Sept 29, 2015;351:h4580).

Avoiding Skin Cancers from Too Much Sunlight
Excessive sunlight exposure causes skin cancers, so any recommendation for increasing sunlight exposure must be accompanied by precautions:
• The people who are most susceptible to skin cancer are those with the lightest skin pigments, so they should limit their exposure to sunlight. On light skin, it takes only a few minutes of exposure of a small area to reap the benefits of sunlight.
• People with dark skin may be at increased risk for diabetes and heart attacks specifically because of their skin color and they may need, and can safely tolerate, larger doses of sunlight exposure.
• 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.

My Recommendations
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 can take up to 1000 IU/day of vitamin D pills if you wish.

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 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. 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
• are athletes with recurrent injuries and decreased performance
• have weak bones, called osteoporosis
• are diabetic, particularly if their harmful 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