If you suffer muscle or tendon injuries, particularly during the winter or early spring, ask your doctor to order a blood test for hydroxy vitamin D. If it is below 30 ng/mL, you probably need more exposure to sunlight or you need to take vitamin D pills.   A review of studies on sports injuries found that lack of vitamin D can be a major cause of recurrent winter-time injuries in athletes and exercisers (Physical Activity and Nutrition, June 30, 2021;25(2):20-25).  In a study of 214 prospective National Football League players, 86 percent of those who missed college games because of lower leg injuries were vitamin D deficient, and 73 percent of those who were deficient in vitamin D had severe lower leg injuries compared to only 40 percent of those who were not deficient in vitamin D (Arthroscopy, Dec. 21, 2017). Low levels of vitamin D can reduce strength and endurance (Sci Sports Exerc, 2018;50:2555-64) and increase injury risk (J Sports Med Phys Fitness, 2019;59:693-9).

My Personal Experience
During my competitive running career, I spent more time injured than training because of wintertime muscle injuries in Boston and Washington, DC. Many years later, I found the cause: my wintertime vitamin D3 blood level was below 20 ng/mL (normal is >30 ng/mL).  I now know that people who are genetically susceptible to vitamin D deficiency are the ones most likely to suffer muscle weakness, injuries and poor athletic performance (Lancet, 2010 Jul 17;376(9736): 180–188). I believe that sunlight offers benefits that cannot be obtained from vitamin D pills.

Other Conditions Associated with Vitamin D Deficiency
About 40 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 (Annals of Int Med, Feb 7, 2023)
• 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

No Need to Take More than 1000 IU of Vitamin D
There is no proven benefit from taking doses of vitamin D greater than 1000 IU/day or having very high blood levels (>30ng/mL). Raising blood levels of hydroxy vitamin D from 20 to 30 ng/ml with high doses of vitamin D pills increases calcium absorption by only one percent and does not increase bone mineral density or physical function, compared with placebo (Curr Opin Endocrinol Diabetes Obes, Dec 2016;23(6):440-444). High doses of vitamin D can cause nausea, constipation, kidney stones, frequent urination, muscle weakness, irregular heartbeats and possibly arteriosclerosis. See my report on Vitamin D Research

Confusion about Vitamin D Deficiency
It is difficult for doctors to identify patients with vitamin D deficiency because most commercial laboratories do not offer a test for the active form of vitamin 1,25 dihydroxy vitamin D. Instead, laboratories measure an inactive form called hydroxy vitamin D. The massive doses of vitamin D recommended by some doctors do not raise the active form of vitamin D because your own body tries to protect you from poisoning by breaking down the active form. You cannot develop vitamin D poisoning from too much sunlight because sunlight breaks down vitamin D just as it helps your body to make it, so as you keep exposing your skin to sunlight, blood levels reach a certain level and do not rise higher. Massive doses of vitamin D pills such as 150,000 IU every three months fail to raise blood levels of the active 1,25 hydroxy vitamin D (J Adolesc Health, Jul 2015;57(1):19-23).

Many studies show that people with dark skin have lower levels of the inactive form of vitamin D. It’s a different story when doctors measure blood levels of the active form of vitamin D. People with dark skin are able to use a much higher proportion of the active form of vitamin D (N Engl J Med, Nov 21, 2013;369(21):1991-2000).

You probably do not need to take vitamin D pills if your blood level of hydroxy vitamin D is above 30 ng/ml unless you have a condition that your doctor feels puts you at increased risk for the signs and symptoms of a deficiency. You can take up to 1000 IU/day of vitamin D pills if your blood levels are below 30 ng/ml. Most researchers and clinicians now feel that, with few exceptions, high doses of vitamin D are not beneficial and are potentially harmful. People who could possibly benefit from treating low blood levels of hydroxy vitamin D (<30 ng/ml) 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 (osteoporosis)
• are diabetic, particularly if LDL cholesterol is over 100
• have an auto-immune disease
• are critically ill or debilitated
• suffer from muscle pain from taking statin drugs

My Recommendations
If you are an exerciser who suffers from muscle pain, injuries or weakness:
• Get your hydroxy vitamin D level checked. That is the only available dependable test. If it is below 30 ng/ml (75 nmol/L), you may be deficient. You can try taking vitamin D3 at a dose of up to 2000 IU/day for a month.

• If that does not bring your hydroxy vitamin D level to normal, you can check with your doctor about taking higher doses. However, taking more than 2000 IU/day is highly controversial.

• Some people will have their hydroxy vitamin D levels above a normal 30 ng/ml and still suffer from muscle weakness, fatigue, pain and injuries, and these people may benefit from exposure to sunlight.

• Since skin cancer is caused by cumulative exposure to sunlight over a lifetime, you should restrict exposure to sunlight on your most-often sun-exposed areas: head, face, top of ears, arms and hands. Try exposing your legs and bathing suit areas, and be careful to avoid sunburn.