A review of studies written since January 2015 shows that you do not benefit from taking high doses of vitamin D or having very high blood levels of that vitamin. The Institute of Medicine recommends that adults need only 600–800 IU of vitamin D per day and that blood levels of hydroxy vitamin D do not need to be higher than 20 ng per milliliter (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).
These reports show that high dose vitamin D pills (greater than 2000 IU/day) are not going to help you and may even harm you (N Engl J Med, Nov 10, 2016;375:1817-1820). The authors estimate that the incidence of vitamin D deficiency in North Americans is less than six percent, yet more than 19 percent of adults take vitamin D pills. Previous reports recommending higher intake from pills and trying to attain higher blood levels of vitamin D were based on the idea that it is difficult to get enough vitamin D from sunshine in the winter months or from foods such as fortified milk or oily fish. Overdosing on vitamin D can cause nausea, constipation, kidney stones, irregular heartbeats and possibly arteriosclerosis.
The major expected benefit of vitamin D is helping to keep bones strong, yet an article this month cites many recent studies showing that 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, when compared with placebo (Curr Opin Endocrinol Diabetes Obes, Dec 2016;23(6):440-444).
Vitamin D for Specific Conditions
Several recent studies recommend vitamin D supplementation for specific populations or symptoms:
• People with generalized muscle and joint pain and blood levels of hydroxy vitamin D below 20 ng/ml (International Journal of Rheumatic Diseases, 11/21/2016)
• Women with postmenopausal osteoporosis (Bone, 11/10/2016)
• People with muscle and joint pain from an auto immune disease called lupus who also have a positive anti-nuclear antibody titer and low blood levels of hydroxy vitamin D (Cancer Epidemiology, Biomarkers & Prevention, 11/17/2016)
• Elderly inactive people who do not go outdoors; 56 studies involving almost 100,000 people over 70 showed that taking 800 IU of vitamin D pills is associated with a slightly prolonged lifespan (Cochrane Database Syst Rev, Jan 2014;10;(1):CD007470)
• Hospitalized intensive-care, critically ill adult patients with hydroxy vitamin D levels below 20 ng/ml (Journal of Critical Care, 11/14/2016)
• People with muscle pain from taking statin drugs to lower cholesterol, who also have blood levels of hydroxy vitamin D below 20 ng/ml (Atherosclerosis, 11/22/2016)
• Diabetics with high blood cholesterol (LDL over 100) and low blood hydroxy vitamin D (<20 ng/ml) (Diabetology & Metabolic Syndrome, 11/22/2016)
• Possibly long-term care residents over 70; may help to prevent respiratory infections (Journal of the American Geriatrics Society, 11/22/2016).
Muscle Weakness, Soreness, Exercise Injuries and Vitamin D
Older studies have shown that vitamin D deficiency is associated with muscle weakness (Scandinavia Journal of Medicine & Science in Sports, October 2009) and increased risk for athletic injuries (Current Opinion in Clinical Nutrition & Metabolic Care, November 2009; Molecular Aspects of Medicine, December 2008). People who are genetically at increased risk for vitamin D deficiency are at increased risk for injuries when they try to exercise vigorously in the winter. A report from the University of Toronto showed that genetic factors cause some people to develop signs and symptoms of severe vitamin D deficiency (Clinical Biochemistry, July 2009).
Lack of vitamin D can weaken muscles (Lancet, Mar 1976;20;1(7960):626-9) and taking that vitamin can correct that weakness (Aging-Clinical and Experimental Research, December 2000;12(6):455–460). Vitamin D acts directly on specific receptors in muscles to make them stronger and to help prevent injuries (Scandinavian Journal of Medicine & Science in Sports, April 2010). As people age, they become increasingly susceptible to muscle weakness and falls caused by lack of vitamin D. Muscles are made of thousands of individual fibers that are classified into two types: slow twitch fibers that govern endurance, and fast twitch fibers that govern primarily strength and speed. Vitamin D specifically helps to maintain the function of the fast twitch strength fibers (Calcif. Tissue Int, 2013;92:151–162). A review of the world's literature showed that lack of vitamin D is associated with muscle weakness in older people (Molecular Aspects of Medicine, June 2005). With aging, you lose muscle fibers. For example, the vastus medialis muscle in the front of the upper leg has 800,000 fibers in a 20 year old, but only 250,000 in a 60 year old. Vitamin D slows this loss of muscle fibers, preserves muscle strength and helps to prevent falls, while lack of vitamin D increases loss of fibers, muscle weakness and falls (Pediatric Clinics of North America, June 2010).
1) You probably do not need to take vitamin D pills if your blood level of hydroxy vitamin D is above 20 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 1000 IU/day of vitamin D pills if your blood levels are below 20 ng/ml.
2) Most researchers and clinicians now feel that, with few exceptions, high doses of vitamin D are not beneficial and are potentially harmful. You should not take doses of vitamin D greater than 1000 IU/day unless your doctor diagnoses a specific reason to do so.
3) People who are most likely to benefit from treating low blood levels of hydroxy vitamin D (<20 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