Vitamin D and Bone Health

Vitamin D is among the most important nutrients for both prevention and treatment of osteoporosis. If you don't get enough vitamin D, even the most effective drug treatments won’t work as intended.

What Are the Functions of Vitamin D?

Vitamin D works in partnership with the mineral calcium. Vitamin D is required for normal absorption of calcium from the intestines, and for utilization of calcium by the bones and other parts of the body. You can consume plenty of calcium, but without adequate vitamin D, the calcium cannot provide the usual benefit to your bones.

Vitamin D also has many other functions -- not all of which are understood. For example, researchers are even exploring the possibility that vitamin D can help to avoid catching the flu virus.

Sources of Vitamin D

Vitamin D is naturally produced in the skin by a process that could be thought of as photosynthesis. When skin is exposed to sunlight, a type of cholesterol in the body is converted into a form of vitamin D. Consequently, with increased exposure to sunlight, less dietary vitamin D intake is needed.

But it's not always so simple: People with darkly pigmented skin have a built-in sunscreen, so they may require 10 to 50 times as much sunlight exposure as someone with a light complexion to produce the same amount of vitamin D. Also, as we age, the skin becomes less efficient at vitamin D production and requires greater amounts of sun exposure.

Also, too much sun exposure can increase skin aging along with the risk of developing of skin cancer. Consequently, the American Academy of Dermatology strongly recommends that we obtain vitamin D from foods and supplements rather than exposure to UV light from the sun or tanning beds.

Vitamin D is not widely distributed in foods. The major food sources are fortified milk and various types of fish. United States law requires fresh milk to be fortified with vitamin D. Yogurt, however, is not required to be fortified. Some brands do add vitamin D. Check the label.

Vitamin D Amounts in Common Food Sources

Vitamin D Fortified* Cow's Milk (amount per cup; 237 mL)

• Fresh Milk, all types: 100 IU
• Chocolate Milk, all types: 100 IU

*based on U.S. fortification levels

Fresh Raw Fish (amount per 3 oz; 85 grams)

• Catfish, channel: 425 IU
• Cod: 40 IU
• Herring: 1,380 IU

Canned Fish (amount per 3 oz; 85 grams)

• Salmon: 200 to 530 IU
• Tuna: 200 IU

Recommendations for Vitamin D

So how much do you need? Recommendations for daily vitamin D intake are currently among the hottest topics in nutrition science. The most recent recommendations in the United States were established in 1997 by the Food and Nutrition Board of the Institute of Medicine. They set these values for normal healthy individuals with limited or uncertain sun exposure.

Institute of Medicine Recommendations for Consumption of Vitamin D

Adequate Intake Values

• Birth to Age 50: 200 IU/day
• Age 51 to 70: 400 IU/day
• Over 70 Years: 600 IU/day

Tolerable Upper Intake Levels

• Birth through age 12 months: 1000 IU/day
• Over 12 months of age: 2000 IU/day

For label reading on foods and supplements, when the label indicates 100 percent of the daily value, that means that it contains 400 IU (10 micrograms) of vitamin D.

Vitamin D Toxicity is Possible

Excess vitamin D can cause dangerously high levels of calcium, leading to confusion, vomiting and muscle weakness. Therefore caution is called for when considering what is a reasonable and safe dose. Since vitamin D is a fat-soluble vitamin, the body can store it in fat tissues for future use and the toxic effects of high dosage don’t occur until after high levels accumulate in the body. Consequently, the toxic effects of excess intake can take months or years to occur, depending on the dosage.

To get some perspective, note that when the average young to middle-aged person wearing a bathing suit is exposed to the amount of sunlight that causes a minimum redness to the skin (called one minimal erythemal dose), the vitamin D produced in the body is equivalent to taking an oral dose of vitamin D between 10,000 and 25,000 IU. If these amounts were taken from oral supplements over an extended period of time, toxic effects might eventually take place. Fortunately, however, the body has mechanisms to prevent continual excess production of vitamin D from sun exposure.

The Bottom Line on Vitamin D Needs

Currently, official recommendations for vitamin D intake are considered to be too low by top researchers in bone health. However, until these recommendations are revised, health care professionals will be hesitant to recommend a daily value that exceeds the official recommendations.

Discuss your vitamin D intake with your physician or dietitian before making radical changes.

Sources

Cannell John J. and Reinhold Vieth, et al. “Epidemic Influenza and Vitamin D.” Epidemiology and Infection 134 (2006): 1129-40.

Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride (1997), Food and Nutrition Board, Institute of Medicine, National Academy of Sciences.

Heaney, Robert P. “Barriers to Optimizing Vitamin D3 Intake for the Elderly.” Journal of Nutrition 136 (2006):1123-5.

Heaney, Robert P. “The Vitamin D Requirement in Health and Disease.” The Journal of Steroid Biochemistry & Molecular Biology 97 (2005):13-9.

Holick, Michael F. “Vitamin D: The Underappreciated D-lightful Hormone that Is Important for Skeletal and Cellular Health.” Current Opinion in Endocrinology & Diabetes 9 (2002): 87-98.

Steingrimsdottir, Laufey, et al. “Relationship Between Serum Parathyroid Hormone Levels, Vitamin D Sufficiency, and Calcium Intake.” Journal of the American Medical Association 294 (2005): 2336-41.

USDA Nutrient Data Laboratory Nutrient Database