Vitamin D is essential for the absorption of calcium and building and maintaining strong bones. Individuals who do not get enough vitamin D may develop soft, thin, and brittle bones, a disease called rickets in children and osteomalacia in adults.
Vitamin D is also required for the healthy functioning of muscles, the nervous system, and the immune system.
This essential nutrient may also play a role in the risk of developing many chronic diseases such as diabetes, hypertension, autoimmune disease and some forms of cancer.
How can you make sure you get enough vitamin D?
Vitamin D is unique among the nutrients required for good health because it can be made by the body when our skin is exposed to sunlight. When the ultraviolet – B (UV B) radiation of the sun penetrates our uncovered skin, vitamin D synthesis is triggered.
How much sunlight is too much or too little?
It is hard to say.The amount of sunlight needed to make enough vitamin D depends on a number of factors:
- Time of day: For the sun’s UV B rays to adequately penetrate the skin the sun must be directly over head.
- Your location: Individuals living in more northern latitudes, where the sunlight is less intense, may not synthesize vitamin D in the winter months. But vitamin D synthesized during exposure at other times of the year can be stored in the liver and fat.
- How much of your skin is exposed: The more skin surface area exposed, the shorter the time needed to meet your vitamin D needs.
- How easily you get sun burned: The melanin pigmentation of dark-colored skin prevents UV B light penetration and limits vitamin D synthesis. Darker skin needs more sun exposure, lighter skin less time in the sun is needed to produce vitamin D.
- Factors that block UV rays: Sunscreen, with a SPF of 8 or more, will block the vitamin D – producing UV rays.
According to the National Institutes of Health, less than 30 minutes of sun exposure (without the use of sunscreen) twice a week to arms, legs or back is sufficient to maintain adequate vitamin D levels.
Getting Vitamin D Without the Sun
When sun exposure is limited it is important to include good food sources of vitamin D (e) in the diet or to take a supplement. Fish liver oils are the best sources of vitamin D. Fatty fish, such as salmon, tuna and mackerel are also good sources. Foods of animal origin contain vitamin D3 or cholecalciferol.Most of the vitamin D in our diet comes from fortified milk and breakfast cereals. The milk supply in the United States is fortified with approximately 100 IU vitamin D per 8 ounces. Some brands of breakfast cereal, orange juice, yogurt, margarine and soy beverages contain added vitamin D. Food labels do not list vitamin D unless the food has been fortified.
Vitamin D from planted-based foods is in a different form called vitamin D2 or ergocalciferol. Vegetables are poor sources of vitamin D. Some mushrooms, however, contain vitamin D2. The amount of vitamin D2 in these mushrooms can be increased by exposing them to UV light. A dried ground mushroom powder containing 600 IU of vitamin D per teaspoon is now available and can be added to food to increase dietary vitamin D.
How much dietary vitamin D is needed for good health?
According to the Institute of Medicine1 during the first year of life infants require 400 International Units (IUs) per day. Infant formula is fortified with vitamin D. Since breast milk is not a good source of vitamin D, breast-fed infants require a daily vitamin D supplement.The requirement for vitamin D increases to 600 IU for children one year of age and older and for adults up to 70 years. To maintain healthy bones, the vitamin D requirement goes up to 800 IU in adults 71 years and older.
Over the counter vitamin D supplements are available in a range of doses. Both ergocalciferol and cholecalciferol are effective in raising vitamin D levels. Taking a large amount of vitamin D can be harmful so it is recommended that you check with your health care professional before beginning a vitamin D supplement. The safe upper limit for vitamin D is 4000 IU/day (for children 9 years and older and adults). Vitamin D is also found in most multivitamin supplements and calcium supplements. Be sure to include these sources of vitamin D when calculating the total amount you are taking daily.
Symptoms of vitamin D deficiency in adults include osteomalacia, causing bone pain and muscle weakness. Vitamin D deficiency can be determined by checking the concentration of 25-hydroxyvitamin D in the blood. If a deficiency or insufficient amount is found, a prescription form of vitamin D is required. It takes several months to treat vitamin D deficiency.
Food sources of Vitamin D2
Foods containing Vitamin D and International Units (IUs)/serving
Cod liver oil, 1 Tablespoon 1,350 IUSalmon, sockeye, canned, 3 ounces 715 IU
Dole Portobello Mushroom Powder, 1 teaspoon 600 IU
Mackerel, Atlantic, raw, 3 ounces 547 IU
Swordfish, raw, 3 ounces 474 IU
Sardines, canned, 2 ounces 150 IU
Tuna, light, canned in water, 3 ounces 40 IU
Egg, 1 large (vitamin D found in the yolk) 40 IU
Foods Fortified with Vitamin D
Milk, skim, reduced fat and whole, fortified, 8 ounces 115 - 124 IUYogurt, fortified, 8 ounces 120 IU
Margarine, fortified, 1 Tablespoon 50 IU
Orange Juice, fortified, 1 cup 100 IU
Fortified, Ready to eat cereal ¾ - 1 cup 40 -100 IU
1. Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academy Press, 2010.
2. U.S. Department of Agriculture, Agricultural Research Service, 2011 USDA National Nutrient Database fir Standard Reference, Release 25. Nutrient Data Laboratory Home Page, http://www.ars.usda.gov/ba/bhnrc/ndl
NOTE: While time in the sun is good for vitamin D production, it does increase the risk of developing skin cancer so it is prudent to wear protective clothing and sun screen with an SPF of 8 or more when in the sun for any length of time. Anyone with a history of skin cancer should avoid the sun exposure and protect themselves from UV rays.
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