Vitamin D is a fat-soluble vitamin that is unique in humans because it acts as a prohormone, which is a precursor to a hormone, and it is synthesized during exposure to the sun on our skin. This makes vitamin D one of the few vitamins our body needs that can be produced by our body independent of our diet. Vitamin D is synthesized in the kidneys in the form of calcitriol, before it is released into the body as a hormone. It helps regulate the concentration of calcium and phosphate in the blood stream and control the ratio of these important minerals in our body. This promotes healthy growth and remodeling in our bones.
Vitamin D plays a large role in keeping us healthy. It helps prevent rickets in children and the onset of osteomalacia in adults. It also helps maintain healthy bones, especially if combined with calcium, and helps reduce the risk of osteoporosis later in life. Although most people associate Vitamin D with healthy bones, it also contributes to the function of our neuromuscular and immune systems as well as inflammatiion and cell growth (1,2,3). It also plays a role in modulating genes in the body that regulate cell proliferation, differentation and apoptosis (1).
The recommended dietary intake is about 500 IU daily in a healthy adult for optimal bone and overall health. However, it is often recommended to take up to 5,000 or even 10,000 IU daily, especially if you are above the age of 70 or have a defeciancy. Pregnant women are also advised to increase their Vitamin D intake. Getting regular sunlight is the simplest way to increase your Vitamin D levels, however, many people don’t get sufficient sun, especially if they live in a colder climate or during the winter (you’ll notice that winter is usually associated with cold and flu season). Fortunately, during the winter months or for those living in colder climates, there are plenty of food sources out there that can help support our Vitamin D intake.
It is common in some countries, including the U.S., to come across food that has been artificially fortified with Vitamin D (4). Fatty fish, such as catfish, salmon, mackerel, sardines, eel and tuna, contain significant amounts of Vitamin D. Other food sources are also rich in Vitamin D, such as: whole eggs, beef liver, fish liver oils – including cod liver oil, mushrooms and yeast that have been grown under UV light.
Some Food Sources of Vitamin D
| Food | IU’s per serving |
| Cod liver oil, 1 tablespoon |
1,360 |
| Swordfish, cooked, 3 ounces |
566 |
| Salmon (sockeye), cooked, 3 ounces |
447 |
| Tuna fish, canned in water, drained, 3 ounces |
154 |
| Orange juice fortified with vitamin D, 1 cup (check product labels, as amount of added vitamin D varies) |
137 |
| Milk, nonfat, reduced fat, and whole, vitamin D-fortified, 1 cup |
115-124 |
| Yogurt, fortified with 20% of the DV for vitamin D, 6 ounces (more heavily fortified yogurts provide more of the DV) |
80 |
| Margarine, fortified, 1 tablespoon |
60 |
| Sardines, canned in oil, drained, 2 sardines |
46 |
| Liver, beef, cooked, 3 ounces |
42 |
| Egg, 1 large (vitamin D is found in yolk) |
41 |
| Ready-to-eat cereal, fortified with 10% of the DV for vitamin D, 0.75-1 cup (more heavily fortified cereals might provide more of the DV) |
40 |
| Cheese, Swiss, 1 ounce |
6 |
It is important to maintain a healthy balanced diet rich in Vitamin D but also to get enough sunlight when the weather is nice enough to do so. Vitamin D is not only important for a healthy skeletal system, but also your immune system (5). It may also reduce the risk against cancer (6) and may even reduce the overall mortality risks from any cause (7,8).
References:
1. Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academy Press, 2010.
2. Holick MF. Vitamin D. In: Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, eds. Modern Nutrition in Health and Disease, 10th ed. Philadelphia: Lippincott Williams & Wilkins, 2006.
3. Norman AW, Henry HH. Vitamin D. In: Bowman BA, Russell RM, eds. Present Knowledge in Nutrition, 9th ed. Washington DC: ILSI Press, 2006.
4. DRI, Dietary reference intakes: for calcium, phosphorus, magnesium, vitamin D, and fluoride. Washington, D.C: National Academy Press. 1997. p. 250. ISBN 0-309-06350-7. Nutrition
5. http://web.archive.org/web/20080419071840/http://www.sciencenews.org/articles/20061111/ bob9.asp Accessed October 2011
6. Ingraham, BA; Bragdon, B; Nohe, A (January 2008). “Molecular basis of the potential of vitamin D to prevent cancer”. Current Medical Research and Opinion 24 (1): 139–49.
7. Autier P, Gandini S. Vitamin D supplementation and total mortality: a meta-analysis of randomized controlled trials. Arch Intern Med 2007;167:1730-7.
8. Giovannucci E. Can vitamin D reduce total mortality? Arch Intern Med 2007;167:1709-10.
