You need vitamin D for absorbing calcium, building strong bones, reducing inflammation, regulating blood pressure and secreting insulin. It is also important for immune function and cell growth. Having low levels of vitamin D may put you at higher risk for a number of health conditions, including flu, autoimmune diseases, cancer and heart disease. Many people become vitamin D deficient in the winter, and taking vitamin D supplements can help prevent this.
Recommended Intake and Sources
The recommended dietary allowance for adults is 600 international units of vitamin D per day, with most of your vitamin D coming from food sources. However, few foods naturally contain high levels of vitamin D, so you may have to rely mainly on fortified foods, such as milk, breakfast cereals, margarines, yogurt and fruit juices. Fatty fish, egg yolks, cheese, mushrooms and beef liver are natural vitamin D sources. Your body also can synthesize vitamin D from sunlight under some conditions.
Winter and Vitamin D
Your vitamin D levels in the winter depend in large part on genetic factors, while in the summer they are determined more by environmental factors, according to a study published in October 2010 in the "American Journal of Clinical Nutrition." You are more likely to be vitamin D deficient in the winter because the sun doesn't provide sufficient ultraviolet light for your body to synthesize vitamin D in the winter in most of the U.S. You have to rely entirely on your stores of vitamin D and what you consume through your diet and supplements, while during the summer many people get sufficient vitamin D due to the time they spend in the sun without sunscreen.
Controversy over Recommended Intake
The recommended dietary allowance for vitamin D was set based on the amount of vitamin D necessary to prevent bone problems. However, newer research points to a much wider role for vitamin D, and many scientists believe the RDA should be higher. Vitamin D researchers often take vitamin D supplements in amounts well over the RDA, and in some cases over the tolerable upper intake limit, according to an article published in "The Globe and Mail" in July 2010. Many take between 1,000 IU and 10,000 IU of vitamin D supplements per day.
Considerations
You should discuss the proper amount of vitamin D to take during the winter with your doctor. He may test your vitamin D levels to see if you are deficient in vitamin D, in which case you would need to take higher amounts of vitamin D to get your blood levels of vitamin D back to the normal range. As much as 70 percent of the U.S. population may not have sufficient levels of vitamin D, notes Linus Pauling Institute researcher Adrian Gombart. Amounts up to 4,000 IU per day are considered safe for healthy adults.
References
- Linus Pauling Institute; Vitamin D; Jane Higdon, Ph.D.; March 2004
- ScienceDaily; Multiple Health Concerns Surface as Winter, Vitamin D Deficiencies Arrive; November 2009
- "American Journal of Clinical Nutrition"; Heritability and Seasonal Variability of Vitamin D Concentrations in Male Twins; Cristina Karohl, et al.; December 2010
- Office of Dietary Supplements: Vitamin D
- "The Globe and Mail"; Scientists Taking Vitamin D in Droves; Martin Mitteltaedt; July 2010
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