Vitamin D-How Much is Enough?
Short Answer: The Institute of Medicine (IOM) has set a Daily Value of 400 IUs per day. The Adequate Intake is 200 IUs per day (this was set in 1997). However many health professionals are recommending higher levels of 800-1000 IUs per day. Stay tuned because the IOM is releasing new recommendations next year!
Long Answer: There is controversy about both Vitamin D requirements and Vitamin D status. I mentioned the Vitamin D requirements above, but what about Vitamin D status?
A blood sample can test Vitamin D status by measuring levels of 25 hydroxy vitamin D, or 25(OH)D. Vitamin D sufficiency is defined as blood levels greater than 30ng 25(OH)D per mL. Vitamin D insufficiency is 21-29 ng 25(OH)D per mL. Vitamin D deficiency is 20ng 25(OH)D per mL or less.
Now comes the tricky part. How do we measure Vitamin D produced in from sunlight? Like most things Vitamin D-related, I don't know if there is a concrete answer for this. Although Vitamin D is found in a limited number of food sources (salmon, mackerel, tuna, fortified milk, fortified cereal), our skin makes Vitamin D from sunlight. Factors such as skin pigmentation, sunscreen, and weather make the situation very confusing. The Institue of Medicine is taking this into consideration as they update their Vitamin D recommendations (scheduled for release in May 2010).
flickr photo by _mandrew_
Daily Sunshine Dose
During rotation at the Joslin Diabetes Center on December 14th, I heard a research presentation on Vitamin D status in children with Type 1 Diabetes. At Joslin a group of researchers is comparing levels of Vitamin D in children who have Type 1 Diabetes compared to children who do not. Vitamin D receptors have been found on many cells, extending D's function in the body beyond calcium absorption and bone strength. This explains much of the recent Vitamin D hype. Joslin is analyzing Vitamin D's role in immunity, particularly in the auto-immune factors of Type 1 Diabetes. Cool, right? Their study is not completed, so I can't comment on the preliminary results.
The UV Index
Vitamin D comes from sunlight. But not just any old sunlight. In order to absorb Vitamin D through the skin, the sunlight must be above UV level 3. The researcher at Joslin presented these nifty weather charts that show UV Index by month. Any month with UV less than 3 means that Vitamin D cannot be absorbed through the skin. Where I'm living, the forecast looks a little gloomy for sunny D.
Boston, my New England home. Low UV levels November through February.
Sacramento, close to my hometown in California. Low UV from November through January.
Honolulu, dare I even compare? I guess my grandpa doesn't have to worry.
Japan, a country dear to my heart. UV levels are similar to Boston.
Check out your city at Climate Guides-Weather 2 Travel.In October I started taking a calcium + vitamin D pill. The world of vitamin supplements is a slippery slope. I know some dietitians who promote them, and others who won't touch them. I believe it is best to get vitamins from the food that we eat. However I decided to start some Vitamin D pills this winter since I don't drink milk, I hardly eat fish, and I live in New England.
Where do you live? What do you think?