Many of us are deficient in vitamin D, especially at higher latitudes in the winter. Until recently, vitamin D was thought to mainly play a role in calcium metabolism. But recent investigations have discovered that it plays many important roles in the health of our immune system as well. Chronic vitamin D deficiency leaves us more vulnerable to infectious diseases as well as cancer and perhaps even heart disease. The correlation of colds and flu to winter and rainy season around the world may be a prime example of vulnerability to infectious disease because of vitamin D deficiency. The correlation of many cancers by latitude may also be an indication of the effects of vitamin D deficiency in causing cancer (though correlation does not prove causation).
Our early human ancestors in Africa had plenty of sun to maintain optimal vitamin D levels most of the year. Today, people living in this area who get good sun exposure are generally able to achieve about 40 to 60 nanograms of vitamin D per milliliter (ng/ml) of blood and this appears to be near the optimal level. Ultraviolet (UV) light forms vitamin D from a cholesterol-derived precursor in the top layers of the skin (another reason why cholesterol is good). Sun angle, cloud cover, altitude above sea level, skin exposure area, skin color, sunscreen usage, obesity, and age are all factors that affect the body's ability to make and utilize vitamin D from sunlight. With optimal conditions, it only takes about 10 minutes of full-body high-angle direct sun exposure for the body to produce about 10,000 International Units (IU) of vitamin D. Darker skin color and sunscreen both block UV light and reduce the vitamin D rate of formation. Obese and elderly people also need more sun exposure to achieve optimal blood levels of vitamin D. Above about 35 degrees latitude, the winter sun is inadequate to provide enough vitamin D to maintain optimal levels, even with full and frequent exposure.
Strong sunlight on our skin is necessary to produce vitamin D but also ages our skin. Diets high in polyunsaturated fat especially make the skin more vulnerable to damage from UV rays in sunlight - another good reason to limit polyunsaturated fat to ancestral intake levels of around 4% or less. Sunburn especially damages the skin, by literally burning the skin and can cause first, second, or third degree burns. So, those with fair skin need to be very careful to avoid sunburn when getting vitamin D from sun exposure.
Supplementing Beyond Sunshine
When adequate sunshine is not available, vitamin D levels need to be maintained by diet and/or supplements for optimal health. The foods highest in vitamin D are mostly animal seafoods, as shown in the table below. The problem is, there are many confounding factors in determining how much dietary vitamin D is necessary to achieve optimal blood levels. Part of this problem is that vitamins D, A, and K2 all interact synergistically and increasing one without increasing the others is more likely to lead to toxicity problems. Getting plenty of calcium and magnesium are also important to prevent toxic effects from high intake of vitamin D. Dairy and animal seafood are excellent sources of these minerals.
Vitamin D in FoodAt high latitudes in winter, getting as much as 1,000 to 4,000 International Units (IU) of vitamin D by diet or supplement may be neccessary to achieve optimal blood levels. Some people may need that much from food or supplements year-round. Others may need little extra vitamin D from diet. The best way to be sure how much is needed is through blood testing.
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(click to enlarge)
Blood Testing for Vitamin D
The best blood test to indicate vitamin D status is the 25(OH)D (25-hydroxyvitamin D) test. Ideally, 25(OH)D levels should be at least about 30 to 40 ng/ml for optimal health. The cost for this test ranges from about $50 to $150 and is usually not covered by insurance - even though vitamin D status is probably as much or more important than most covered parameters (a sad comment on the poor state of our modern "health care" system). Ideally, testing should be done at least twice a year, once in summer and once in winter.
Update 2010 December 5
There is controversy over the optimal blood levels of 25(OH)D and unfortunately we are a long way away from having good long-term scientific studies to help us better understand what is optimal. For an excellent review of the science to date, see this article by Chris Masterjohn. Some researchers have speculated that optimal 25(OH)D blood levels should be in the 50 to 60 ng/ml range or even higher, but so far the science does not clearly support that contention. Also, considering that our ancestors got their vitamin D from sunshine and nutrient dense foods, this ancient approach seems best for promoting optimal vitamin D status since it includes other co-factors necessary for proper vitamin D metabolism.
References and Further Reading
From Seafood to Sunshine - A New Understanding of Vitamin D Safety
Are Some People Pushing Their Vitamin D Levels Too High?
On the Trail of the Elusive X-Factor
Use of vitamin D in clinical practice
The Miracle of Vitamin D
Naked at Noon
Here's a little sunshine to hopefully brighten up your day :)
Walking on Sunshine - Aly and AJ