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We’ve been hearing a lot about vitamin D deficiency and its role in osteoporosis, depression, heart disease and stroke, cancer, diabetes, thyroid problems, immune function — even weight loss. So what’s the big deal?

Vitamin D is a fat-soluble vitamin, meaning that it is best absorbed when eaten with fat (as in full-fat milk, eggs, or meat such as salmon. This is one of the many reasons why fat free milk is bad). Vita D is found in food but also can be made in your body after exposure to ultraviolet rays from the sun. If you are wearing sunscreen SPF 8 or higher, you will NOT be able to make vita D in the sun. So don’t fall for all the hype about covering every exposed part of your body when outdoors. 15 minutes in direct sunlight is what the (alternative) doctor ordered. The benefits far outweigh the risks.

The major biologic function of vitamin D is to maintain normal blood levels of calcium and phosphorus. Vitamin D aids in the absorption of calcium, helping to form and maintain strong bones. Interestingly, D is a vitamin that actually functions as a steroid hormone. This is a fancy way of saying that it has several important metabolic jobs: normal cell growth and differentiation throughout the body (cancer prevention), reduce tumor growth, mineral absorption and bone-building, insulin sensitivity, inflammation reduction, to name a few.

How much do you need? Well, in the olden days when we were doing what humans are supposed to do – being active outside planting and harvesting and hunting and gathering – sun exposure and limited dietary sources were enough. But now that we spend 10 hours daily in front of the computers or video games, requirements have changed. We need about 400 IU per day, but the source makes a difference: the body gets the most usable form from sunlight exposure, so try to get outside for 15-20 minutes in direct sunlight each day. Salmon, milk, organ meats and egg yolks are good dietary sources, but beware of the vitamin D fortified foods, which contain forms that are not as bioavailable to the body. Make sure you get your levels tested before taking vitamin D: because it is fat soluble, it is stored in the body and not excreted (like water soluble vitamins), so it is possible to overdose, leading to health problems. Your body is wise: it is impossible to make too much vitamin D from sun exposure.

Some symptoms of D deficiency are muscle pain, weak bones/fractures, low energy and fatigue, lowered immunity, depression and mood swings, and sleep irregularities. How’s that for vague? Ask your doctor to check your blood levels, especially if you’re in the upper latitudes (like here in Northern CA!) or don’t get much sun. Consider supplementing during the winter months. Cod liver oil sure is tasty!

Follow a balanced whole foods diet, get outside, and get your blood analyzed every year. When you do, add vitamin D to the list (thyroid, liver function, lipid profile, and iron are also important). This is true preventive medicine.