Let the sunshine in

A regular source of vitamin D is absolutely essential for efficient calcium absorption in the body. Fortunately, it’s in plentiful supply, from both endogenous (from within) and exogenous (from without) sources. Exogenous vitamin D is found in food – including fortified milk and butter, egg yolks, fatty fish and fish liver oils – and vitamin supplements.

With supplements, it’s not a case of "if a little bit’s good, a lot must be better." You need sufficient D, but too much can be dangerous. The dietary division of The Osteoporosis Society recommends 400 International Units (IU) per day for the average Canadian, more (up to 800 IU per day) for seniors or those who are housebound. If you’re experiencing headaches, loss of appetite and/or diarrhea, you might be overloading.

The word "housebound" is a clue to our other, endogenous, source of vitamin D: It’s actually contained in our skin, in the form of a vitamin D precursor cell called 7 dehydrocholesterol that’s converted to vitamin D by exposure to a specific wavelength of UV B radiation (between 290 and 315 mm). That’s right, science students: direct sunlight. We only need about 20 minutes’ worth a day tturn the trick, but it’s got to be direct exposure: If you slather on a sunscreen with a sun protection factor of 15 or better, that will effectively stop the skin from synthesizing vitamin D. And you can’t get your daily dose of D by staying indoors, even if you sit till you swelter in your sun porch: UV B is blocked by glass windows.

Winter’s another problem. Way up here in the Great Canadian North, the sun’s rays are filtered through the atmospheric ozone layer at such an oblique angle they’re too weak to activate vitamin D production in our skin. So, from October to March (the end of March north of Edmonton), anyone living north of the 42nd parallel (every Canadian who isn’t resident in a tropical embassy), isn’t getting enough UV light to produce sufficient amounts of vitamin D – and that’s where foods and vitamin D supplements come in. If you’re not sure you’re getting enough, or you’re worried you may be getting too much, have your diet checked by a doctor or nutritionist.