Last November, Dr. Reinhold Vieth, a professor in U of T’s departments of nutritional sciences, and laboratory medicine and pathobiology, was one of several medical professionals who participated in a conference on the “vitamin D deficiency crisis” in North America. Dr. Vieth has been studying the health effects of vitamin D for more than 30 years. He spoke recently with U of T Magazine editor Scott Anderson.
What do we know about the health benefits of vitamin D?
In the 1990s, clinical trials indicated that calcium and vitamin D reduce bone fracture rates. Higher vitamin D levels are also associated with a lower risk for multiple sclerosis, breast and colon cancers, and juvenile and adult-onset diabetes, as well as lower rates of cardiovascular disease.
How much vitamin D needs to be present in the body for positive health effects to occur?
More than 75 nanomoles per litre (nmol/L). The average Canadian has about 65, although it varies by season. It also varies by skin type. People with darker skin tend to have less.
One of the main sources of vitamin D is sunlight. Yet we’ve been warned that spending too much time in the sun causes skin cancer. What is the best course of action?
In summer, sunbathing for 10 minutes on your front and 10 minutes on your back makes about 10,000 units of vitamin D. This is the equivalent of 100 glasses of milk or 25 vitamin pills – and enough for most people to produce the desired 75 nmol/L of vitamin D in their blood. We require – and are designed to survive – a certain amount of sunshine. People just need to remember to cover up before they burn.
“Crisis” is a strong word. Why do you think there is a vitamin D deficiency crisis in Canada?
I fear we will see a sharp rise in disease due to vitamin D deficiency, particularly among new Canadians. People native to northern latitudes have lighter skin so they can absorb more vitamin D from sunlight. Many new immigrants have moved from a southern environment, for which their skin colour is optimized, to Canada, for which their skin colour is not optimized.
Young people of non-European ancestry living in Canada have low vitamin D levels, predisposing them to a long-term risk for some diseases. People from India or equatorial Africa require six times the sun exposure to make the same amount of vitamin D as a white person. At Canadian levels of exposure, they don’t have to worry about skin cancer. Unfortunately public health messages try to “keep it simple.” So we end up telling the black African person to keep out of the sun just as much as the pale Scottish person.
What’s the solution?
I think everybody would benefit from taking vitamin D supplements in the winter. There’s no harm in taking 2,000 units. For a dark-skinned person it’s appropriate to take more than that, but no one’s telling them to.
Health Canada’s recommended levels are much lower, are they not?
Health Canada recommends 200 units a day for people under 50. But the food and nutrition board in the U.S. is reviewing dietary guidelines for vitamin D and by late May is expected to announce a new recommended daily allowance. This board is jointly sponsored by Health Canada, so any changes announced there will also be made here.
Should we have our vitamin D level measured each time we go to the doctor?
I believe we should pay as much attention to vitamin D as we do to cholesterol.