5 Myths and Truths About Vitamin D

These days you’re wise to wonder if vitamin D is what keeps the doctor away rather than the proverbial apple—everywhere you look there’s another health story starring vitamin D. But is it really all it’s cracked up to be? What is it and what are the best sources of vitamin D, aside from the sun? We help you sort fact from fiction about vitamin D with the help of Reinhold Vieth, a leading expert on vitamin D and director of the Bone and Mineral Laboratory at Toronto’s Mount Sinai Hospital.

Myth: Clinical trials have proven that vitamin D prevents cancer.

Truth: Recent research shows links between vitamin D deficiency and a host of diseases including obesity, heart disease, multiple sclerosis, certain cancers, and depression. However, there are no placebo-based clinical controlled trials linking vitamin D with any disease other than osteoporosis, says Vieth. This means that the current evidence we have about vitamin D and disease prevention is based on epidemiological evidence (evaluating rates of disease based on population data and often, self-reported vitamin D intake); not the conclusive evidence governments and experts prefer for making preventive health recommendations. “This is one reason why there is no bottom line on how much vitamin D one should supplement with in order to get the best health benefit,” Vieth adds. “We just don’t know yet.”

Myth: Your body can produce enough vitamin D on its own.

Truth: Most of the vitamin D in people comes from exposure to sunlight. Your skin produces vitamin D in response to exposure to UVB rays. The reason why many people suffer low blood levels of vitamin D in northern climates is because of the limited sunlight, especially during winter months. People with dark skin pigmentation are typically more deficient in vitamin D because they require more exposure to sunlight to produce the same amount as people with fair skin.

Vitamin D supplements are currently recommended in cases where extended exposure to the sun is a concern, for all adults over 50 or for those who do not get enough vitamin D from their diets.

Myth: You don’t need a supplement if you spend a lot of time outside.

Truth: If you work outside year-round, you may not need a vitamin D supplement, says Vieth. But since the majority of us cannot be guaranteed to receive consistent, safe exposure to UVB rays, it’s best to ask your doctor whether taking a vitamin D supplement is right for you.

Myth: There’s no way to tell how much vitamin D I need.

Truth: The amount of vitamin D in your blood determines how much supplementation you need, and you can ask your physician for a simple blood test to find out your levels. The cost of the vitamin D test isn’t covered by many provincial healthcare plans (unless you have been diagnosed with certain conditions such as osteoporosis), so you may have to pay for it out of pocket.

But will knowing your vitamin D levels help prevent disease? All anyone can say with certainty is that to prevent bone fracture and lower your risk of osteoporosis you need about 75 nanomoles per litre (nmol/L) or higher of vitamin D in your blood, says Vieth. “The average fair-skinned Canadian has about 60 or 65 nmol/L of vitamin D. If you take 1,000 IU of vitamin D over the long-term [with no end point] you can raise your blood levels by [an average of] 25 nmol/L,” he adds. (It takes several months for vitamin D to build up to adequate levels in your system.)

Myth: You shouldn’t take more that 200 IU of vitamin D in supplement form every day.

Truth: The Institute of Medicine of the National Academies’ guidelines recommend a daily intake of 600 IU of vitamin D for people under 70 years of age and up to 800 IU per day for people 71 and older. Also interesting: The Institute says most North Americans are getting enough vitamin D.

That being said, other health authorities already advise higher amounts. For example, Osteoporosis Canada recommends between 400 and 1000 IU daily for adults under 50 and between 800 and 2000 IU daily for adults over 50. The Canadian Cancer Society advises adults take 1,000 IU daily in fall and winter months (or all year round if they are older, have dark skin, don’t go outside often and wear protective clothing that covers most of their skin). Your best bet is to ask your doctor how much vitamin D supplementation is right for you.

This story was originally published in November 2010.

Next, learn which supplements every woman should ask her doctor about.