There's little argument that a healthy diet helps fight disease. Large studies have consistently shown that a diet based on whole grains, fruits, vegetables, legumes and vegetable oils helps prevent high blood pressure, heart disease, stroke, osteoporosis, Type 2 diabetes and certain cancers.
If your diet is less than stellar, you might be inclined to reach for a supplement to fill the gaps. According to Statistics Canada, nearly half (42 per cent) of Canadian adults have taken a vitamin and mineral supplement in the past month.
But depending on which supplement you choose - and how much you take - you might be doing more harm than good.
As I wrote in last week's column, there's little, if any, evidence that antioxidant supplements - vitamins C and E, beta carotene and selenium - ward off heart disease or cancer. What's more, there's some suggestion that high doses of vitamin E and beta carotene can be dangerous for certain people.
Antioxidants are not the only supplements that have been called into question. Recent reports have questioned the safety of multivitamins and folic acid - supplements routinely recommended by the medical community, myself included.
When it comes to multivitamins, most people who pop one a day do so as a form of health insurance. Yet there's no evidence that taking a one-a-day formula will prevent future disease.
A 2006 review conducted by the U.S. National Institutes of Health found too few studies to give multivitamins a go-ahead for heart disease and cancer prevention. Despite the lack of evidence for benefit or harm, it's still prudent for some segments of the population to reach for a multivitamin to fill nutritional gaps.
There is clear evidence that taking a multivitamin with 400 micrograms of folic acid (a B vitamin) before and during pregnancy prevents neural tube defects, birth defects that affect the brain and spinal cord.
When it comes to iron, it's challenging for menstruating women to meet their daily requirement (18 milligrams) from food alone. This is especially true for vegetarians and women who follow a low-calorie diet.
With age, we have more difficulty absorbing vitamin B12 from food. That's why the U.S. Institute of Medicine advises adults over 50 to get the nutrient from a supplement or fortified foods (e.g. soy beverages).
Vitamin B12 is found only in animal foods, so strict vegetarians must also rely on a multivitamin to meet their daily needs.
A multivitamin can provide a safety net if you don't get your full complement of nutrients from diet. And despite good intentions, not all of us follow Canada's Food Guide to the letter. Stress, workplace demands and lack of time and energy are common barriers to eating healthfully.
But more is not better. A 2007 study published in the Journal of the National Cancer Institute found that taking a multivitamin more than seven times per week was linked with a greater risk of advanced prostate cancer and risk of death from the disease.
The link was strongest in men with a family history of prostate cancer and in men who also took zinc, selenium or beta carotene supplements. Regular multivitamin use (no more than once daily) did not boost prostate cancer risk. (The researchers were unable to determine which ingredient was responsible for the association.)
There's also some concern that high doses of folic acid, a B vitamin thought to guard against colon and breast cancer, might do more harm than good.
