It's estimated that 82 per cent of Canadians households use nutritional supplements, and among them, two-thirds take a multivitamin every day in an effort to stay healthy. Yet according to a report released last month by the U.S. National Institutes of Health, there's scant evidence that popping a multivitamin pill will prevent disease.
The expert panel assessed findings from randomized trials to determine whether vitamins and minerals were effective at lowering the risk of heart disease and cancer or delaying the progression of eye diseases, including cataract and age-related macular degeneration. (Macular degeneration is the leading cause of severe vision loss in older adults.)
Only three situations were identified in which vitamin supplements seem to offer clear benefit. The report recommended calcium and vitamin D for postmenopausal women to promote bone health, antioxidants plus zinc for people with early-stage macular degeneration and folic acid for women of childbearing age to prevent birth defects.
The panel also evaluated five studies that used combinations of up to seven vitamins and/or minerals, but concluded the evidence was too thin to make a recommendation for or against taking multivitamins. Some have criticized the report for considering only randomized controlled trials, because other types of studies can also provide important insights. (Randomized trials assign people to receive a vitamin supplement or a placebo pill.) For instance, the Nurses' Health Study, which followed almost 90,000 healthy women for 15 years, found that long-term use of a multivitamin with folic acid significantly reduced colon-cancer risk.
Even if a multivitamin and mineral supplement doesn't ward off cancer or heart disease, there are other reasons why you might consider taking one. If you're like many time-crunched Canadians who eat on the go, or you're following a low-calorie diet, you're probably not getting all the nutrients your body needs. If you're a menstruating female, it can be challenging to consume a day's worth of iron (18 milligrams) from food alone. (Consider that three ounces (about 90 grams) of red meat, one of the best sources, has no more than three milligrams of iron.) And the task is nearly impossible for vegetarians, because iron is absorbed less efficiently from plant foods.
If you're an older adult, you may not be absorbing enough vitamin B12 from foods.
Folic acid is another reason why women should take a one-a-day formula. This B vitamin, which is needed to help prevent birth defects, occurs naturally as folate in spinach, lentils and asparagus, but only half can be absorbed. Folic acid, the synthetic version in supplements and fortified foods, is almost 100 per cent absorbed.
A multivitamin will also boost your intake of vitamin D, a nutrient that most Canadians don't get much of, particularly in the winter. It is produced in the skin in response to sunlight. Vitamin D helps keep bones healthy and may play a role in cancer prevention.
How do you know if you need a multi? The answer depends on your eating habits. With the growing trend toward nutrient-fortified foods -- fruit juices, breakfast cereals, energy bars and meal replacements -- you might be consuming the equivalent of a multivitamin and mineral pill without knowing it.
If you decide to take a multi, you need to know which ingredients are hype and which ones you really need. A good multi should supply 100 per cent of the daily recommended intake for most vitamins and minerals. In Canada, you won't find the recommended daily intakes printed on supplement labels. Here's a list of important nutrients adults should look for -- in milligrams (mg), micrograms (mcg) and international units (IU) -- and what to avoid:
Vitamin A: You don't need more than 3,300 IU.
Beta-carotene: Many multis have beta-carotene, some of which the body converts to vitamin A. Look for no more than 15,000 IU. Very high doses (more than 30,000 IU) may increase the risk of lung cancer in smokers. You're better off eating more beta-carotene rich foods like spinach, broccoli, carrots and sweet potatoes.
Vitamin D. Choose a product with 400 IU, especially if you're over 50. The official recommended intakes are 200 IU for people aged 19 to 50, 400 IU for 51-70 year-olds, and 600 IU if you're over 70.
Vitamin E. The recommended daily intake is 22 IU. There's no evidence that taking more will ward off heart disease or cancer. In fact, research has linked a daily intake above 100 IU with a greater risk of death during the study periods. Choose a multi with less that 100 IU a day.
Vitamin K. The recommended daily intake is 90 mcg for women and 120 mcg for men but daily intakes of 150 to 250 mcg may help prevent hip fractures. Until recently, vitamin K was not allowed in multivitamins. Now 120 mcg a day is permitted. But it's still not included in many. If you take blood thinners, ask your doctor about taking a multi with vitamin K.
B vitamins. Look for at least 100 per cent of the recommended intakes for B1 (1.2 mg), B2 (1.3 mg), B3 (14 - 16 mg), B6 (1.3 - 1.7 mg), B12 (2.4 mcg) and folic acid (0.4 mg).
Iron. If you're a premenopausal woman choose a multi with 10 to 18 mg; men and postmenopausal women don't need more than eight , although products with 10 mg are fine). If you have been diagnosed with elevated iron stores, choose an iron-free formula.
Calcium. This mineral is too bulky to fit a whole day's worth into a one-a-day formula. Get what you can from foods and take a separate calcium supplement if you need more.
Magnesium. Men need 420 mg a day; women need 320 mg. It's found in whole grains, legumes, nuts, seeds, dates and leafy greens. To boost your intake, pick a multi with 100 mg. Taking more than 350 mg of supplemental magnesium can cause diarrhea.
Other minerals. Look for 10 mg of zinc, two mg of copper, 25 to 35 mcg of chromium, and 55 mcg of selenium.
Leslie Beck, a Toronto-based dietitian at the Medcan Clinic, is on CTV's Canada AM every Wednesday. Visit her website at lesliebeck.com.