The Harvard School of Public Health, Boston, USA
"A diet that includes plenty of fruits, vegetables, whole grains, good protein packages, and healthy fats should provide most of the nutrients needed for good health. For those who eat a healthy diet, a multivitamin may have little or no benefit. But not everyone manages to eat a healthy diet. When it comes to micronutrients, many Americans get less than the adequate amounts, according to criteria set by the Institute of Medicine. For example, more than 90 percent of Ameri-cans get less than the Estimated Average Requirement for vitamin D and vitamin E from food sources alone (1). Many older people have trouble absorbing vitamin B12 from food; the 2010 Dietary Guidelines for Americans, in fact, recommends that people over the age of 50 eat foods fortified with vitamin B12 or take vitamin B12 supplements (2). Getting enough folate is especially important for women who may become pregnant, since adequate folate can help lower the risk of having a baby with neural tube defects. That's why the Center for Disease Control and Prevention recommends that all women of childbearing age (ages 15 to 45) consume 400 micrograms per day of folic acid (3).
Some scientists believe there is not enough evidence to recommend for or against taking a daily multivita-min, because there isn't yet enough data from randomized controlled trials (4). That's a reasonable but short-sighted point of view, since it may never be possible to conduct clinical trials that are long enough to test the effects of multiple vitamins on the risk of various types of cancer, Alzheimer's disease, and other degenera-tive conditions. Looking at evidence from epidemiological studies on diet and health, and biochemical studies on the detailed mechanisms of disease, the health benefits of taking supplements to reach an adequate micronutrient intake appear to outweigh the risk of overdose (5). However, as with anything we take, it is the dose that makes things good or bad for the health. Thus, official intake recommendations should be followed.
To consumers, getting informed about research on vitamin supplements can be a frustrating task. Different studies on the same vitamins often present conflicting information. A closer look at the study design often reveals the cause of these discrepancies and helps to put conflicting results into context:
- What vitamin dose did study participants take and how long did they take it for?
The most obvious source of conflicting findings is the fact that different studies test different doses of vitamins, for different lengths of time. A short vitamin supplement trial may not show any benefit simply because it takes a long time for a disease to develop or for the vitamin's protective effects to kick in.
- Who were the study participants and how healthy were their lifestyles?
Lifestyle characteristics such as diet, smoking and exercise can have a dramatic effect on our health and can also have an effect on how our bodies respond to vitamins. A supplement is only useful to people whose diets are lacking in that specific nutrient; a clinical trial that gives vitamin pills to well-fed participants may not show any results. Similarly, people who smoke may have greater need for certain vitamins, so a study conducted on smokers could have different results from one conducted on people who never smoked or who have kicked the habit.
- When did study participants take the supplement?
A supplement may only be beneficial at one stage of a disease or condition and not another, so studies done at different stages (e.g., preventing the development of a disease or treating an existing disease) may have different results.
- How did researchers measure the supplement's effectiveness?
Studies often differ in how they measure their outcomes – that is, how they measure whether a supplement had any benefit. Heart disease, for example, covers a wide range of conditions, including heart attack, stroke, or peripheral vascular disease. If a study measures the effect of a vitamin supplement on heart disease overall, it may miss a supplement's protective effect against stroke.
Our knowledge about optimal intakes of vitamins and minerals is not set in stone, so it is important that we continue to research the relationships between vitamins, minerals, and chronic disease over long periods of time. This may mean more confusing news headlines along the way, as the science develops.”
Based on: Harvard School of Public Health. The Nutrition Source – Nutrition Insurance Policy: A Daily Multivitamin. Published online 2012.