Multivitamins: Should You Take Them? What Studies Show

The three largest available studies answer yes, with measured and moderate benefits. In 14,641 physicians followed for eleven years, a daily multivitamin reduced the overall incidence of cancers by 8%. In more than 5,000 seniors from the COSMOS study, it slowed cognitive decline by an amount equivalent to two years of less aging. And in 10,698 American adults, its regular use virtually eliminated insufficient nutrient intake for 15 out of 17 nutrients. The benefit is most clear in people whose diet does not meet all their needs and after age 60.

A supplement that combines in a single dose most essential vitamins and minerals, at levels close to recommended daily intakes. The idea is not to provide high doses of one specific nutrient, but to fill all at once the small deficiencies that accumulate in an ordinary diet. It is the most consumed supplement in the world, taken by at least one in three American adults.

This is what the analysis of 10,698 American adults from the NHANES survey shows. Compared to diet alone, taking a multivitamin, at any frequency, was associated with a lower prevalence of insufficient intakes for 15 of the 17 nutrients examined. Among the most regular users, at least 21 days out of 30, the insufficiencies were virtually eliminated, except for calcium, magnesium, and vitamin D, whose requirements exceed what a capsule can contain. Blood biomarkers of deficiency were also significantly lower.

In the largest randomized trial ever conducted on this subject, yes, modestly. The Physicians' Health Study II followed 14,641 American physicians over age 50 for an average of eleven years, double-blind against placebo. The multivitamin group had 17.0 cancers per 1,000 person-years compared to 18.3 under placebo, a reduction of 8%, statistically significant. Among the 1,312 men who had already had cancer, the reduction reached 27%. The authors conclude a modest but significant reduction in total cancer risk.

This is the most recent result and one of the most solid. The COSMOS study randomized 21,442 Americans over age 60, and three cognitive sub-studies tested more than 5,000 of them. The meta-analysis of these three sub-studies shows a clear benefit of daily multivitamin on overall cognition and on episodic memory, that of lived events. The authors estimate that the magnitude of the effect is equivalent to reducing cognitive aging by two years.

Because they are on an individual scale, and that's normal for a supplement taken by people who are overall well-nourished. An 8% reduction in cancer risk is not felt; it is counted at the population level, where it represents thousands of cases avoided. Two years of less cognitive aging are not noticed day to day; they matter at age 80. These effects are modest and real, which is exactly what you can expect from a daily gesture that is inexpensive and carries no notable risk.

The studies draw a profile. People over 60 years old, in whom absorption decreases and where the cognitive benefit has been measured. Those whose diet is not varied, restrictive, or irregular, in whom NHANES shows the most frequent deficiencies. Vegetarians and vegans for B12 and iron. People who eat little, experiencing loss of appetite or on a calorie-restricted diet. And those who have already had cancer, in whom the physicians' trial showed the strongest reduction.

In a person who eats varied foods every day, deficiencies are rare, and the multivitamin adds little. The problem is that few people eat varied foods every day. The NHANES survey shows frequent insufficiencies for several nutrients in the diet alone of the general population. The multivitamin does not replace a good diet; it covers the days when your diet wasn't good, and there are more of them than people think.

Pregnancy has specific needs, in folic acid, iron, iodine, which general formulas do not cover in the right proportions, and some contain vitamin A at doses to avoid. There are formulas designed for pregnancy, to be chosen with the midwife or doctor. A standard multivitamin should not be taken during pregnancy without this advice.

Three criteria. Doses close to recommended daily intakes, around 100%, rather than megadoses. Well-absorbed forms: methylcobalamin or cyanocobalamin for B12, vitamin D3 rather than D2, bisglycinate for minerals. And a complete composition, with minerals and not just vitamins, since the positive trials used vitamin and mineral formulas. The presence of a batch number and the origin of raw materials are indicators of quality.

With a meal, preferably in the morning or at midday. Vitamins A, D, E and K are fat-soluble and are better absorbed in the presence of dietary fats. The meal also reduces the digestive discomfort that some experience on an empty stomach, particularly with iron and zinc. B vitamins having a slight stimulating effect, evening is not the ideal time for most people.

Yes, that's what the data shows. In NHANES, the benefit on insufficiencies increased with frequency, and it was among users of 21 days or more per month that deficiencies were virtually eliminated. The trials on cancer and cognition tested daily use over years. Regularity matters more than dose.

It depends on what you're expecting. Nutritional deficiencies are corrected in a few weeks, and a person who had them may notice an improvement in energy or skin on this timescale. The measured effects on cancer and cognition appeared over years of daily use, and are not felt. The multivitamin is a foundational gesture, not a quick fix.

Rarely. The NHANES analysis shows it: vitamin D, calcium, and magnesium are the three nutrients for which the multivitamin is not sufficient to fill the gaps, because the required amounts exceed what a capsule can contain. In winter, or in case of measured deficiency, dedicated vitamin D supplementation remains useful in addition to the multivitamin. Our vitamin D guide details the dosages.

Yes, provided you avoid duplicates. A multivitamin plus omega 3, collagen, magnesium, or an herbal supplement poses no problem, as these products do not contain the same nutrients. A multivitamin plus a B-complex, plus zinc, plus vitamin A adds up to the same molecules and can exceed limits. The rule: one source per nutrient, except for vitamin D, calcium, and magnesium which are lacking in the multivitamin.

With a multivitamin dosed at recommended intakes, no. The NHANES analysis shows that users exceeded the upper safety limit for 7 nutrients, but in no more than 4% of them, and only slightly. The risk appears when you combine a multivitamin with several isolated supplements containing the same nutrients, for example a multivitamin plus a B-complex plus zinc. A single multivitamin per day, without duplicates, remains in the safety zone.

Very few at recommended intake levels. The most frequent is digestive discomfort on an empty stomach, which disappears when taken with a meal. Urine may turn bright yellow because of vitamin B2, which has no consequence. Trials with 14,641 and 21,442 participants over more than ten years did not identify any concerning safety signals.

That the three largest studies converge: fewer cancers in 14,641 physicians followed for eleven years, two years less cognitive aging in COSMOS seniors, insufficiencies virtually eliminated in 10,698 adults. That the effects are modest on an individual scale and real over a lifetime. That the benefit is most clear after age 60 and in those whose diet does not cover everything. And that a multivitamin dosed at recommended intakes, taken daily with meals, without duplicates, is a simple and safe gesture.