A new systematic review of existing data and trials published between January 2012 and October 2017 has found that popular multivitamins, vitamin C, vitamin D, and calcium — some of the most common choices — had no real advantage to people's health, and there was no evidence taking them reduced the risk of cardiovascular disease, heart attack, stroke, or early death.
The study, published in the Journal of the American College of Cardiology, was led by researchers at St. Michael's Hospital and the University of Toronto.
"We were surprised to find so few positive effects of the most common supplements that people consume," said Dr. David Jenkins, the lead author of the study. "Our review found that if you want to use multivitamins, vitamin D, calcium or vitamin C, it does no harm — but there is no apparent advantage either."
There were, however, a few apparently advantageous supplements. Folic acid and B vitamins with folic acid were found to possibly reduce the risk of cardiovascular disease and stroke. Niacin (a form of vitamin B3) and antioxidants, meanwhile, were actually associated with a higher risk of death for any cause, albeit a very small increase.
The researchers say these higher risks could be related to niacin's adverse effects on blood sugar levels, or how when taken in high doses, antioxidants could also be harmful. Alternatively, it could be something to do with the fact that people sometimes think taking vitamins can be a substitute for a healthy, balanced diet when they are not.
"These findings suggest that people should be conscious of the supplements they're taking and ensure they're applicable to the specific vitamin or mineral deficiencies they have been advised of by their healthcare provider," said Jenkins.
The vitamins the team reviewed were A, B1, B2, B3 (niacin), B6, B9 (folic acid), C, D, and E, carotene; calcium; iron; zinc; magnesium; and selenium.
Research has shown how some vitamins are more useful than others. For example, zinc has been linked to shortening the effects of a cold (vitamin C doesn't do this, despite people thinking it does.) Vitamin D can also be difficult to get from your food, so if you're deficient, supplements may also be effective.
"In the absence of significant positive data — apart from folic acid's potential reduction in the risk of stroke and heart disease — it's most beneficial to rely on a healthy diet to get your fill of vitamins and minerals," Jenkins said. "So far, no research on supplements has shown us anything better than healthy servings of less processed plant foods including vegetables, fruits, and nuts."