In a large study which surveyed US adults about their supplement intake, then followed them for around 6 years, researchers found that most dietary supplements don't appear to help avoid death. Not only that, but where there was a benefit to health during life, it was only where the nutrient came from food, not supplements.
The study was published in the Annals of Internal Medicine .
Researchers studied 31,000 U.S. adults who answered survey questions about their supplement intake in the past 30 days and about their diet over the past 24 hours. During a median 6 years' follow-up, 12% died.
They did find that those taking supplements had a reduced chance of death from any cause, but that seemed more due to them having healthier lifestyles, having had higher education and incomes. There was only a benefit from a supplement called Lycopene, which is in high levels in tomatoes.
Adequate intake of vitamin K and magnesium was tied to lower mortality risk, but this was restricted to intake from food (vegetables, nuts and seeds are good sources). Excess calcium intake was actually associated with a 62% higher chance of dying from cancer, but they believed this was due to people taking high-dose supplements. (this is not an uncommon finding - calcium from food is vital, but supplemental calcium in high doses should only be taken under medical advice)
The researchers concluded: "The evidence suggests that adequate nutrient intake from foods was associated with reduced mortality and excess intake from supplements could be harmful."