To figure that out, the study's researchers used data from a massive long-term research project called NHANES, which tracks Americans’ diets and health and had relevant information on 27,725 people. Of those, just over half took some kind of supplement and more than one third took a multivitamin specifically. Those people tended to be older, whiter, more highly educated, and wealthier. The researchers analyzing this data made sure to control for these factors, but they note in the study that it’s likely these confounding variables that make people who take supplements appear to be healthy. It’s not the act of taking vitamins that makes them live longer; rather, people who are already likely to live longer tend to take vitamins.