Researchers in the USA analyzed the medical records of nearly 400,000 adults who did not suffer from major long-term illnesses to see if multivitamins reduced their risk of death over the next two decades, writes The Guardian.

Instead of living longer, those who took daily multivitamins were slightly more likely than those who did not to die during the study period, leading government researchers to comment that "using multivitamins to improve longevity is not supported".

Almost half of adults in the United Kingdom take multivitamins or dietary supplements once a week or more, contributing to a domestic market worth over half a billion pounds annually.

The global supplement market is estimated to be worth tens of billions of dollars each year. In the USA, one-third of adults use multivitamins hoping to prevent diseases.

But despite their popularity, researchers have questioned the health benefits of multivitamins and even warned that supplements can be harmful. While natural food sources of beta-carotene protect against cancer, for example, beta-carotene supplements may increase the risk of lung cancer and heart disease, suggesting that supplements lack important ingredients. At the same time, iron, which is added to many multivitamins, can lead to iron overload and increase the risk of cardiovascular disease, diabetes and dementia.

For the latest paper, Dr. Erikka Loftfield and her colleagues at the National Cancer Institute in Maryland analyzed data from three major health studies in the USA. All were launched in the 1990s and gathered details on participants' daily multivitamin use. The records covered 390,124 generally healthy adults who were followed for more than 20 years.

Researchers found no evidence that taking daily multivitamins reduced the risk of death and reported instead a 4% higher mortality risk among users in the early years of follow-up. The higher risk of death may reflect the damage that multivitamins can cause or a tendency for people to start taking daily multivitamins when they develop a serious illness. Details are published in JAMA Network.

Dr. Neal Barnard, a professor of medicine at George Washington University and co-author of a commentary published alongside the study, stated that vitamins are useful in specific cases. Historically, sailors were saved from scurvy by vitamin C, while beta-carotene, vitamins C and E, and zinc seem to slow down age-related macular degeneration, a condition that can lead to severe vision loss.

It is also possible that vitamins are beneficial without reducing the risk of early death. A preliminary study conducted in 2022 found evidence that multivitamins might slow cognitive decline in old age, but more research is needed.