More details are needed about this specific research, but this has been said before: foods are good, but supplements can be problematic. Here extra vitamins and minerals are linked to higher rates of cancer. From Science Daily:
While dietary supplements may be advertised to promote health, a forum at the American Association for Cancer Research (AACR) Annual Meeting 2015 by University of Colorado Cancer Center investigator Tim Byers, MD, MPH, describes research showing that over-the-counter supplements may actually increase cancer risk if taken in excess of the recommended dietary amount.
"We are not sure why this is happening at the molecular level but evidence shows that people who take more dietary supplements than needed tend to have a higher risk of developing cancer," explains Byers, associate director for cancer prevention and control at the CU Cancer Center.
The line of research started 20 years ago with the observation that people who ate more fruits and vegetables tended to have less cancer. Researchers including Byers wanted to see if taking extra vitamins and minerals would reduce cancer risk even further. "When we first tested dietary supplements in animal models we found that the results were promising," says Byers. "Eventually we were able to move on to the human populations. We studied thousands of patients for ten years who were taking dietary supplements and placebos."
The results were not what they expected."We found that the supplements were actually not beneficial for their health. In fact, some people actually got more cancer while on the vitamins," explains Byers. One trial exploring the effects of beta-keratin supplements showed that taking more than the recommended dosage increased the risk for developing both lung cancer and heart disease by 20 percent. Folic acid, which was thought to help reduce the number of polyps in a colon, actually increased the number in another trial.
"This is not to say that people need to be afraid of taking vitamins and minerals," says Byers. "If taken at the correct dosage, multivitamins can be good for you. But there is no substitute for good, nutritional food." Byers says that people can get the daily recommended doses of vitamins and minerals in their diets by eating healthy meals and that many adults who take vitamin supplements may not need them."