Can vitamins cause cancer? Viral claims of risk increase may be bunk

An inflammatory new claim that taking vitamin supplements will increase your risk of cancer has ruffled feathers throughout the medical community.

Dr. Mohammad Muneeb Khan, an oncologist with the UK’s National Health Service (NHS), has proclaimed that multivitamins should be labeled with the same warnings now found on cigarette packages.

“Synthetic pills contain obscenely high and wholly unnecessary volumes of micronutrients that far exceed what the average human body requires,” Khan said, according to a statement from South West News Service.

“The problem is that these excess multivitamins are readily available to feed the hundreds of cancer cells that are made in our body every day,” Khan added.

“Normally, our body has the capability to destroy these cancer cells effectively, but this becomes a challenge when they are well-fed and able to increase in number quickly.”

His views set off alarms among the estimated 52% of US adults who use at least one dietary supplement — and the 31% who report using a multivitamin-mineral supplement.

Khan bases his claim on a study from 1996 of more than 18,000 smokers, former smokers and workers exposed to asbestos.

The study, named the Beta-Carotene and Retinol Efficacy Trial (CARET) and published in the New England Journal of Medicine, tested the ability of high doses of beta carotene and retinol (vitamin A) to prevent lung cancer.

After an average of four years, “the combination of beta carotene and vitamin A had no benefit and may have had an adverse effect on the incidence of lung cancer and on the risk of death from lung cancer,” the study authors warned.

As a result of those alarming results — in particular, a 28% increase in the incidence of lung cancer among participants — the CARET study ended almost two years earlier than planned.

“Studies looking at the daily use of supplements including vitamin A and vitamin B complex (including vitamins such as B1, B6 and B12) have, likewise, shown a correlation in increased risk for different types of cancer,” Khan said.

“And while research is likewise lacking at present for other vitamins, it’s fair to assume they will also have similar effects.”

There are a handful of other studies finding adverse effects among people who take certain doses of multivitamins.

For example, a 2022 study found “slightly higher risks of overall, prostate and lung cancer, as well as leukemia, were observed for greater multivitamin use in men, with a higher oropharyngeal cancer risk in women.”

The U.S. Preventative Services Task Force recommends avoiding beta-carotene and vitamin E supplements, which can increase the risk of certain types of cancer and death among healthy adults without special nutritional needs.

But, overall, most vitamin supplements are relatively harmless — at safe doses — and can provide some benefit, other experts say. “Vitamins do not cause cancer!” screamed a 2015 headline from the Linus Pauling Institute at Oregon State University.

“Vitamins and minerals taken at doses recommended by the Institute of Medicine, also known as the Recommended Dietary Allowance or RDA, are the ‘remedy’; the same vitamins and minerals taken at doses far exceeding the RDA may have adverse health effects and, hence, could become a ‘poison,’” wrote the authors of the OSU report.

The authors also point to one of the largest, longest and most carefully conducted trials on multivitamins, the Physicians’ Health Study II, which found that the daily consumption of multivitamins over 11 years reduced total cancer incidence by 8% — better than any pharmaceutical drug to date.

Other respected medical groups agree.

“The problem with focusing on this supplement debate is that the real issue is being ignored,” said dietitians at the University of Michigan Rogel Cancer Center.

“We are so focused on an easy fix for cancer and this is supported by statistics. In one study, 50% of breast cancer patients were taking a multivitamin, but 70% of these individuals had an unhealthy BMI, one of the main risk factors for increased risk of breast cancer,” they continued.

“The real focus should not be on supplements, but on eating a healthy diet focused on whole grains, vegetables, fruits and legumes that will nourish your body with all the nutrients you need to prevent cancer and other chronic diseases.”