PHILADELPHIA — LAST month, Katy Perry shared her secret to good health with her 37 million followers on Twitter. “I’m all about that supplement & vitamin LYFE!” the pop star wrote, posting a snapshot of herself holding up three large bags of pills. There is one disturbing fact about vitamins, however, that Katy didn’t mention.

Derived from “vita,” meaning life in Latin, vitamins are necessary to convert food into energy. When people don’t get enough vitamins, they suffer diseases like scurvy and rickets. The question isn’t whether people need vitamins. They do. The questions are how much do they need, and do they get enough in foods?

Nutrition experts argue that people need only the recommended daily allowance — the amount of vitamins found in a routine diet. Vitamin manufacturers argue that a regular diet doesn’t contain enough vitamins, and that more is better. Most people assume that, at the very least, excess vitamins can’t do any harm. It turns out, however, that scientists have known for years that large quantities of supplemental vitamins can be quite harmful indeed.

In a study published in The New England Journal of Medicine in 1994, 29,000 Finnish men, all smokers, had been given daily vitamin E, beta carotene, both or a placebo. The study found that those who had taken beta carotene for five to eight years were more likely to die from lung cancer or heart disease.