But unlike fads that sizzled and fizzled, the evidence this time is strong and keeps growing.
If it bears out, it will challenge one of medicine's most fundamental beliefs: that people need to coat themselves with sunscreen whenever they're in the sun. Doing that may actually contribute to far more cancer deaths than it prevents, some researchers think.
The vitamin is D, nicknamed the "sunshine vitamin" because the skin makes it from ultraviolet rays. Because sunscreen blocks vitamin D's production, some scientists are questioning the long-standing advice to always use it.
The reason is that vitamin D increasingly seems important for preventing and even treating many types of cancer. In the last three months alone, four separate studies found it helped protect against lymphoma and cancers of the prostate, lung and, ironically, the skin. The strongest evidence is for colon cancer.
Many people aren't getting enough vitamin D, and it's hard to get from food and fortified milk; supplements are problematic.
So the thinking is this: Even if too much sun leads to skin cancer, which is rarely deadly, too little sun may be worse.
No one is suggesting that people fry on a beach, but many scientists believe that "safe sun" — 15 minutes a few times a week without sunscreen — is a healthy thing to do.
One is Dr. Edward Giovannucci, a Harvard University professor of medicine and nutrition who laid out his case in a recent lecture at a major cancer research meeting.
His research suggests that vitamin D might help prevent 30 deaths for each one caused by skin cancer.
"I would challenge anyone to find an area or nutrient or any factor that has such consistent anti-cancer benefits as vitamin D," Giovannucci told the cancer scientists. "The data are really quite remarkable."
The talk so impressed the American Cancer Society's chief epidemiologist, Dr. Michael Thun, that the society is reviewing its sun protection guidelines. "There is now intriguing evidence that vitamin D may have a role in the prevention as well as treatment of certain cancers," Thun said.
Even some dermatologists may be coming around. "I find the evidence to be mounting and increasingly compelling," said Dr. Allan Halpern, dermatology chief at Memorial Sloan-Kettering Cancer Center in New York, who advises several cancer groups.
The dilemma, he said, is a lack of consensus on how much vitamin D is needed or the best way to get it. Even if sunshine were to be recommended, the amount needed would depend on the season, time of day, where a person lives, skin color and other factors. Thun and others worry that folks might overdo it.