A similar dearth of evidence can also be found in a more popular pill -- the daily multivitamin supplements that provide one day's serving of vitamins.
"It is the most popular dietary supplement," Council for Responsible Nutrition spokeswoman Judy Blatman said.
But while the pills may be popular, the benefits of taking them remain largely unproven.
While health experts are in general agreement that multivitamins may provide some benefit in insuring people get at least minimal daily intakes of vitamins and minerals, they have not shown any long-term benefits, according to Howard Sesso, an epidemiologist at Brigham and Women's Hospital and assistant professor of medicine at Harvard Medical School who has researched the topic in a number of large studies.
"We know, for the most part, that multivitamins are seen as a kind of insurance against any deficiency that might be present," he said.
But any benefits beyond that are purely speculative.
"We still don't know definitively whether taking a standard multivitamin does anything in the context of preventing chronic disease," Sesso said.
Suzanne Murphy, an epidemiologist with the Cancer Research Center of Hawaii who has looked into vitamins and cancer prevention, came to a similar conclusion.
"Most people in the United States eat fairly well, they eat well enough to avoid deficiency, Murphy said. "They may not be eating optimal diets, but you're not going to really replace some of those poor dietary habits with a multivitamin pill. There's something about fruits and vegetables that goes beyond the nutrients, at least the ones that we've identified."
"I don't think a multivitamin, at a reasonable level, that just supply's a days worth of vitamins and minerals, is harmful," she said. "If that's something you want to spend your money on and sort of makes you feel you have some insurance, that's fine."
Shao said vitamins are needed because people don't weigh out their food daily to analyze the nutrients.
"I don't know for sure if I'm getting all the nutrients that I need from my diet," he said, noting that he is more acquainted than most people with the dietary guidelines.
"I can't eat the way that dietary guidelines say I need to eat," Shao said. "Frankly, it's virtually impossible. That's where using a multivitamin to help fill in gaps that we know are there."
He noted that vitamins have not been found to prevent chronic diseases, but he doesn't expect them to be.
"Nobody has teased out ... the single cause for a chronic disease, because there isn't a single cause. It's a number of different things," Shao said.
While whether to take a daily multivitamin supplement remains up in the air, one thing researchers agreed on was that the pills were no substitute for eating better foods.
"A multivitamin is not a substitute for a good diet," Schardt said.
He said they may provide insurance, but cannot be a frontline diet change. "If you have automobile insurance, that doesn't mean you can drive like a maniac. You still have to exercise common sense."
But other researchers believe that an emphasis on adding vitamins isn't tackling the bigger health issues.