In recent years, both research on and the availability of vitamin supplements has increased, drawing increased attention to the possibility that a daily pill can enhance health.
In particular, it is a message that is resounding with men. According to the Council for Responsible Nutrition, 53 percent of American adults take a multivitamin supplements, and 50 percent of adult men 18 and older take a multivitamin, according to a 2008 survey.
But preliminary encouraging findings can often hit a wall when they are refuted by larger studies that lead to increased scrutiny on products whose claims overstated the benefits of that early research.
The latest product to enter the crosshairs of a watchdog group is Bayer's One A Day Men's Health Formula supplements, which are promoted as containing "Selenium to support a healthy prostate."
Those claims came under fire from the watchdog group the Center for Science in the Public Interest after two studies on selenium came out in June in the Journal of Clinical Oncology, one of which showed selenium provided no benefit and one which showed that for men with a certain gene variant -- one found in three quarters of men -- selenium supplements made the prostate cancer worse.
The authors of the second study, from the University of California, San Francisco, concluded that their findings "indicate caution against broad use of selenium supplementation for men with prostate cancer."
CSPI released a statement earlier this week in conjunction with a complaint to the Food and Drug Administration.
"Bayer must be stopped from promoting its selenium-containing products as a means of reducing prostate cancer risk and promoting prostate health," senior nutritionist David Schardt said in the statement. "Not only does selenium not prevent cancer, supplementation with selenium may be harmful."
Schardt told ABC News that other supplements have similar problems, such as supplements that contain gingko, whose claims for increasing memory and concentration have not been confirmed by studies.
At this point, he said, the very notion of a men's supplement is faulty.
"Do you need to take a men's supplement?" Schardt said. "A basic adult multivitamin does well for both men and women. The nutrient needs are not that much different."
As of press time, Bayer HealthCare had not responded to a request for comment.
But Andrew Shao, a nutritionist and vice president of scientific affairs for the vitamin trade group the Council for Responsible Nutrition, countered that while claims of selenium preventing prostate cancer have not been proven, they are not completely baseless, having been displayed by some observational studies.
He said that the randomized trials necessary for proof are expensive and, therefore, difficult to conduct. But "That doesn't mean there are no data to support the relationship."
Dr. Donald Hensrud, chair of preventive medicine at the Mayo Clinic, said that without that evidence, there is no clear health benefit from men's supplements.
"One popular men's vitamin contains a little more of certain nutrients to help the prostate, heart, and blood pressure," he said. "However, although there may be some mild evidence associating those nutrients with these conditions, there are no randomized controlled trials showing better health outcomes in subjects who took those nutrients."
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.
"It's rather few cases ... where multivitamins have been put to test in a clinical trial to date. None have really shown benefits for any clinical outcome like cancer or heart disease," said Ross L. Prentice, a biostatistician with the Fred Hutchinson Cancer Research Center and the University of Washington who has been involved in some long-term studies of the effects of multivitamins on the risk of disease.
"There may be some short-term benefits people report but, in terms of chronic diseases, I don't think so," Prentice said.
Prentice said he is skeptical of the vitamin-based approach to improving health, despite the number of primary care physicians -- including his own -- who have recommended taking more vitamins.
"I think it's a large, expensive industry without convincing data. Billions of dollars in expensive urine so far," he said.
"I think we should be looking into commonplace things like how many calories we consume ... how much physical activity we get," he said. "Those are difficult topics to study reliably, but they're likely the keys to our obesity epidemic and obesity-related diseases."
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