If you tuned into The Daily Show earlier this month, you would have heard Jon Stewart's guest, David Agus, a physician and author of the new best-selling book The End of Illness, fret about what could be called America's vitamin abuse problem.
There have been 50 large-scale studies on supplements, he said, and not one has shown a benefit in heart disease or cancer. "I don't get it," he said. "Why are we taking these?"
Agus is not alone in his frustration. Other experts liken buying vitamins to flushing money down the toilet. In some cases, they mean it literally: If the body gets more of certain vitamins than it needs, it often excretes the excess in urine.
That doesn't stop Americans from spending about $28 billion a year on dietary supplements, including vitamins and herbal supplements.
In some cases, people may be spending money only to put their health at risk. "As Americans, we think more is better, but that's not the case with vitamins," says Dee Sandquist, a registered dietician and spokesperson for the Academy of Nutrition and Dietetics. Here are three popular vitamin supplements that prove you can, in fact, get too much of a good thing.
Vitamin E. Supplement skeptics often point to the story of vitamin E, which was once considered a promising tool for cancer prevention. The National Cancer Institute was so hopeful that vitamin E supplements would decrease rates of prostate cancer that in 2001 it funded a study designed to test the theory. Instead, the findings revealed that the men who took vitamin E were 17 percent more likely--not less--to develop the disease.
While vitamin E is a key player in immune function and cell communication, it's best obtained through diet--in foods like wheat germ, sunflower seeds, and broccoli--and worst when taken regularly in high doses. Like many vitamins, it appears to lose its main benefits when taken in excess.
Vitamin A. Vitamin A is what gives carrots their good-for-your-vision reputation. Found in both animal and plant-based products, it's also important for reproduction, bone health, and immune function. Supplements can be important for people with certain conditions that hinder fat absorption, including celiac disease, Crohn's disease, and pancreatic disorders.
But vitamin A deficiency is uncommon among healthy Americans. And partly because the nutrient can build up to toxic levels in the body, taking more than you need over time can lead to serious liver problems, birth defects, and disorders of the central nervous system.
A form of vitamin A called beta-carotene is thought to help prevent cancer--but perhaps only when obtained through the diet. In pill form, it seems to do just the opposite. Much as the pivotal vitamin E study backfired, so did the Alpha-Tocopherol Beta-Carotene Cancer Prevention Study, which found that male smokers who took beta-carotene supplements were 18 percent more likely to develop lung cancer, and 8 percent more likely to die, than the ones who did not.
Gerard Mullin, director of integrative gastrointestinal nutrition services at the Johns Hopkins Hospital and author of The Inside Tract: Your Good Gut Guide to Great Digestive Health, has cared for patients who developed liver fibrosis because they overdosed on vitamin A. "A lot of people don't know it can be dangerous," he says. "They think it fights infections."
Vitamin C. Infection-fighting prowess is often attributed to vitamin C, as well. From orange-flavored chewables to Emergen-C packets, mega-doses of vitamin C are staples in many American medicine cabinets. While the natural form of the vitamin supports immune function, there is only a weak scientific link between regular use of vitamin C supplements and shorter or less severe colds. There's no good evidence that vitamin C pills can prevent a cold altogether.
Unlike vitamin A, vitamin C is water soluble, which means that if you take more than your body can use, the excess is usually excreted without causing harm. However, Sandquist says, adverse reactions like diarrhea, stomach cramps, and nausea can occur.
Adds Mullin, "In high-enough doses, vitamin C can cause kidney stones." Any amount larger than 500 milligrams per day can be enough to cause a problem, he says. That's only half a 1-gram packet of Emergen-C. "It rarely happens, but there have been case reports."
Reasonable Doses. Sandquist recommends that healthy people abide by the Institute of Medicine's "Tolerable Upper Intake Levels," which indicates the maximum daily intake of a vitamin you should consume through a combination of diet and supplements. Taking more than that amount means the risks likely outweigh the benefits. The recommended amount is often less than the limit.
"When the IOM makes their recommendations, they look at all the available research," she says, so its conclusions are more reliable than any single study, even one that gets a lot of publicity.
Because taking supplements is second nature for many consumers, vitamins are often overlooked as a potential culprit for symptoms like headaches or diarrhea, Sandquist says. It's important to be conscious of what you're consuming--in natural, supplement, and fortified forms--and to tell your doctor about every last one.
"The best strategy is to follow the 'choose my plate' method," she says, referring to the U.S. Department of Agriculture's healthy food guide. If people do that, she says, "then they probably wouldn't have to worry about a vitamin supplement unless they have a specific medical condition."
[Next story: 4 Herbal Supplements Your Doctor Hates]