8 Vitamin Secrets Doctors Tell Their Friends

Clifford Rosen, MD, directs the Center for Clinical and Translational Research at the Maine Medical Center Research Institute in Scarborough, Maine.

He also knows a thing or two about vitamin D: He was part of an Institute of Medicine committee that set recommendations for the “sunshine vitamin.”

So he’s astounded when he learns that friends are popping as much as 5,000 IU of the vitamin each day—far higher than the 4,000 IU established as the safe upper limit.

“Probably 80 percent of the people I know take vitamin D,” says Dr. Rosen. “When I ask them why, they say, ‘It’s not harmful.’ But that’s not necessarily true.”

In fact, the latest research in vitamin science suggests that many previously lauded supplements may be riskier than once thought. And dangers may be greater for those who are savviest about nutrition.

“People who take supplements tend to eat better and have higher nutrient intakes than people who don’t,” says Paul R. Thomas, RDN, a scientific consultant at the National Institutes of Health’s Office of Dietary Supplements.

“Adding supplements on top of a healthy diet increases the risk of getting more than you need.”

Yet it’s tough to judge the value of supplements when news headlines seesaw between recommendations and warnings. So we asked some of the nation’s top supplement experts a simple question: What advice do you give your friends and family about vitamins? Their answers may make you rethink what’s in your medicine cabinet.

“Vitamin C? It merely gives you expensive urine.”

Mark Levine, MD, laughs when asked if he takes a vitamin C supplement. A researcher at the National Institute of Diabetes and Digestive and Kidney Diseases, Dr. Levine has done meticulous studies of how the body uses vitamin C. Although some research indicates that it may protect against cancer, cardiovascular disease, age-related macular degeneration, and—most famously—the common cold, studies that isolate vitamin C from the diet generally don’t find that taking it alone protects against disease. “The best evidence for vitamin C comes from studies where people get it from fruits and vegetables,” Dr. Levine says. What’s more, Dr. Levine’s research shows that the body tightly regulates vitamin C levels, so it’s futile to load up on the megadoses found in popular supplements. “The body works very hard to absorb low amounts of vitamin C,” Dr. Levine says. “But as the dose goes up, you absorb much less, and you excrete the extra vitamin C through your urine in a matter of hours.”

So should you take a supplement? Probably not. Even amounts higher than the RDA of 90 mg for men and 75 mg for women are easy to get from your diet. In fact, only 6 percent of the population is deficient in vitamin C, according to the Centers for Disease Control and Prevention.

“If you eat fish twice a week, you don’t need omega-3 supplements.”

“People often ask me about omega-3s,” says Dariush Mozaffarian, MD, Dean and Jean Mayer Professor at the Tufts Friedman School of Nutrition Science and Policy in Boston. Dr. Mozaffarian studies these unsaturated fatty acids which are found in many types of fish. “I tell them that decades of studies show that eating two servings of fish a week lowers the risk of death from heart disease, but getting more than that doesn’t make a big difference.”

So should you take a supplement? “If you don’t eat much fish, it’s reasonable to take a 1-gram fish oil capsule a day, especially if you’re over 40 or have heart disease,” Dr. Mozaffarian says. But avoid taking more; some studies suggest that doses upwards of 2 to 3 grams may actually raise levels of LDL cholesterol.

“Unless you have osteoporosis, skip calcium pills.”

Women have been told for years to take calcium preventively for strong bones, so Dr. Rosen isn’t surprised when people cop to popping calcium pills three or four times a day. But many are shocked by the advice he shares now: “If your bones are healthy, I don’t think there’s any need for supplements, because they might not help and may be harmful.” Besides, he says that it's in many common foods such as milk, yogurt, broccoli, and fortified orange and cereal. “You need only about 1,000 mg of calcium a day, and the typical dietary intake is about 850 mg, so most people already consume a relatively decent amount,” Dr. Rosen says.

So should you take a supplement? “There’s no reason to take one unless you have osteoporosis—and even then, take only one 500-mg pill a day,” Dr. Rosen says.

“Got achy, arthritic knees? Glucosamine/chondroitin is worth a try.”

Although his own research concluded that glucosamine/chondroitin—renowned for its purported ability to relieve osteoarthritis symptoms—had no effect on joint pain or function, Allen D. Sawitzke, recommends it to friends with moderate pain anyway. The reason: “Some people do really well on it, even though others don’t,” says Sawitzke, an associate professor at the University of Utah Hospital and Clinics in Salt Lake City and an investigator for the national Glucosamine/Chondroitin Arthritis Intervention Trial (GAIT).

So should you take a supplement? If you have moderate arthritis pain, consider trying glucosamine/chondroitin for two to three months. If you don’t see a benefit by then, you’re probably not going to, Dr. Sawitzke says.

“Vitamin D has been oversold.”

You’ve probably heard about many studies linking low levels of vitamin D with higher risks of such wide-ranging problems as cancer, heart attacks, infections, Alzheimer’s disease, autoimmune conditions, depression, and obesity. When his friends cite these reports, Dr. Rosen points out that the studies don’t show cause and effect. “These associations can be really misleading,” he says. “Without more trials, we just don’t have a good sense of how vitamin D supplements help.” The one exception: Vitamin D intake has conclusively been shown to promote strong bones. You need vitamin D to absorb calcium—that’s why the two are often packaged together. But too much vitamin D (above the safe upper limit of 4,000 IU for adults) can lead to nausea, vomiting, constipation, and weakness. And because vitamin D boosts calcium absorption, amounts above the upper limit can increase risks from both nutrients.

So should you take a supplement? Most adults probably don’t need one. But after age 70, when your bones need more vitamin D, your body begins to make less of it from the sun, so Dr. Rosen recommends taking an extra 400 IU a day.

“Fiber supplements may help, but they’re not a license to skip veggies.”

In an ideal world, everyone would get their share of fiber from huge, crunchy salads and snacks of fresh fruit. But Joanne Slavin, PhD, RD, who has studied fiber and was on the committee that researched the 2010 federal Dietary Guidelines for Americans, knows that’s not going to happen. Most people get only about half the recommended intake of at least 25 grams of fiber a day, a bigger shortfall than with most nutrients, says Slavin, a professor in the University of Minnesota’s Department of Food Science and Nutrition, in St. Paul. That’s why she recommends some friends take supplements to help make up the difference. “You still need to eat fiber-rich foods like fruits, vegetables, whole grains, and legumes,” Slavin says. “But if you can’t swap these for what you normally eat, a supplement can help you get what you need.”

So should you take a supplement? First, try to boost fiber intake by tweaking your diet. If you still fall short, take 3 grams of fiber once a day to start (to avoid gas and bloating). If you tolerate the change after three days, increase to 3 grams three times a day. Check with your doctor if you have diabetes or take certain drugs. Fiber can lower blood sugar and reduce the absorption of some meds.

Two vitamins to think twice about

There are plenty of good reasons to get more vitamins A and E from your diet. However, taking them in pill form may not be beneficial.

Vitamin A: Excessive amounts of vitamin A in supplement form may be harmful. According to the National Institutes of Health, too much vitamin A can lead to nausea, headaches, dizziness, coma, and even death. It's not advisable for pregnant women either; high doses of preformed vitamin A in pregnant women may cause birth defects in their babies.

Vitamin E: The National Institutes of Health notes that high doses of vitamin E, in supplement form, may increase bleeding risk and stroke. If you're taking warfarin, this may especially increase bleeding risks. Antioxidant supplements like vitamin E may also reduce cancer chemotherapy or radiation's effectiveness