Dr. Clifford Rosen knows vitamin D: He was part of an Institute of Medicine committee that recently 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, who directs the Center for Clinical and Translational Research at the Maine Medical Center Research Institute in the U.S. “When I ask them why, they say, ‘It’s not harmful.’ But that’s not necessarily true.”

When do vitamins become harmful?

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, 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 top U.S. 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

Dr. Mark Levine 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. The benefits likely come from the interaction of a range of nutrients in C-rich foods such as citrus fruits, red and green peppers, broccoli, strawberries, and Brussels sprouts.

While some evidence suggests that taking 200 mg or more of vitamin C a day might shorten a cold by a few hours—“12 hours at the most,” says Dr. Levine—popping a supplement after symptoms start does no good.

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.”

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 Dr. Dariush Mozaffarian, associate professor of medicine at Harvard Medical School, who studies these unsaturated fatty acids 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.”

Omega-3s reduce inflammation and make plaques in arteries more stable so they’re less prone to trigger a heart attack or stroke. They may also alleviate depression and protect skin from ultraviolet radiation.

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.”

Calcium keeps bones strong and helps muscles, nerves, blood, and hormones do their jobs. You especially need it when you’re young and building bone or when you’re older (over 50 for women and over 70 for men) and your body absorbs less of the mineral. But it’s easy to get enough from foods such as milk, yogurt, broccoli, and fortified orange juice 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.

He’d rather see friends make up this small deficit by switching out junky processed foods for healthier, calcium-rich ones than take supplements, which can quickly push people dangerously close to the upper limit of 2,000 to 2,500 mg—beyond which calcium may start to harm health. Too much calcium can cause constipation and increase the risk of kidney stones.

Newer research, though still controversial, shows that people who take calcium supplements suffer heart attacks at a greater frequency than those who don’t.

There’s more: Dr. Rosen says that many people assume that calcium supplements prevent osteoporosis, but the latest independent review of research found there’s not enough proof to show that pills help people who do not have the bone-weakening condition. It’s a bit of a “damned if you do, damned if you don’t” situation: for postmenopausal women, doses below 1,000 mg don’t help, but doses above that amount may increase risks.

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, Dr. 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 Dr. Sawitzke, an investigator for the national Glucosamine/Chondroitin Arthritis Intervention Trial (GAIT).

Experts think these chemicals, which occur naturally in joints, help build cartilage and supporting tissue and work best as a duo. The trial—which compared the supplement with an anti-inflammatory pain reliever and a placebo—showed that, on average, people experienced no benefit from either treatment after two years. But Dr. Sawitzke points out the fine print: Glucosamine/chondroitin didn’t help all, but some people who took it lost ten times less cartilage. And the compounds significantly eased pain for 79 percent of participants during the first two months.

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.

While blood levels of vitamin D are widely reported to be abysmally low, the latest research shows that most people have enough for healthy bones (above 20 ng/ml). Since intake from food sources, such as fortified milk, fatty fish, eggs, and mushrooms, is subpar, most of us get surprisingly meaningful amounts from the sun, which triggers D production in the skin.

“Even ten minutes of casual exposure on the hands and feet seems to make a difference,” Dr. Rosen says.

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. Supplements may also be worthwhile for teens (who are in their prime bone-building years), people with disorders such as Crohn’s or celiac disease (which can affect D absorption), and obese people (whose excess body fat may stockpile the nutrient, so less circulates in blood).

 

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, 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. 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.”

Fiber—plant substances that the body doesn’t easily digest—lingers in the GI tract, where it nourishes good bacteria and slows digestion. It may lower the risk of heart disease and stroke, hypertension, obesity, and some cancers. Although much of what’s known about fiber’s health impact comes from studies of overall dietary patterns, clinical studies based on isolated fiber like that found in supplements also reported important benefits, like lowering cholesterol.

“Not all dietary fibers are alike,” Slavin says. For example, studies suggest that psyllium (used in liquid supplements like Metamucil) and wheat bran are effective at promoting regularity; inulin can increase healthy bacteria in the gut; and oat bran and barley bran have been shown to lower cholesterol.

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, since fiber can lower blood sugar and reduce absorption of some meds.

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