Lozenges, supplements and nasal sprays that contain the mineral claim to boost immunity, and there is some evidence that they might do so. In an effort to stay well, though, we might be making ourselves sick. Consistently taking excessive
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Zinc: An article in Monday's Health section on the dangers of excess zinc incorrectly spelled the name of dietitian Ruth Frechman as Ruth Frenchman. —
amounts of zinc, according to early evidence, could lead to learning and memory problems, nerve damage, urinary tract problems and other negative effects.
With supplements that provide many times the recommended daily intake, cold medicines that are loaded with zinc and an abundance of fortified foods -- on top of the zinc already in a healthy diet -- overdoing it might be easier than you think.
"Everyone pays attention to zinc deficiency, and we need to get the story out that that's there, but that doesn't mean we need to gobble up boatloads of this stuff," says Jane Flinn, a psychologist at George Mason University in Fairfax, Va., who specializes in the effects of metals on learning, memory and Alzheimer's disease. Working with mice, she was one of the first to spot zinc's potential dangers. "You can fortify too much."
So far, side effects from zinc appear to be largely limited to people who have sprayed it deep into their sinus passages, slathered lots of denture cream on their gums, taken huge doses of supplements for medical conditions or drunk water from private wells that use galvanized pipes or tanks, especially in high-zinc areas. But because many Americans think that if a little of something is good then a whole lot might be better, some doctors and nutrition experts are raising red flags now.
For most people, zinc is a good thing, but troubles come with both too little and too much of it, says Ruth Frechman, a registered dietitian in Burbank and a spokeswoman for the American Dietetic Assn. Some Americans may need to work harder to get enough zinc, she says, including people older than 60, vegetarians, alcoholics, pregnant women, exclusively breastfed babies between 7 and 12 months old, and people with certain diseases. People who pound supplements, on the other hand, are at danger of getting too much.
Zinc keeps bodies humming in all sorts of ways. It's involved in DNA repair, wound healing and the sense of smell. It allows about 100 enzymes to do their jobs.
Zinc deficiency is a major public health crisis among poor people in developing countries. Not getting enough of it can damage memory, eyesight, taste buds and the immune system. Among other symptoms, zinc deficiency can cause growth retardation and diarrhea, especially among children. Zinc supplementation in these places can go a long way toward saving lives.
In North America, though, scientists are starting to recognize that getting too much zinc might be a bigger problem. The recommended daily intake is 11 milligrams for men and 8 milligrams for women. In a nation of plenty, it's easy to exceed those amounts.
There are more than 75 milligrams of zinc in six oysters, nearly 9 milligrams in a 3-ounce serving of cooked beef shanks, more than 3 milligrams in a cup of baked beans, 15 milligrams in a cup of some fortified cereals and 15 milligrams in many multivitamins.
All that zinc adds up. Studies show that consuming at least 50 milligrams a day for a few months could lead to copper deficiency, which can cause anemia, bone loss, nerve damage and other problems. Taking in 80 or 100 milligrams or more for months or even years can cause bigger problems, some irreversible. A typical, over-the-counter zinc supplement contains 50 milligrams. There are 13 milligrams in one popular brand of zinc lozenges.
In one of the first papers pointing to zinc's potential dangers, Flinn and colleagues fed zinc-enhanced water to pregnant rats and to their babies after birth. The water contained hundreds of times as much zinc as normal tap water. Three months later, it took longer for the zinc-fortified rats to learn how to find a submerged platform in an underwater maze, compared with rats that weren't full of zinc.
After a six-month break from testing, the same rats (who had continued to drink zinc-enhanced water) were much worse at remembering where the platform was. They grew more anxious during the test, the researchers reported in 2005 in the journal Physiology & Behavior. The study also showed increased levels of zinc in the rodents' brains, further suggesting that zinc was causing cognitive harm.
And in November, in the Journal of Alzheimer's Disease, Flinn and colleagues reported Alzheimer's-like memory problems in mice that were fed a zinc-enriched diet.
Zinc, copper and iron are all found in the plaque that builds up in brains of Alzheimer's patients. And while researchers try to figure out what that means, data suggest that removing zinc from the brain slows mental decline, says Imre Lengyel of University College London's Institute of Ophthalmology who has published studies on links between zinc and macular degeneration.