That zincing feeling.

You'd think it were a miracle cure.

If you believe everything you hear, zinc can jump-start your immune system, soothe your swollen prostate, quench an oncoming cold, and keep you from going blind.

It can also restore your faded sense of taste, clear up your pimples, make your wounds heal more quickly, and (wink, wink) help you perform better in the you-know-what department.

The scientific evidence tells a different tale. If you're deficient in the mineral, taking it could help with some of those problems. But it's almost impossible to tell if you're deficient. And if you're not and you take too much zinc, you could end up hurting yourself.

ZINC BASICS

"Zinc is necessary for the translation of DNA instructions, for the reproduction of cells, for the activity of the immune system, for growth and sexual development, and much more," says researcher Harold Sandstead of the University of Texas Medical Branch in Galveston.

"Deficiencies could be widespread in the U.S.," he adds, echoing the opinion of some other researchers. But does "widespread" mean five percent or 25 percent of the population? Scientists can't say, because they can't easily identify who's deficient and who isn't.

"We don't have a simple measure of zinc status," explains Ananda Prasad of Wayne State University in Detroit. "Blood levels can be misleading because they fluctuate for reasons having nothing to do with available zinc in the body."

"Perhaps the best indicator of whether you're getting enough zinc is whether you eat enough protein," adds Sandstead. Meat, poultry, shellfish, milk, yogurt, cheese, and beans are ail rich in the mineral. Groups that might not get enough include young children, the elderly, and some premenopausal women.

Why premenopausal women? "A lot have cut back on foods like red meat for health reasons," says Sandstead.

THE IMMUNE SYSTEM

"Zinc is absolutely essential for a good, working immune system," says Prasad.

He recently detected signs of depressed immunity in 36 of the 118 elderly white women in Detroit whom he found to be zinc-deficient. (He measured the concentration of zinc in their white blood cells.) Giving some of the 36 women 30 mg a day of zinc raised their levels of thymulin and interleukin 1, two signals that their immune systems were more able to fight off disease.[1]

But, Prasad cautions, "there is no evidence that people who aren't deficient gain any benefit from taking extra zinc" (see "Zinc 101").

COLDS

When inventor George Eby of Austin, Texas, gave his ailing daughter a zinc lozenge more than 15 years ago, it seemed to cure her cold within a few hours.

Could something as simple as zinc do what the manufacturers of Contac and Nuquil can't? Probably not. Since Eby's observation, three studies have failed to find any significant benefit from zinc. Two others did, but they were flawed.

In the best of the two, students at Dartmouth College who sucked on lozenges with 23 mg of zinc every two hours for the length of their colds reported that their illnesses ended about a day earlier than the colds of students who were given (lookalike but supposedly inactive) placebos.[2] But the placebos may not have been inactive, because students who took them earlier seemed to get better more quickly than students who took them later. Another study using similar lozenges has been completed, but the results haven't yet been published.

"If zinc were really effective, you wouldn't see such inconsistent results in the research," says Arnold Manto, a respiratory diseases expert at the University of Michigan.

THE PROSTATE

It's a burr in the saddle for half of middle-aged and older men. If the prostate becomes inflamed, it's called prostatitis. If it becomes enlarged, that's benign prostatic hypertrophy (BPH). Either spells discomfort ... not to mention annoyingly frequent trips to the john.

"Zinc is essential for some functions of the prostate," says William Fair, who is chief of urology at the Memorial Sloan-Kettering Cancer Center in New York.

That may explain why the mineral keeps turning up in "men's" formulas like Super Male-Plex and Maxi Pros Plus. Don't waste your money.

In studies conducted at Stanford University 20 years ago, Fair found that men with BPH or inflammatory prostatitis (the most common type) didn't have any less zinc in their prostatic fluid than men without prostate trouble.[3]

"We did find that men with bacterial prostatitis, which is a much less common form, had lower levels of zinc," he explains. "But the levels didn't change--and their conditions didn't improve--when we gave them 50 to 100 milligrams a day of zinc for three to six months."[3]

Fair's conclusion: "I don't know of any good scientific studies that show that zinc supplements are helpful in treating any disease of the prostate."

EYE TROUBLES

Macular degeneration is a progressive deterioration of the retina. It's the leading cause of blindness among older Americans. It's also incurable.

After a 1988 pilot study suggested that about 80 mg a day of zinc might slow the disease, some companies started adding the mineral to over-the-counter eyecare products. But the pilot study was small, and even its authors cautioned against reading too much into the results.[4]

The National Eye Institute and several pharmaceutical firms are sponsoring trials to see if zinc plus other nutrients can reduce the risk of macular degeneration and cataracts. Results won't be available for at least a few years.

OTHER CLAIMS

* Taste. "The protein that stimulates the growth of taste buds, which is called gustin, must have enough zinc available to do its job," explains Robert Henkin, director of the Taste and Smell Clinic at Georgetown University in Washington, D.C. "Little zinc, little gustin. Little gustin, impaired taste."

That's apparently why healthy college students who were fed low-zinc diets had trouble distinguishing one taste from another.[5]

But "taste loss can occur from a variety of causes, many unrelated to zinc deficiency," explains Henkin. "So giving zinc supplements to anyone who complains of a taste loss is probably of little value." And, adds Prasad, "zinc won't do a thing for those who are already getting recommended levels."

* Wound Healing. Poor wound-healing is a sign of a possible zinc deficiency, especially in the elderly, says Harold Sandstead.

In one study, the bed sores of elderly hospital patients with low levels of zinc in their blood cleared up more quickly when they were given 245 mg a day for six weeks. Patients with normal blood levels of zinc healed no quicker when given zinc, though.[6]

* Acne. "The jury's still out on using zinc supplements to treat acne," says Alan Shalita, chair of the department of dermatology at the State University of New York Health Sciences Center in Brooklyn.

"In our study, we found that zinc didn't improve ordinary inflammatory acne," he adds. "But some European dermatologists insist that zinc works on the pustular kind," which is less common.

* Sex Drive. So is it true what they say about oysters? Do they or other rich sources of zinc keep men virile?

"There is no evidence whatsoever that zinc supplements will enhance sexual function or potency," says Marc Goldstein, director of the Center for Male Reproductive Medicine and Microsurgery at New York Hospital-Cornell Medical Center.

RELATED ARTICLE: Zinc 101

The Recommended Dietary Allowances (RDAs) for zinc are: 12 milligrams (mg) a day for women, 15 mg a day for men and pregnant women, and 16 to 19 mg a day for breastfeeding women.

Middle-aged Americans average 8 to 12 mg a day from their food, while seniors average 6 to 9 mg. Most multivitamins contain 15 mg--enough, say experts, to prevent a deficiency.

But just because a little is good, a lot isn't necessarily better. "Don't regularly consume more than 50 milligrams of zinc a day from food and supplements combined," cautions Ananda Prasad.

At 50 to 100 mg a day or more (from supplements and food combined), zinc can weaken your body's antioxidant defenses, cause your HDL ("good") cholesterol to drop, and make your immune system less able to fight off disease.

RELATED ARTICLE: THE BOTTOM LINE

* Zinc is most abundant in oysters, fortified cereals, meat, poultry, dairy products, beans, and peanuts. So if you get enough protein (and most Americans do), you probably get enough zinc.

* While it's difficult to diagnose a zinc deficiency, see a doctor if you have: wounds that take long to heal, recurring infections that don't clear up easily, or diminished taste.

* For insurance, it wouldn't hurt to take a daily multivitamin-and-mineral supplement that contains the USRDA, or DV, for zinc--15 mg. Just make sure that it also has at least 2 mg of copper (which zinc can impair your body's ability to absorb).

* Don't take more than 30 mg a day of zinc from supplements unless a doctor tells you to.

[1] Nutrition 9: 218, 1993. [2] J. of Int'l. Medical Research 20: 234, 1992. [3] W. R. Fair and W.D.W. Heston, pp. 129-140, in Zinc Metabolism: Current Aspects in Health and Disease, eds. G. J. Brewer and A. S. Prasad. (New York: Liss, 1977). [4] Archives of Ophthalmology 106:192, 1988. [5] J. of Clinical Investigation 82: 1202, 1988. [6] Lancet 2: 780, 1972.