At Issue: Experts differ when it comes to recommending zinc supplements

Issue: October 1998

A: No, evidence is inconclusive

Srinivas Natrajan, DVM, PhD, OD, FAAO: Zinc, an essential trace element, is one of the most commonly found elements in the human body. It is a component of numerous enzyme systems, including ocular tissues, which in turn play an important role in the metabolism of proteins, lipids and carbohydrates.

The recommended dietary allowance of zinc is 15 mg/day for adult males, and 12 mg/day for adult females. Major dietary sources of zinc are shellfish (especially oysters), meats, liver, poultry, eggs and dairy products.

In recent years, attention has been focused on the role of zinc in halting the progression of AMD because the retina, the retinal pigment epithelium and choroid contain high concentrations of the trace element zinc. The data of a few pilot studies have indicated that the use of oral zinc sulfate (100 mg twice a day) in people with AMD was associated with a retardation of visual loss. Also, these studies pointed out the fact that taking excess zinc could be associated with potentially toxic side effects and complications such as copper-induced microcytic anemia and gastrointestinal irritation. A few studies, on the other hand, did not find a significant correlation between AMD and the levels of zinc in the serum.

Hence, it is very clear that the studies conducted so far are rather inconclusive in nature or weakly supportive, and there is no strong evidence to show that supplementing the diet with zinc would be beneficial in AMD patients. There fore, in my opinion, it would be unwise at this time to recommend the use of oral zinc in patients suffering from AMD until conclusive evidence from well-controlled clinical trials is published. However, I would recommend patients suffering from AMD to take a vitamin-mineral supplement and eat a lot of fruits and vegetables that provide antioxidants.

A: Take recommended daily doses

William L. Jones, OD, FAAO: Zinc, one of the most common trace minerals in our bodies, is highly concentrated in the eye, with greater concentrations in the tissues around the macula. Zinc is an integral part of more than 100 enzymes in the body (some of which are found in the retina); therefore, it is an important component of retinal metabolism. Because zinc is important for the health of the retina, some eye care professionals believe that supplements of zinc in the diet may slow down the process of macular degeneration.

Numerous studies concerning the effect of nutrition on macular degeneration have focused on the importance of zinc. Some studies have shown that older people have low levels of zinc in their blood due to either poor diets or decreased absorption of zinc from food sources. However, scientific studies are not conclusive at this time and there is no universal agreement among eye doctors about the value of zinc supplements.

There is some concern that too much zinc may interfere with other trace mineral such as copper. High doses of any element or vitamin may not be completely safe; therefore, caution should be used in prescribing high doses. Patients should be encouraged to take supplements with the recommended daily dose of zinc.

A: Yes, nutrition slows degeneration

Mark Sawamura, OD, FAAO: Zinc is a mineral found in high concentration in the choroid and retina. It scavenges toxic free-radicals and interacts with vitamin A to generate visual pigment in the photoreceptors.

In 1988, a study by Newsome involving 151 patients evaluated the use of 200 mg of zinc sulfate in patients with early AMD. Over 2 years, significantly more eyes in the treated group remained visually stable or accumulated less drusen. How ever, zinc supplementation in AMD patients remains some what controversial. Since Newsome's study, more research has supported the role of carotenoids and nutrients in stabilizing AMD. Current investigations are specifically targeting the effects of zinc upon the various stages of AMD; however, we are still eagerly awaiting the results. Improving nutritional intake can slow the development of degenerative diseases, so the use of vitamin supplements is beneficial.

Zinc is naturally found in meats, green leafy vegetables, oysters, whole cereals and eggs. Acquisition of vitamins and minerals should occur through a healthy, balanced diet, although that is not always possible. In certain patients - the elderly, those with high fiber intake, those with diarrhea - normal zinc absorption is poor. Zinc supplementation also may be affected by these conditions. Ocular vitamin supplements such as Ocuvite Extra (Bausch & Lomb Surgical, St. Louis) and ICAPS (Alcon, Fort Worth, Texas) provide 40 mg of elemental zinc per tablet, which is 267% of the U.S. recommended daily allowance. The suggested dosage for such preparations is one to two tablets per day at mealtimes.

Excess zinc may also cause gastrointestinal upset and, in high doses, result in copper intake deficiency (anemia). Time-released preparations release the nutrients at a slower rate, thus reducing intestinal irritation.

Patients who are diagnosed with AMD can benefit from zinc supplementation. Early intervention therapy in these patients should also be considered in those with a strong family history of AMD or those who are prone to skin cancer. Counseling should also address smoking, uncontrolled blood pressure and dietary intake, which all have a negative influence upon the disease.

For Your Information:

Srinivas Natrajan, DVM, PhD, OD, FAAO, is a professor of physiology, pharmacology and nutrition at The New England College of Optometry. He can be reached at The New England College of Optometry, Boston, MA 02115; (617) 236-6226; fax: (617) 424-9202; e-mail: natrajan@ne-optometry.edu. He has no direct financial interest in any products mentioned in this article, nor is he a paid consultant for any companies mentioned.

William L. Jones, OD, FAAO, can be reached at (505) 265-1711; e-mail: wm_jones@msn.com. Dr. Jones has no direct financial interest in any products mentioned in this article, nor is he a paid consultant for any companies mentioned.

Mark Sawamura, OD, FAAO, is an assistant professor of the Eye Care Clinic, Optometric Center of Fullerton, an affiliated clinic of the Southern California College of Optometry. He can be reached at SCCO, 2575 Yorba Linda Blvd., Fullerton, CA 92831; (714) 449-7407; fax: (714) 992-7811; e-mail: msawamura@scco.edu. Dr. Sawamura has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.