CLINICAL

  NUTRITION

GET IN SYNC WITH ZINC

THE MINERAL PLAYS A CONTROVERSIAL ROLE IN SUPPLEMENTATION

THIS COLUMN is the third in a series on individual nutrients often found in multivitamin/mineral supplements and their link with eye health. This month’s focus: zinc, an incredibly versatile mineral not only in the eye, but also throughout the body.

Here, I provide an overview of the systemic and ocular effects of this mineral, along with the controversy surrounding supplementation.

OVERVIEW

Systemically, zinc is involved in more than 300 different enzymatic catalytic reactions. In addition, the structure of cell membranes and a number of proteins is dependent on zinc. Also, the naturally occurring antioxidant, superoxide dismutase, requires Zinc (and copper) to function. Further, zinc plays a central role in several other aspects of the immune system, including the development, replication, activation and function of many of the cells and signaling chemicals involved in immunity. In fact, taking zinc gels or lozenges at the onset of cold symptoms is a popular practice, with some evidence supporting the notion that it will reduce the duration and severity of symptoms. Finally, zinc regulates several functions of gene expression, cell signaling, hormone release and nerve transmission, facilitates the release of stored vitamin A from the liver and is a component of retinol-binding protein, which is necessary for the transport of vitamin A in the blood.

In terms of ocular health, zinc is involved in the enzymatic conversion of retinol into retinal, which is used to synthesize rhodopsin in the photoreceptors. In the eye, zinc is stored in the RPE cells and is released under conditions of oxidative stress. It plays several primary and supporting roles in this environment, including neutralization of free radicals. Several notable studies also report that zinc reduces progression of AMD and vision loss in doses ranging from 40mg to 200mg per day. AREDS and AREDS2 investigators concluded that the optimal dose of zinc in patients with category 3 AMD was 80mg when combined with 500mg vitamin C, 400 IU vitamin E and carotenoids (10mg lutein and 2mg zeaxanthin, replacing beta-carotene in the original AREDS supplement).

The current US RDA for zinc is 11mg for men and 80mg for women. This is a common range that is included in MVM supplements. High doses of zinc on a long-term basis (more than 40mg a day) can lead to copper deficiency, so supplemental copper (1mg to 2mg per day) is needed. Food sources of zinc include shellfish, eggs, beef and other red meats (higher bioavailability), as well as plant sources, such as nuts and legumes. Absorption is more limited from plant-based sources, due, in part, to the phytic acid content in plants containing zinc, which inhibits bioavailability. True zinc deficiency is rare, and zinc insufficiency is more common in developing countries than in the U.S. However, marginal deficiency — the numbers vary widely — may be more common than previously thought, especially in vegetarians and in patients with diabetes, in whom frequent urination may lead to zinc (and other nutrient) depletion.

SUPPLEMENTATION CONTROVERSY

Zinc is also involved in the complex reactions of the complement system within the photoreceptors and Bruch’s membrane. Zinc binds with complement factor H (CFH). CFH is a critical substance that can downregulate inflammation.

Exactly how zinc functions in this way is not fully understood, but the binding of zinc and CFH may allow for the accumulation of inflammatory substances within drusen bodies themselves. On the other hand, zinc is required for superoxide dismutase types 1 and 3, which are powerful antioxidants. So zinc may act as a pro-inflammatory substance in one cellular environment and an anti-oxidant in another. Whether a dose-response is at play, or whether genetic alleles for CFH can predict a proper zinc intake level remains to be determined. As is the case with many controversies in nutrition, it is challenging to isolate a single nutrient as being “good” or “bad” for all patients. Dose-response issues, absorption, dietary preferences, genetics, lifestyle choices, such as smoking (which is also known to have a significant deleterious effect on CFH) and other factors, which may not even be identified yet, may all contribute to the puzzle.

When deciding which supplement to recommend to your AMD patient, remember that the generally accepted standard of care includes a supplement that contains zinc — not zinc as a stand-alone supplement. Evidence is lacking that definitively concludes that AREDS or AREDS 2 supplements are harmful when used in the recommended combination.

This presents a tricky situation for you and your patients: Should you or shouldn’t you test them for risk alleles? Advocates of genetic testing — and I am in this camp — cite full informed consent as a guiding principle; if the technology exists for patients to have additional information, they should be informed of its availability, and partner with the doctor to determine whether testing is right for them. On the other hand, those opposed to universal genetic testing say the additional cost and potentially “bad” news are reasons to opt out of routine testing, at least until more evidence supports it. OM

KIMBERLY K. REED, O.D., F.A.A.O. is a professor at the NOVA Southeastern University College of Optometry in Fort Lauderdale, Fla., a member of the Ocular Nutrition Society and author of numerous articles on ocular nutrition, disease and pharmacology. She is also a frequent continuing education lecturer. To comment on this column, email Dr. Reed at Kimreed@nova.edu, or visit tinyurl.com/OMcomment to comment on this article.