Zinc is an essential trace mineral that functions as part of about 300 different enzymes. As such, zinc plays a role in numerous biochemical pathways and physiological processes. More than 90% of the body’s zinc is stored in the bones (30%) and muscles (60%), but zinc is also found widely distributed in small amounts in virtually all body tissues. The richest dietary sources of zinc are seafood (especially oysters), meat, fish, eggs, and poultry. Because of the varied roles of zinc in the body, claims for zinc-containing dietary supplements are numerous, including those for improved wound healing, general immune system support (including reduced severity and length of colds and upper respiratory tract infections), and support of various “male” aspects of health (supporting a healthy prostate gland, preventing benign prostatic hyperplasia, and increasing fertility via enhanced sperm production.
Zinc lozenges have become one of the most popular natural approaches to treating the common cold and the scientific evidence generally supports this use for short periods of time (1-2 weeks). Zinc lozenges appear to reduce cold symptoms such as sore throats, hoarseness and coughing – and may even be able to shorten the duration of colds by a full day or so. Like vitamin C, zinc is an essential nutrient for optimal functioning of the immune system – and they both possess significant antiviral activity when consumed at elevated levels for a short period of time. It also appears, however, that some forms of zinc lozenges may be more effective than other forms (due to the total amount of ionized zinc that the lozenge actually releases into the mouth and throat). At least one study has shown that lozenges containing zinc gluconate plus citric acid, sorbitol or mannitol may not deliver high enough levels of ionized zinc – whereas those lozenges which contained glycine (an amino acid) appeared to deliver a higher quantity of ionized zinc.
Because zinc is an essential part of nearly 300 different biochemical pathways, “structure/function” claims can be made for the nutrient’s role in a wide variety of processes including digestion, wound healing, energy production, growth, cellular repair, collagen synthesis, bone strength, cognitive function, carbohydrate metabolism (glucose utilization and insulin production), and reproductive function. Even mild zinc deficiency has been associated with depressed immunity, decreased sperm count and impaired memory. Perhaps the most popular claim for zinc is for its role in immunity, where zinc delivered in lozenge form may interfere with the replication of the cold virus (rhinovirus).
Zinc deficiency is common in developing countries and in some elderly and athletic populations (Bogden et al. 1990, Bunker et al. 1994, Johnson and Porter 1997, Mahalanabis et al. 2002, Penny et al. 1999, Sazawal et al. 1998, Singh et al. 1994) and in these populations, zinc supplementation at dosages of 10-25mg/day – improves most markers of immune function and duration of illness (diarrhea and upper respiratory tract infections).
Evidence exists to support the use of zinc lozenges in reducing the duration and severity of colds. Although concentrated zinc lozenges can help kill cold viruses in the mouth and throat, it is important to begin using them as soon as possible following the onset of cold symptoms (ideally within the first 24-48 hours). Test tube studies have shown that zinc can block the cold virus from replicating – an effect that could help the body’s natural immune defenses “get a jump” on killing the viruses. Most studies of the effect of zinc lozenges (typically zinc gluconate or zinc acetate) on the common cold have shown that subjects in the supplement group tend to have fewer “symptomatic” days (on average 2-3 fewer sick days) compared to subjects receiving a placebo (measured in terms of coughing, sore throat, nasal congestion and headache).
Occasionally, high dose zinc supplements are recommended to diabetic patients. Such patients commonly suffer from increased loss of zinc and reduced body stores of zinc. High doses of zinc have been shown to mimic the effects of insulin in reducing blood sugar and promoting wound healing. These effects, however, should be considered preliminary and high dose zinc supplements are not recommended for diabetics except on the advice of their personal physician.
Exercise performance has also been associated with adequate zinc status – especially in athletes who avoid red meat, concentrate their diets too much on carbohydrates or follow an overly restricted dietary regime. Low zinc intake (below 3mg/day) has been linked to reduced activity of a zinc-containing enzyme in red blood cells called carbonic anhydrase (which helps red blood cells transport carbon dioxide from tissues to the lungs to be exhaled). Mild to moderate zinc deficiency can lead to significant reductions in the body’s ability to take up and use oxygen, remove carbon dioxide and generate energy during high intensity exercise.
Safety / Dosage
The short-term use of zinc at therapeutic doses for cold relief (see below) is assumed to be safe and chronic supplementation with zinc at levels 2-3 times the current RDA should not be expected to pose any significant adverse side effects. However, high doses of zinc are not recommended for periods of more than two weeks due to concerns of immune system suppression, interference with copper absorption and other long-term health effects such as increased risk for heart disease. High doses of zinc (gram levels) can cause nausea, diarrhea, and vomiting.
The Daily Value for zinc is 15mg per day – a level that should be adequate for support of bone metabolism and optimal physical performance. As therapy for colds, however, higher levels are required – with levels in the range of 13-23mg (in lozenge form) taken every 2-4 hours for no more than 2 weeks. These levels appear to be quite effective for reducing duration and severity of cold symptoms compared to not taking zinc lozenges. It is also important to note that other supplements, particularly high levels of calcium and iron can decrease zinc absorption, while complexation (chelation) with various amino acids (such as glycine, histidine and aspartate) or other organic compounds (such as gluconate or picolinate) may increase zinc bioavailability. Zinc supplementation can also reduce absorption of copper (Bonham et al. 2003) and iron (Donangelo et al. 2002), but may also potentiate the effect of supplemental vitamin A on night vision and immune parameters (Christian et al. 2001).
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EDITOR'S NOTE: This monograph can be found in The Health Professional's Guide to Dietary Supplements (Lippincott, Williams & Wilkins) by Shawn M. Talbott, PhD and Kerry Hughes, MS.