Please note:

Any dietary or drug treatment with high doses of micronutrients may override the body's own control mechanisms; therefore, micronutrient therapies may be associated with potential side effects and toxicities. High-dosed micronutrients should not be used without medical supervision. 

 

Common cold

The use of zinc lozenges within 24 hours of the onset of cold symptoms, and continued every two to three hours while awake until symptoms resolve, has been advocated for reducing the duration of the common cold.

 

Among ten randomized controlled trials, five studies found that zinc lozenges reduced the duration of cold symptoms in adults, whereas five studies found no difference between zinc lozenges and placebo lozenges with respect to the duration or severity of cold symptoms.

 

A meta-analysis of published randomized controlled trials on the use of zinc gluconate lozenges in colds found that evidence for their effectiveness in reducing the duration of common colds was still lacking (38).

 

Despite numerous well-controlled trials, the efficacy of zinc lozenges in treating common cold symptoms remains questionable. Some experts have recommended that a person who does not show clear evidence of improvement of cold symptoms after three to five days of zinc lozenge treatment should seek medical evaluation (39).

 

Intranasal zinc preparations, designed to be applied directly to the cells lining the nasal passages, are also marketed as over-the-counter cold remedies. While two randomized controlled trials found that intranasal zinc gluconate modestly shortened the duration of cold symptoms (40, 41), three other placebo-controlled studies found intranasal zinc to be of no benefit (42, 43, 44). Of serious concern are several case reports of individuals experiencing loss of the sense of smell (anosmia) after using intranasal zinc as a cold remedy (45).

 

Age-related macular degeneration

Zinc is found in the retina at high concentrations declining with age; additionally, the activities of some zinc-dependent retinal enzymes have been shown to decline with age. Thus, zinc is hypothesized to play a role in the development of age-related macular degeneration (AMD). However, scientific evidence that zinc intake is associated with the development or progression of AMD is limited.

 

Observational studies have not demonstrated clear associations between dietary zinc intake and the incidence of AMD (46, 47, 48).

 

A randomized controlled trial provoked interest when it found that 200 mg/day zinc sulfate (81 mg/day elemental zinc) over two years reduced the loss of vision in patients with AMD (49). However, a later trial using the same dose and duration found no beneficial effect in patients with a more advanced form of AMD in one eye (50).

 

A large randomized controlled trial of daily supplementation with antioxidants (500 mg vitamin C, 400 IU vitamin E, and 15 mg beta carotene) and high-dose zinc (80 mg zinc and 2 mg copper) found that the antioxidant combination plus high-dose zinc, and high-dose zinc alone, both significantly reduced the risk of advanced macular degeneration compared to placebo in individuals with signs of moderate to severe macular degeneration in at least one eye (51).

 

Data from smaller trials have generally not observed a protective effect of vitamin and mineral supplementation on AMD (52).

 

At present, there is little evidence that zinc supplementation would be beneficial to people with early signs of macular degeneration, but further randomized controlled trials are warranted (53).

 

Diabetes mellitus

Moderate zinc deficiency may be relatively common in individuals with diabetes mellitus. Although zinc supplementation reportedly improves immune function in diabetics, zinc supplementation of 50 mg/day adversely affected blood glucose control in insulin-dependent (type 1) diabetics in one study (54). In a more recent study, supplementation of type 2 diabetics with 30 mg/day zinc for six months reduced a non-specific measure of oxidative stress without significantly affecting blood glucose control (55).

 

Presently, the influence of zinc on glucose metabolism requires further study before high-dose zinc supplementation can be recommended for diabetics (4).

 

HIV infection / AIDS

Sufficient zinc is essential in maintaining immune system function and HIV-infected individuals are particularly susceptible to zinc deficiency. In HIV-infected patients, low blood serum levels of zinc have been associated with a more advanced stage of the disease and also with increased mortality (56, 57).

 

In one of the few zinc supplementation studies conducted in AIDS patients, 45 mg/day zinc for one month resulted in a decreased incidence in opportunistic infections compared to placebo (58).

 

However, the HIV virus also requires zinc, and excessive zinc intake may stimulate the progression of HIV infection: in an observational study of HIV-infected men, increased zinc intake was associated with more rapid disease progression, and any intake of zinc supplements was associated with poorer survival rates (59).

 

Further research is necessary to determine optimal zinc intakes for HIV-infected individuals (20, 60).