Upper Respiratory Infection: Nutritional Considerations

Diet has a significant effect on immune function. Notably, high–fat diets can decrease the effectiveness of the immune system, while certain micronutrients play important roles in promoting immune function. Unfortunately, many Americans regularly consume too much fat, and up to 50 percent get less than half the Recommended Dietary Allowance for many micronutrients. 

Although diet changes (e.g., increasing beta–carotene intake or reducing fat intake) have been found to stimulate immune function, these improvements have mainly been identified in clinical trials of nutrient supplements, rather than in trials of healthy diets. Clinical trials comparing the effects of various diets (e.g., high–fat vs. low–fat, omnivorous vs. vegetarian) have not yet been done, so potential benefits of dietary changes for the treatment or prevention of colds remain speculative.

The role of certain micronutrients in the prevention or treatment of URIs is discussed below:

• Vitamin mineral supplements: Older individuals are often deficient in a number of vitamins and minerals, putting them at risk for decreased immune function.  Some studies suggest that multiple vitamin–mineral supplements may reduce sick days and antibiotic use. The minerals zinc and selenium are known to be important in immune function and may be responsible for this effect.
• Zinc: Zinc is known to decrease viral growth and may enhance functioning of the immune system. Consequently, zinc lozenges can reduce the duration of colds. The formulation of zinc lozenges seems to influence their effectiveness. Many (e.g., zinc aspartate, zinc glycinate, and zinc orotate) are ineffective. In comparison, studies using other forms of zinc (e.g., zinc gluconate and zinc acetate) have found them to be an effective treatment. Several studies have also confirmed that intranasal zinc gel was effective for reducing the duration of colds.
  
  Zinc lozenges are maximally effective when used every two hours. Mild irritation of the mouth and stomach is common with zinc lozenges.
• Vitamin C: The utility of vitamin C for preventing or treating colds is widely accepted in the general population. However, most evidence supports the efficacy of megadoses for URIs only for individuals who are under significant physical or environmental stress, such as marathon runners, skiers, soldiers, and people exposed to severe cold. In these persons, the risk for developing colds was reduced by 50 percent vitamin C supplements, compared with individuals not using the supplements. Among the same groups, those taking vitamin C had 80 to 100 percent reductions in risk for pneumonia, compared with persons given placebo treatment.
• Vitamin E: High doses of vitamin E may improve immune function. In a study of elderly nursing home residents, 200 IU of vitamin E per day significantly reduced the incidence of common colds and the number of persons who got colds. Further research is required to determine whether vitamin E can reduce the risk of infections.
  
  Vitamin E supplementation may have the opposite effect in persons with established respiratory infections: supplements of 200 milligrams/day caused longer illness duration, more symptoms, and higher fever frequency.
• Echinacea: Anecdotal reports support the effectiveness of the common botanical echinacea, but clinical trials to date have been negative or inconclusive.
• Alcohol: Excessive alcohol intake can increase the risk of infection. Although small amounts (one to two drinks per day) do not appear to decrease immunity, alcohol abuse is known to significantly increase the risk of infection by depressing the immune system.