Upper Respiratory Infection: Nutritional Considerations

Upper respiratory infections are caused not simply by the presence of an invading microorganism, but also by the failure of the immune system to eliminate the intruder. Diet is a significant modulator of immune function. Notably, high–fat diets are immunosuppressive, while certain micronutrients play important roles in immune function. Unfortunately, many Americans regularly consume too much fat, and up to 50% get less than half the Recommended Dietary Allowance for many micronutrients.¹¹ Deficiencies of these nutrients are known to impair immune function.¹²

Although diet changes (eg, increasing the intake of carotenoid–containing foods or reducing fat intake) have been found to stimulate immune function,^13,14 these improvements have mainly been identified in clinical trials of nutrient supplements, rather than in trials of therapeutic diets. Clinical trials comparing the effects of various diets (eg, high–fat vs. low–fat, omnivore vs. vegetarian) are not yet available, so potential benefits of dietary alterations 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, predisposing them to a blunting of the innate and the adaptive immune responses.^15,16 Some studies suggest that multiple vitamin–mineral supplements may reduce sick days and antibiotic use. The trace elements zinc and selenium, known to be important in immune function, may be responsible for this effect.¹⁵

Zinc. Zinc ions inhibit rhinoviruses through several mechanisms: prevention of viral replication;¹⁷ potentiation of the antiviral action of native human interferon; and stimulation of T–cells¹⁸. Consequently, zinc lozenges can significantly reduce the duration of colds.¹⁹ The formulation of zinc lozenges seems to influence their effectiveness. Many (zinc aspartate, zinc glycinate, zinc orotate) bind zinc tightly and do not release the positively charged zinc ions that are the active principle, or they release negatively charged zinc ions that may actually increase the duration of colds. In comparison, studies using other forms of zinc (eg, zinc gluconate, zinc acetate) have found them to be an effective treatment.^17,19,20 Several studies have also confirmed that intranasal zinc gel was effective for reducing the duration of colds.¹⁷

Patients should be cautioned that irritation of the oral mucosa and mild gastrointestinal complaints are common with zinc lozenges, while nasal irritation occurs more frequently with gel and spray forms. In addition, zinc lozenges are maximally effective only when used every 2 hours. Nasal gel/spray formulations are most effective when used every 4 hours.

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 upper respiratory infections only for individuals who are under significant physical or environmental stress, such as marathon runners, skiers, soldiers, and people exposed to severe cold.^21,22 In these persons, the relative risk for developing colds was reduced by 50% when they took vitamin C supplements, compared with individuals not using the supplements.²¹ Among the same groups, those taking vitamin C had 80% to 100% reductions in pneumonia incidence, compared with persons given placebo treatment.²²

Vitamin E. In pharmacologic amounts, vitamin E reduces the production by cyclo–oxygenase of prostaglandin E2, a suppressor of T–cell function, and enhances lymphocyte proliferation and interleukin–2 production.²³ In 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.²⁴ Further research is required to determine whether vitamin E can reduce the incidence of infections.

Vitamin E supplementation may have the opposite effect in persons with established respiratory infections. Supplements of 200 mg/day caused longer illness duration, more symptoms, and higher fever frequency.²³

Echinacea. Anecdotal reports support the effectiveness of the common botanical echinacea, but clinical trials to date have been negative or inconclusive. A lack of standardization of purportedly active ingredients is another stumbling block that must be overcome before echinacea can be recommended for URI.²⁵

Excessive alcohol intake can increase susceptibility to infection. Although small amounts (1–2 drinks per day) do not appear to adversely affect immunity,^26,27 alcohol abuse increases the incidence of infectious diseases through depleting circulating lymphocyte populations and suppressing production of cytokines important in antimicrobial immunity.²⁶

Orders

See Basic Diet Orders chapter.

What to Tell the Family

The common cold is easily transmitted within the household. Family members should be encouraged to cover their mouths and noses while coughing and sneezing, and to promptly wash their hands. Refraining from touching the eyes and nose may also help prevent respiratory infections. Vitamins and other supplements may be beneficial in some, but not all, individuals.