Asthma: Nutritional Considerations

The following factors are under investigation for their roles in asthma:

Maintenance of ideal body weight. Studies have found that a higher body weight increases the risk of asthma in both children and adults.¹²

Modifying fatty acid intake. Recent reviews¹³ and prospective studies^14,15 have implicated omega-6 fatty acids (found in animal products and in margarine and other vegetable oils) as a possible risk factor for asthma. Consumption of these fatty acids has increased in Westernized societies along with a rise in asthma incidence, possibly due to their being precursors of leukotrienes with bronchoconstrictive effects. Studies have implicated margarine consumption as a risk factor for current asthma in both young adults¹⁶ and an older adult population.¹⁷ Also, a high ratio of omega-6 to omega-3 fatty acid intake was significantly associated with the risk for asthma in a pediatric population.¹⁴

Clinical interventions have not consistently demonstrated a benefit of adjustment in fatty acid intake in asthmatic patients. Increasing dietary intake of the omega-6 fat linoleic acid did not result in asthma exacerbation in 1 study,¹⁸ possibly indicating that animal sources of omega-6 fat (ie, arachidonic acid) may be responsible for these effects. Although fish intake has been associated with a lower risk for childhood asthma in certain studies,¹⁹ others have found associations between greater fish intake and an increase in asthma risk.^20,21 In spite of some data revealing improvement in exercise-induced asthma symptoms in individuals given omega-3 fatty acid supplements,²² intervention trials have not yet definitively established the benefit of this approach in asthmatic patients.²³

Controlled clinical trials have not yet established that a reduction in the intake of fat from margarine, other sources of omega-6 fatty acids, or fish is an effective treatment for asthma. Nevertheless, other reasons exist for limiting dietary fat intake in asthma patients, such as the current obesity epidemic. Obese patients may be at risk for acquiring asthma.²⁴ As mentioned in the Obesity chapter, the worldwide prevalence of overweight is directly related to the percentage of fat in the diet, and low-fat diets have been consistently shown to promote moderate weight loss.²⁵ For individuals who are underweight or may not benefit from a low-fat diet, the use of judicious amounts of monounsaturated fat from olive or canola oil may be a healthier alternative to vegetable oils, due to the latter's inconvertibility to pro-inflammatory leukotrienes.

Avoidance of salty foods. Pulmonary function improves with low-salt diets.²⁶ In persons with exercise-induced asthma, following a low-salt diet (1,500 mg/day sodium restriction, equivalent to about 3.7 g NaCl/day) reduces post-exercise asthma severity to below the diagnostic limit of a 10% fall in FEV1.²⁷

Fruits, vegetables, and other foods high in antioxidants. Several studies have found relationships between higher fruit and vegetable intakes and reduced risk for asthma.^13,28 In some studies, patients with asthma were found to have lower dietary intakes or blood levels of antioxidants.^29,30 Elevated levels of lipid peroxides indicative of oxidative stress have been found in these patients³¹ and appear to correlate with symptom severity.³² The Nurses' Health Study, a prospective study in adults, found that women in the highest quintile of vitamin E intake from food (not from supplements) had a 47% lower risk of adult-onset asthma than those in the lowest quintile. Studies have also found that supplemental antioxidants in the form of carotenoids (high-dose beta-carotene, lycopene, and other carotenoids) or combinations of vitamin C and vitamin E significantly improved exercise-induced asthma.²⁷

Avoidance of allergenic foods, beverages, and preservatives. Food-induced bronchospasm occurs with the intake of certain foods in 2% to 24% of persons with asthma. Foods implicated most often as a cause include peanuts, milk, eggs, tree nuts, soy, wheat, legumes, beans, and turkey.³³ The presence of both sulfating agents and histamine in wine may aggravate asthma,³⁴ and several studies have found that asthma may be induced by green tea.³⁵ Avoidance of trigger foods improves peak expiratory flow rate in asthmatic children.^36,37

When dairy products are omitted from the diet, calcium may be obtained from calcium-fortified soymilk or juices, green leafy vegetables, beans, and calcium-precipitated tofu. Allergy testing should be considered in patients who appear to experience exacerbation of asthma in relation to certain foods or food groups.

Alternatively, patients can attempt to determine if a food triggers asthma by eliminating all common potentially allergenic foods and then reintroducing them one at a time. Patients should keep careful records of food intake and any change in symptom frequency to confirm that a given food is provoking an exacerbation of asthma. 

Vegetarian and vegan diets. In a study of 27,766 Seventh-day Adventists,³⁸ vegetarian women reported a lower incidence of asthma, compared with women on nonvegetarian diets.³⁸ The theoretical basis for the value of vegan diets is the absence of potential triggers, particularly dairy products and eggs, as well as a relative lack of arachidonic acid. Observational studies have produced conflicting results, including some that suggest a protective effect of dairy product use in relation to asthma.³⁹ However, in a clinical trial of a vegan diet, 22 of 24 asthma patients noted significant improvements in vital capacity, FEV1, and physical working capacity after 1 year on a vegan diet.⁴⁰ Additional clinical trials are required to investigate the role of vegetarian and vegan diets.

Preventive measures. Measures recommended to decrease the risk for developing asthma include breast-feeding for the first 4 to 6 months of life and avoiding the following foods until children reach the specified ages: dairy products until at least 1 year old; eggs until at least 2 years old; and peanuts, nuts, and fish until at least 3 years old.⁴¹

Orders

Avoid allergenic foods per patient history.

Vegetarian diet, nondairy, may be tried on a prospective basis.

What to Tell the Family

Asthma can generally be well managed with diet and medications. Family members can help by encouraging a diet high in fruits, vegetables, and whole grains, and by minimizing fats and oils. Adopting such a diet themselves may help protect family members from later health problems, as well as make it easier for the patient to adhere to the dietary changes.