Asthma: Overview and Risk Factors
Asthma is a chronic respiratory disease marked by inflammation and obstruction of the airways, leading to shortness of breath, wheezing, chest tightness, and cough. In severe cases, additional symptoms may include difficulty taking deep breaths and difficulty finishing sentences.
Asthma is usually triggered by allergens. Other triggers include respiratory infections, inhaled irritants (particularly occupational exposures and tobacco), stress, exercise, cold temperatures, and medications (e.g., beta-blockers, aspirin, and nonsteroidal anti-inflammatory drugs such as ibuprofen).
About 5 percent of people in the United States have asthma. Although the disease commonly begins in childhood, up to 40 percent of patients first develop asthma as adults. An increase in the global rate of asthma over the past 30 years has been attributed to climate change, allergen exposures, urbanization, and air pollution, among other factors, but the precise reasons for this increase are not clear.
Asthma is more common in developed countries, leading to the question of whether dietary factors, chemical exposures, or overuse of antibiotics may be the cause. While air pollution is known to cause flare-ups of asthma, it is not clear whether it can cause the disease.
Types of Asthma
- Mild intermittent asthma: Symptoms occur only intermittently. This type is treated on an as-needed basis with inhaled medications, such as albuterol, that widen the airways.
- Mild persistent asthma: Symptoms are mild, but occur on a regular basis. This type is treated by daily use of inhaled steroids, along with albuterol when symptoms occur. Inhaled steroids have been shown to decrease the risk of flare-ups and hospitalizations, and reduce the need for albuterol. Common inhaled steroid medications include budesonide, fluticasone, triamcinalone, and beclomethasone.
- Moderate persistent: Symptoms are harsher and occur on a regular basis. This type is treated with an increased daily dose of inhaled steroids, in addition to other medications (e.g., salmeterol, montelukast, theophylline, and cromolyn) as well as albuterol during flare-ups. Failure to control symptoms with the use of two of the above medications suggests the individual may have severe asthma or perhaps another diagnosis.
- Severe asthma: Severe asthma requires high-dose inhaled steroids or oral steroids, along with other controller medicines. This type of disease is serious, and patients may require frequent hospitalization.
In children, asthma occurs more commonly in boys. Among adults, however, the disease is most common in women over 40. African-Americans tend to have more severe disease, compared with whites.
Other risk factors include:
- Other allergic diseases (e.g., eczema or food allergy)
- Family history: About 75 percent of children with two asthmatic parents also have asthma.
- Environmental and occupational factors: These factors include tobacco smoke, animal dander, dust mites, plants, pollen, mold, enzymes, chemicals, and metals.
- Obesity: A large study, the Nurses' Health Study II, revealed that women with the highest body mass index had the greatest risk of asthma. The most obese had nearly three times the risk, compared with individuals of normal weight. Another study showed improvement in asthma severity and control for those who lost weight.
- The evaluation begins with a medical history and a physical examination.
- Lung function tests are used to diagnose asthma. Individuals are asked to blow forcefully into a tube and the force of exhalation is measured by a computer. During flare-ups or in patients who have had asthma for many years, the tests may show decreased force of breathing, obstruction to airflow, and decreased lung space.
- Blood testing is sometimes recommended, particularly in severe cases that require hospitalization.
- Skin testing may be used to identify allergens that may cause flare-ups.
With optimal asthma management, individuals should have no symptoms or exercise limitation, no flare-ups or need for oral steroids, no need for albuterol inhalers, and, overall, minimal medications and side effects.
- Smoking cessation and avoidance of heavily polluted areas are beneficial.
- Commonly used medications include steroids, albuterol, salmeterol, leukotriene antagonists (e.g., montelukast), and theophylline. In addition, omalizumab is a new medication that may be useful for individuals with advanced asthma. In general, inhaled steroids are the most important medication, with additional use of albuterol during flare-ups.
Asthma: Nutritional Considerations
The following factors are under investigation for their roles in asthma:
- Maintenance of healthy body weight: As noted above, studies have found that a higher body weight increases the risk of asthma in both children and adults. (Calculate your BMI with our online calculator.)
- Modifying fatty acid intake: Recent reviews and studies 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. Studies have implicated margarine consumption as a risk factor for current asthma in both young adults and an older adult population. Also, a high intake of omega-6 fatty acids compared with omega-3 fatty acids was associated with the risk for asthma in children.
However, clinical studies 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 flare-up in one study. Although fish intake has been associated with a lower risk for childhood asthma in certain studies, others have found associations between greater fish intake and an increase in asthma risk. In spite of some data revealing improvement in exercise-induced asthma symptoms in individuals given omega-3 fatty acid supplements, clinical trials have not yet definitively established the benefit of this approach in asthmatic patients.
- Avoidance of salty foods: Lung function appears to improve with low-salt diets. In persons with exercise-induced asthma, following a low-salt diet (1,500 milligrams per day of sodium) reduces asthma severity significantly.
- Fruits, vegetables, and other foods high in antioxidants: Several studies have found relationships between higher fruit and vegetable intakes and reduced risk for asthma. In some studies, patients with asthma were found to have lower dietary intakes or blood levels of antioxidants. The Nurses' Health Study found that women who had the highest vitamin E intake from foods (not from supplements) had a 47 percent lower risk of adult-onset asthma than those who had the lowest intake. Other 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.
- Avoidance of allergenic foods, beverages, and preservatives: Food-induced asthma occurs with the intake of certain foods in 2 to 24 percent of persons with asthma. Foods implicated most often as a cause include peanuts, milk, eggs, tree nuts, soy, wheat, legumes, beans, and turkey. The presence of both sulfur chemicals and histamine in wine may aggravate asthma, and several studies have found that asthma may be induced by green tea. Avoidance of trigger foods improves lung function in asthmatic children.
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 individuals who appear to experience flare-ups of asthma in relation to certain foods or food groups.
Alternatively, individuals 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 a flare-up of asthma.
- Vegetarian and vegan diets: In a study of 27,766 vegetarians, vegetarian women reported a lower incidence of asthma, compared with women on nonvegetarian diets. The theoretical basis for the value of vegan diets is the absence of potential triggers, particularly dairy products and eggs.
Observational studies have produced conflicting results, including some that suggest a protective effect of dairy product use in relation to asthma. However, in a clinical trial of a vegan diet, 22 of 24 asthma patients noted significant improvements in lung function after one year on a vegan diet. 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 four to six months of life and avoiding the following foods until children reach the specified ages: dairy products until at least one year; eggs until at least two years; and peanuts, nuts, and fish until at least three years.