As cases of asthma have worsened around the world, many people have begun to wonder if there is a relationship between diet and asthma. Wouldn't it be great if you could simply change your diet and your asthma would improve? Let's examine the effects of diet and asthma more closely.
You will notice a couple of things as you read about the relationship of diet and asthma. Diet appears to impact asthma when people are asked in general about the types of food they eat. For example, people eating a lot of fruit generally have less asthma or better controlled asthma than people who eat less fruit. On the other hand, when scientists treat asthmatics with the individual components of fruit believed to impact asthma, such as vitamins, there is not as much of an effect on asthma symptoms or prevention.
While the overall effect of clinical trials seems to indicate that supplementation with individual nutrients does not have a tremendous impact on asthma, there are other possible explanations. It may be that a combination of nutrients is essential to impact asthma. Alternatively, scientists may be studying the wrong nutrients -- there may be a compound in food that we have not yet identified that improves asthma.
Several studies surrounding the Mediterranean diet support this idea. The Mediterranean diet consists of increased intake of fruit, vegetables, nuts, whole grains, and unsaturated fatty acids. Patients adhering to the "complete cocktail" of a Mediterranean diet have been found to:
- Achieve better asthma control as measured by both symptoms and objective measures of lung function
- Have decreased allergy symptoms
- Have decreased development of asthma
This overall healthy diet seems to improve asthma among those people who stick to it.
Vitamins and Asthma
Let's take a look at individual vitamins and how they might impact asthma. A number of cross-sectional studies suggest that increasing the amount of fruit you eat decreases your risk of developing asthma and decreases asthma symptoms, such as:
In light of these findings, scientists began to look at relationships between asthma and specific vitamins in fruit that are thought to have antioxidant and other anti-inflammatory properties. These include:
Vitamin C: Vitamin C is a potent antioxidant that is thought to provide some anti-inflammatory properties in the pathophysiology of asthma. Foods such as broccoli, bell peppers, oranges, strawberries, and lemons contain significant amounts of vitamin C. Vitamin C is found in high concentrations in the lining of the lung.
Vitamin C deficiency is associated with lung dysfunction in both adults and children, and this may be due to possible increases in inflammation. Similarly, asthmatic children have lower vitamin C levels compared to children without asthma, which points to the possibility that lower levels of vitamin C lead to increased inflammation and increased risk or symptoms of asthma. However, vitamin C supplementation has not consistently led to significant improvements in asthma symptoms or objective measurements such pulmonary function tests or spirometry.
- Vitamin E: While vitamin E is less studied in asthma research compared to vitamin C, higher vitamin E levels are associated with less skin sensitization, lower IgE levels, and decreased inflammation in asthma pathophysiology. People with increased dietary, but not supplemental, intake of vitamin E less commonly develop asthma. However, randomized controlled clinical trials of supplemental vitamin E in asthmatics have not consistently demonstrated that taking supplemental vitamin E improved or prevented asthma or asthma symptoms.
- Vitamin A and Beta-carotene: Like the other vitamins discussed, vitamin A and beta-carotene are thought to have antioxidant properties that may improve inflammation in asthma. While these have been the least studied vitamins in asthma research, two small studies have shown that one week of eating foods high in these vitamins or taking supplements can help prevent exercise induced asthma. Because the studies were small, however, there are no current recommendations for vitamin supplementation in the prevention of exercise induced asthma.
- Zinc: Low levels of zinc have been found in patients with asthma and zinc deficiency is associated with inflammation, but may or may not actually impact asthma.
Diet and Asthma - Flavones and Flavenoids
Flavones and flavenoids are other antioxidant compounds found in foods such as citrus fruits, tea, red wine, and dark chocolate. These compounds are thought to stabilize mast cells, a type of cell prominent in the pathophysiology of asthma. While at least one study has shown that intake of flavones and flavenoids can improve chronic obstructive pulmonary disorder (COPD), a disease with a number of components similar to asthma, no studies have directly looked at asthma.
Diet and Asthma - Minerals
- Selenium: As with other antioxidants, cross-sectional studies have identified low selenium levels in asthma patients compared to non-asthmatics. You might then think that in parts of the world where people commonly have selenium deficiencies, asthma would be more common, but this is not the case. Patients taking selenium supplements report improved asthma control, but these results are not substantiated with improvements in objective asthma measurements like pulmonary function testing or airway hyperresponsiveness.
- Magnesium: Magnesium is a bronchodilator that has long been used in the acute treatment of asthma exacerbations. However, magnesium supplementation has not been scientifically demonstrated as a useful supplement to improve or prevent asthma.
- Sodium: While there is evidence that high-sodium diets are associated with airway hyperresponsiveness and increased smooth muscle contraction, studies restricting sodium intake have not provided evidence that this practice might improve asthma.
Diet and Asthma - Fatty Acids
Fatty acids, such as omega-3 polyunsaturated fatty acids from fish oils, are thought to stabilize cell membranes and provide leukotriene inhibition. Cross-sectional studies have demonstrated that increased fatty acid intake may protect against asthma, but individual trials with fatty acids have not demonstrated much protective effects.