We aren't aware of an asthma diet that controls asthma by reducing airway inflammation. For example, eliminating dairy products is unlikely to improve asthma control. However, beverages such as strong coffee that contain caffeine provide some relief for an hour or two since caffeine is a mild bronchodilator. But taking a rescue asthma inhaler is much more effective for the temporary relief of asthma symptoms.
There is some evidence that people who eat diets higher in vitamins C and E, beta-carotene, flavonoids, magnesium, selenium, and omega-3 fatty acids are less likely to have asthma. Many of these substances are antioxidants, which protect cells from damage. One study showed that children who grew up eating a Mediterranean diet -- high in nuts and fruits like grapes, apples, and tomatoes -- were less likely to have asthma-like symptoms. On the other hand, eating omega-6 fats and trans fats, found in some margarines and processed foods, may worsen asthma.
For more information, read WebMD's Asthma and Diet: What You Should Know.