Diet changes can partly explain the high burden of asthma in industrialised nations. Findings from experimental studies have stimulated many observational studies of the association between vitamins (A, C, D, and E) or nutrients acting as methyl donors (folate, vitamin B12, and choline) and asthma. However, observational studies are susceptible to several sources of bias; well conducted randomised controlled trials (RCTs) are the gold standard to establish whether diet has an effect on asthma. Evidence from observational studies and a few RCTs strongly justifies ongoing and future RCTs in three areas: vitamin D for the prevention or treatment of asthma, choline supplementation as adjuvant treatment for asthma, and vitamin E to prevent the detrimental effects of air pollution in patients with asthma. At present, insufficient evidence exists to recommend supplementation with any vitamin or nutrient acting as a methyl donor to prevent or treat asthma.