This review article examines the efficacy of flavonoids in allergies. Randomized and controlled trials were systematically evaluated. This review found that various sources of flavonoids were associated with a reduction in symptoms in patients with allergic diseases. Although the synthesis is of low to moderate quality evidence, the results suggest that the use of flavonoids is a viable strategy for alleviating allergic symptoms. Future research is needed to determine the optimal dose, composition and duration of treatment, as well as the best source of flavonoids. In this context, more method-based clinical trials are needed to provide high-quality clinical evidence on the use of flavonoids in the treatment of allergic diseases.
Many allergic diseases, including asthma, allergic rhinitis and atopic dermatitis, have a multifactorial etiology and similar risk factors. These diseases are a major cause of morbidity worldwide and have a negative impact on health and medical systems in both developed and emerging economies. The estimated prevalence of allergic rhinitis is one in seven adults and children in the US (14% and 13% respectively), 7% in Latin America and 9% in the Asia-Pacific region. According to the Global Asthma Report 2018, asthma affects up to 339 million people worldwide and kills around 1,000 people every day. While there are effective medications used to treat allergic diseases, conventional therapeutics often do not completely eliminate symptoms and can have adverse effects with continued use. Therefore, new adjuvant therapies are needed that can target the triggering mechanisms of the allergic inflammatory response and improve patients’ quality of life. The development and progression of allergic diseases are related to an increased inflammatory response; therefore, some foods and nutrients with anti-inflammatory effects, such as flavonoids, may be useful in the treatment of these diseases. Due to their antioxidant, anti-inflammatory, antiviral and anticarcinogenic properties, flavonoids are part of a variety of nutraceutical, pharmaceutical and cosmetic applications. Preclinical and clinical reports have demonstrated the efficacy and safety of flavonoids. Flavonoids, including flavones, flavonols, flavanones, isoflavones and anthocyanins, may be effective in allergic diseases. Thus, patients with allergic diseases may benefit from flavonoid therapy alone or in combination with anti-allergic medications. To contribute in this way to a better evaluation of current clinical trials, we conducted a critical analysis of randomized clinical trials investigating the effect of flavonoids on allergic diseases. Our aim was also to improve the quality of the reports and prevent the spread of methodological errors that could affect the development of future studies and efficient clinical approaches.