Ruth Jessen Hickman, MD
Can asthma be prevented or controlled through a patient’s diet? At this point, the jury is still out on the extent to which nutrition affects asthma, although some studies have yielded interesting results warranting additional research. This research indicates that dietary influences on asthma pathophysiology are additive, synergistic, and complex — and that for a truly profound change in patients’ disease course, comprehensive dietary interventions and related systemic social changes may be needed.
Guidelines from the Global Initiative for Asthma (GINA) cite evidence suggesting a diet high in fruits and vegetables may lower the risk for asthma and for declining lung function in people with asthma, and that increased intake of fruits and vegetables may lead to improved asthma control. The guidelines also recommend that practitioners ask patients and family members about diet and exercise and encourage them to eat a diet containing many fruits and vegetables for its overall health effects.1
Recent research strongly suggests an association between diet and asthma, said Yueh-Ying Han, PhD, a research associate professor of pediatrics at the University of Pittsburgh School of Medicine, who has studied multiple aspects of the intersection of diet and asthma. In light of this research, she urges all clinicians caring for patients with asthma to provide education on the potential positive impact of diet on asthma.
Micronutrients and Asthma
In assessing the potential effects of diet on asthma, many researchers have focused on individual micronutrients. This approach has the advantage of being potentially easier to study compared with full dietary analyses. The ideal goal of such micronutrient research: to identify a supplement providing asthma-related benefits that could be administered as a simple pill.2
But this magic bullet has yet to be found, said Dr Han. “Currently, we don’t really have very good and robust recommendations for a specific micronutrient,” she noted. There is no definitive evidence pointing to a recommended pill or supplement that can aid in preventing asthma or decrease asthma symptoms, she explained.
Nevertheless, research suggests that micronutrients have a potent role of in the pathophysiology of asthma. For example, research has shown that vitamin D may affect asthma through regulating gene expression, altering immune responses, decreasing impact from viral illnesses, and enhancing responsiveness to steroids.3 Vitamins with antioxidant properties such as A, C, and E may decrease oxidative stress and decrease inflammation; in people with and without asthma, serum levels of such antioxidants have been positively associated with lung function measured by forced expiratory volume in 1 second (FEV1).3
Yet results from human trials for vitamin D and other micronutrients have been mixed.2 The authors of a recent systematic review that included both prospective observational studies and clinical trials could not identify a causal relationship between vitamin D intake and risk for asthma in young children; notably, many of the studies included in the review were of poor quality and had potential for bias.4
The effect of micronutrient intake during pregnancy and childhood has been another focus of research into diet and asthma prevention. Maternal vitamin D deficiency in pregnancy was linked to an increased risk for childhood asthma in several studies, and 2 large randomized controlled trials demonstrated the primary preventive effects of high-dose vitamin D until age 3 years, but not at age 6 years.5
Other research has explored how asthma is affected by methyl donors (eg, folate, vitamin B12, choline), long-chained polyunsaturated fatty acids, and other micronutrients. Thus far, however, the experimental studies done haven’t translated into consistent human findings that might lead to clear micronutrient interventions in asthma.2,5
Macronutrients and Asthma Prevention
Over the past decade or so, researchers have started looking more closely at the effect of overall diet on asthma and on other chronic diseases, said Dr. Han. With this research, instead of looking at specific elements of a patient’s diet, researchers are assessing the overall impact of diet on weight, as adiposity can increase inflammatory signaling molecules and increase the risk and severity of asthma.6 The overall diet may also have a synergistic effect in terms of both micronutrient and macronutrient intake.
For example, researchers have developed a scoring system — the dietary inflammatory index — to estimate the inflammatory potential of patients’ diets, based on data from both animal and epidemiologic studies. High fat meals, for instance, which have been shown to correlate with higher levels of inflammatory cytokines, have higher scores. In contrast, lower scores are assigned to factors such as n-3 polyunsaturated fatty acids (PUFAs), fiber, vitamins E and C, β-carotene, and magnesium, which have been shown to be associated with lower levels of inflammation.7
Using this scoring system, Wood et al found that the diets of patients with asthma were proinflammatory relative to the diets of healthy individuals in a control group, and that individuals with higher-inflammatory diets had greater odds of having asthma, with the odds especially high among those with diets that were inflammatory in the extreme. Wood et al also found that a worse inflammatory score with respect to diet was associated with both increased inflammatory signaling and lower lung function assessed by FEV1.7
Researchers have devoted a significant amount of attention to the Mediterranean diet, characterized by a high intake of fruits and vegetables, nuts, fiber, and olive oil and a low intake of dairy and red meat.5 Findings, although promising, have been somewhat mixed. A 2013 meta-analysis of 8 studies concluded that adhering to a Mediterranean diet during childhood was associated with a reduced risk of wheeze and asthma.5,8 However, a later 2019 meta-analysis of 12 heterogenous observational studies found no conclusive association between Mediterranean diet during pregnancy or early childhood and risk of diagnosed asthma later in childhood, although the Mediterranean diet was found to have a protective effect with respect to wheezing.5,9 Additionally, a large, cross-sectional study of more than 50,000 children did find that consumption of fruits, vegetables, and fish correlated with a reduced asthma prevalence.10
Thus, whereas recent research strongly suggests an association between diet and asthma prevention, future randomized controlled trials are needed to causally confirm this association.
Patient Education and Health Disparities
Dr. Han urged all clinicians caring for patients with asthma to provide education on the potential positive impact of diet on asthma in a practical, encouraging way, making patients aware of food choices that might positively improve their health. However, she noted that it’s important to be realistic and sensitive to the fact that these food choices are not equally available to some patients, such as patients who face overall food insecurity, who live in food deserts, or who otherwise don’t have social or economic access to fresh fruits and vegetables.
In the US, said Dr. Han, individuals from populations with the highest incidence of asthma — including individuals who are Black or Puerto Rican and individuals who are socioeconomically disadvantaged — may face multiple challenges that can affect their disease course, in addition to diet and food availability. Such challenges include decreased access to health care resources, poverty, increased stress, and increased exposure to sources of both indoor and outdoor air pollution.
Currently, GINA guidelines note that people of different racial or ethnic backgrounds may have varying responses to asthma treatment, that these varying responses are not necessarily associated with genetic differences, and that diet is potentially one important factor underlying varying asthma treatment responses.1 To more fully address socioeconomic factors affecting varying treatment responses, there may be a need for broader preventative public health education, a greater amount of clinician time during the patient visit devoted to discussion of this topic, and structural community changes.
Asthma Incidence and Dietary Patterns
If asthma can indeed be prevented through diet, then dietary patterns would be expected to affect the incidence of asthma.
Notably, while the incidence of asthma in industrialized nations has largely plateaued since the 1990s, the incidence in lower to mid- income countries has been rising over the past several decades.11 Some researchers speculate that diet is a factor contributing to this disparity in asthma incidence.2,3,12 These researchers speculate that decreased intake of fruits and vegetables concurrent with increased intake of refined grains, red meat, and saturated fats, may partially account for the increase in asthma incidence in low- to mid-income countries.3
According to Dr Han, it would be challenging to tease apart the reasons behind these asthma incidence trends and to pinpoint the role that nutrition plays in worldwide differences in asthma incidence.
Ruth Jessen Hickman, MD, is a freelance medical and science writer living in Bloomington, Indiana.