# Diet and Asthma: A Narrative Review
## 4 Diet and Asthma
### Salt  
Dietary sodium having an influence on asthma was first proposed in 1987 based on a correlation observed between asthma mortality and table salt purchases [87]. Dietary changes associated with the development of a different, more rushed, lifestyle have been identified as one of the environmental factors that may have contributed to the rise in the prevalence of asthma, with sodium being identified as a dietary constituent that may be implicated in this phenomenon [88]. Data from a cross-sectional study found a lower potassium and a higher sodium intake in severe asthmatics patients compared to healthy controls [48]. It has been proposed that a high sodium intake may lead to hyperpolarization of the bronchial smooth muscle, causing asthma exacerbation [48], and dietary sodium intake has been shown to be positively associated with airway responsiveness [89]. However, a cross-sectional study did not corroborate these findings, as it found no significant associations between 24 h sodium urinary excretion samples and airway reactivity to methacholine in adults [90]. A meta-analysis of six randomized controlled trials that aimed to assess the effect of a dietary sodium reduction in patients with asthma found no evidence of an effect of salt intake on lung function measures in asthmatic adults [91]. Nonetheless, this systematic review suggested that there is a pattern of a small improvement in airway function and a small reduction in bronchodilator use with low-salt diets [91]. Another meta-analysis aiming to observe the effects of salt on asthma control also found no benefits of salt manipulation on asthma [92]. There was, however, some evidence that a low-sodium diet may improve lung function after exercise and possibly baseline lung function, according to the evidence from exercise-induced asthma studies, but these results were based on \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Fats and Fish** & & & \\ \hline
**Author, Year,** & & & \\
**Study Design** & **Sample, Age** & **Dietary Assessment/Intervention** & **Results** \\ \hline [PERSON] et al., 2011, & A total of 174 adult asthmatics. & Dietary intake was obtained by an FFQ. & High \(n\)-6:\(n\)-3 PUFA ratio: (\(-\)) airway inflammation; (\(-\)) asthma control. \\ [PERSON] et al., 2005, & In total, there were 1014 children, with 78 asthmatics. & Current consumption and frequency of fish. & Fish consumption: (+) asthma prevalence. \\ Cross-sectional [81] & Median age: 9 (range 5-14 years). & & Fish consumption: (+) asthma prevalence. \\ [PERSON] et al., 2013, & Adults and children, papers included in the meta-analysis (\(n=8\)), consisting of prospective cohort studies (\(n=8\)). & Fish and fish oil intake. & Fish consumption: (+) asthma prevalence in children; (=) asthma prevalence in adults. \\  & Children, papers included in the meta-analysis & & All fish (lean and fatty) consumption: (+) asthma prevalence prevalence in children up to 4.5 y. \\  & (\(n=23\)): cross-sectional studies & Fish intake. & (+) withdrew fresh consumption: (+) asthma prevalence in children 8 to 14 y. \\ Meta-analysis [83] & (\(n=12\)); case-control studies (\(n=2\)); cohort studies (\(n=9\)). & & Fatty fish consumption: (+) asthma prevalence in children 8 to 14 y. \\  & Papers included in the meta-analysis (\(n=19\)): Meta-analysis [84] & Fish intake during pregnancy & Fish consumption: (=) asthma prevalence. \\ \hline \hline \end{tabular} Notes: Age is described as mean age \(\pm\) standard deviation (SD) in years, unless stated otherwise. N/R, Not reported; FFQ, food frequency questionnaire; FEV1, forced expiratory volume in 1 s; PUFA, polyunsaturated fatty acids; TLR, toll like receptor; mRNA; and microRNA. (+) stands for significant and positive effects on asthma outcomes; (\(n\)) stands for no significant effects on asthma outcomes; and (\(-\)) stands for negative effects on asthma-related outcomes.  
\end{table}