Critical Analysis: The Role of Nutrition in Asthma Prevention and Treatment

Key Issues with the Review Paper 

There is evidence of selective reporting of studies and results.   

• In contrast to systematic reviews which provide a strong level of evidence to inform guidance, narrative reviews such as this one do not have to adhere to any specific process, and are thus more at risk of being subjective and biased. Furthermore, there are expected “best practices” that should be followed with narrative reviews such as presenting a good overview of the evidence (as opposed to being selective AKA “cherry-picking”), presenting opposing studies or results, etc… These “best practices” do not appear to have been followed by this review paper.

Does not represent an accurate overview of the current state of the evidence.

• Despite the goal stated which was to review the “current literature,” the studies that were included in relation to the role of dairy and asthma are largely older (i.e. 1990’s and early 2000’s) and/or have weaker designs (e.g. cross-sectional studies). Several studies that were published in later years, many of which have stronger designs (e.g. prospective or longitudinal studies) and which show a beneficial role for dairy, have not been included. 

The authors misrepresent and/or misinterpret many of the studies they include such as omitting results that are beneficial and/or neutral for dairy (see section titled Studies on Dairy Included in the Review Paper). 

For more information, please see Milk Products and Asthma.  

Studies on Dairy Included in the Review Paper

A case-control study by Han et al² of 678 Puerto Rican children 6 to 14 years old in San Juan examined the association between diet and asthma in Puerto Rican children. High consumption of grains was associated with lower odds of asthma, whereas frequent consumption of dairy products was associated with higher odds of asthma.

• This study did not distinguish children with milk allergy which may have been a confounder. 

An intervention “pilot” study by Yusoff et al³ examined the effects of excluding eggs and milk on the occurrence of symptoms in children with asthma. Twenty-two children between 3-14 years and clinically diagnosed as having mild to moderate asthma were included in the study. Excluding milk and eggs caused a decrease in the concentrations of IgG in the blood and was associated with functional and clinical improvements. These beneficial changes tended to be lost when the diet was stopped.

• This study did not disentangle eggs and milk and included kids that may have had allergy to eggs and possibly milk. Also, parents allocated their kids into the experimental or control group which is an important limitation. 

A cross-sectional study by Woods et al⁴ of 1601 young Australian adults (mean age ~35 y) examined the food and nutrient intakes in individuals with asthma compared those without asthma. Whole milk and butter appeared to protect against current asthma, doctor-diagnosed asthma, bronchial hyperreactivity and atopy while ricotta and low-fat cheese increased risk of some of these measures. Other dairy products (cheese, ice-cream and yogurt) were not associated with asthma. Soy beverage was associated with an increased risk of current asthma and doctor-diagnosed asthma. 

• The review paper by Alwarith et al did not report results of this study which showed protective associations (i.e. whole milk and butter) or neutral associations (i.e. cheese, ice cream, yogurt) with some dairy foods, nor did they report on the harmful association with soy beverage. 

In a randomized, double-blind, placebo-controlled cross-over trial by Woods et al⁵, 20 adults with asthma were challenged with 300 ml of cow’s milk compared with placebo (i.e. rice beverage). A positive reaction was defined as a 15% reduction in both forced expiratory volume in 1 second (FEV1) and peak expiratory flow (PEF). For both FEV1 and PEF there were no statistically significantly differences in group means between active challenge and placebo challenge. The authors’ concluded that: “It is unlikely that dairy products have a specific bronchoconstrictor effect in most patients with asthma, regardless of their perception.” 

• The review paper by Alwarith et al misrepresents this study by suggesting an overall adverse effect with milk which is not in line with the findings and conclusions of this study. 

In a randomized, double-blind placebo-controlled cross-over trial by Nguyen et al⁶ of 25 adults with mild asthma, subjects were randomly assigned to a solution containing cow milk powder or a placebo solution. Forced vital capacity (FVC), forced expiratory volume in I second (s) (FEV1), and FEV1/FVC were measured. There were no significant differences in the groups.

• The review paper by Alwarith et al suggests adverse effects with milk which is not in line with the findings of this study. 

A randomized controlled trial by Haas et al⁷ compared the effects of ingesting 16 oz. of whole milk, skim milk, and water on several respiratory function measures: forced expiratory volume in 1 second (FEV1), forced expiratory flow (FEF), and pulmonary diffusing capacity (Dlco) in 11 asthmatic and 10 non-asthmatic subjects. The two milk types did not significantly change FEV1 or FEF in either group, indicating that the amount ingested did not change airway resistance sufficiently to alter airflow parameters. In the asthmatic group, however, Dlco decreased progressively over the 3 hours after whole milk but not after water or skim milk.

• The review paper by Alwarith et al does not include the finding that skim milk did not have any adverse impact on Dlco.  

Conclusions

This review paper, although published in a credible nutrition science journal, has several important weaknesses and limitations including selective reporting of studies and results and should therefore be interpreted with caution. 

The overall evidence to date does not support the assertions of this review paper related to dairy products. In fact, there is evidence to support that dairy products, especially regular fat dairy products, may decrease the risk of asthma and/or improve asthma control.