Asthma has increased in prevalence in recent decades¹, and now affects approximately 14% of youth worldwide². Over a similar period, there has been a shift from a traditional or prudent diet, to a western diet. Individual nutrients , and foods^4,5 are often studied as exposures for asthma outcomes, thereby potentially resulting in different conclusions than if diet quality was the exposure⁴. Asthma has many phenotypes. Consideration is warranted to two phenotypes: allergic asthma and non-allergic asthma, as atopy (described below) may infl uence diet and asthma⁷.

A study of diet quality and asthma in youth

The study was conducted to determine if diet quality is crosssectionally associated with asthma amongst youth. Data were collected from youth enrolled in a nested case-control study of the 1995 Manitoba, Canada Birth Cohort Study of Allergy, Genes and the Environment (SAGE)⁸. From responses to a food frequency questionnaire (FFQ) based on the FFQ used in the Nurses’ Health Study⁹, and adapted to suit the study, diet quality scores were created based on the Youth Healthy Eating Index (YHEI)¹⁰. The YHEI includes both a total diet score and component scores, based on recommendations from the United States’ Department of Agriculture’s Dietary Guidelines for Americans¹¹ and which are similar to those from Canada’s Food Guide to Healthy Eating¹². Youths’ scores were classified as either low or high, with a cut-off at the median.

In the study, 26.2% of youth had asthma, according to pediatric allergists’ assessments. Youth were skin prick tested to common allergens to determine atopy status. Youth with asthma were classifi ed as having either allergic asthma (asthma + atopy, n=107) or non-allergic asthma (asthma, no atopy, n=38).

Airway hyperresponsiveness (AHR) was also considered, which is informative in the presence of asthma-like symptoms but the absence of obvious airway obstruction¹³. AHR was classifi ed as mild, or moderate/severe, and compared to non-AHR. Of youth assessed, 44.3% were non-AHR, 23.5% had mild AHR and 32.2% had moderate/severe AHR.

Diet quality is relatively poor

FFQ were completed by 476/489 youth (mean age 12.6 ± 0.5 years). The median diet score was 33.5/85, which is suggestive of relatively poor diet quality. Component scores for whole grains, vegetables, fruit, dairy, snack foods and fried foods were around the 50th percentile. Component scores for multivitamin use and fish were very low.

Diet quality is not statistically signifi cantly associated with asthma, but high vegetable intake protects against allergic asthma

Diet quality and asthma were not statistically signifi cantly associated. With consideration to asthma phenotypes, the results showed that high vegetable intake was associated with ~50% reduced odds of allergic asthma (p<0.02). No such associations were found for total diet quality, or other components and allergic asthma, or between diet quality and non-allergic asthma.

Diet quality is not statistically signifi cantly associated with AHR, but high vegetable intake protects against moderate/severe AHR

Similar to the results for diet quality and asthma phenotypes, only high vegetable intake was protective against moderate/severe asthma, again by ~50% (p<0.02). No associations were found between diet quality and mild AHR.

Clinical Implications and Conclusions

Although vegetable intake was associated with reduced odds of allergic asthma and moderate-to-severe AHR, it is too soon to advise patients that high vegetable intake protects against these conditions. Like other youth¹⁴, youth in the study had relatively poor diet quality. High vegetable intake should be encouraged as it is associated with many other health benefits.

Based on: Protudjer JL, Sevenhuysen GP, Ramsey CD, Kozyrskyj AL, Becker AB. Low vegetable intake is associated with allergic asthma and moderate-to-severe airway hyperresponsiveness. Pediatr Pulmonol. 2012 Dec;47(12):1159-69.