Abstract:
Background:
An increase in body mass index (BMI) z-score and an unhealthy diet rich in sugar are known
to be positively associated with asthma incidence, although the evidence is mixed. The link
between scabies and asthma has never been investigated before.
Aim:
I explored the relationship between BMI z-score, diet, and the incidence of childhood asthma
in a cohort study group. The link between scabies and asthma was also investigated.
Design:
I conducted a study of 6,854 children participating in the “Growing Up in New Zealand”
cohort. Outcomes were diagnosis of “current” and “severe” asthma according to the
International Study of Asthma and Allergies in Childhood (ISAAC) parental questionnaire
when children were eight years old. Exposures were BMI z-score at two, 54 months, eight
years, and dietary intake information collected using a food frequency questionnaire (FFQ)
administered when the children were two years and 54 months old. Scabies diagnosis was
derived from either hospital diagnosis or prescription for permethrin. Food frequency
questionnaire information was summarised into two principal components. Potential
confounding factors, including gender, ethnicity, socioeconomic status, house dampness and
mould, maternal smoking and breastfeeding, were included in the analysis.
Results:
The prevalence of current and severe asthma was 10.9% (745/6,854) and 3% (207/6,854). The
diagnosis of these conditions was more likely in boys, Māori children, and children living in
the most deprived areas. Current asthma was also more likely in Pacific children. There is also
a strong association between mothers who reported smoking cigarettes and both conditions.
A t-test showed a statistically significant association between BMI z-score, measured in
children 54 months and eight years old, with current asthma. The mean difference in BMI zscore
in children 54 months old was – 0.1311 (95% CI: -0.2306, -0.0316, p-value=0.009), which
translated to a 221 grams difference for an average 54-months-old child of average height.
The crude odds ratio (OR) for BMI z-score at age 54 months and current asthma was 1.09
(95% CI:1.02-1.17, p-value=0.008), but the association was no longer statistically significant
after adjustment (OR=1.04, 95%CI: 0.97-1.12, p-value=0.255).
The mean difference in BMI z-score at age eight years was –0.2267, which represents, for an
average eight-year-old child of average height, a difference in weight of 2,070 grams between
children with current asthma and children with no such diagnosis (p-value <0.001, 95% CI: -
0.319, -0.0862). The crude and adjusted ORs for BMI z-score at age eight years were
respectively 1.13 (95% CI 1.07-1.20, p-value <0.001) and 1.08 (95% CI 1.02-1.16, pvalue=
0.016). The adjusted measure represents a 1,600 gram difference for an eight-year-old
child of average height. This positive association remained statistically significant in crude and
adjusted analyses after gender, Māori ethnicity, and maternal smoking were introduced into
the model.
No association was identified between BMI z-score at two years and current asthma or
between BMI z-score, assessed at two years, 54 months and eight years, and severe asthma.
A chi-square test showed a positive association between unhealthy diet, represented by the
highest quintile of the first two principal components derived from the food frequency
questionnaire administered at two years and 54 months, and both current and severe asthma
(p-value<0.001). Multiple logistic regression confirmed the same positive trend of association,
although neither crude nor adjusted ORs were statistically significant.
The prevalence of scabies in the cohort study group was 0.6% (41/6,854). No link between
scabies and asthma was found.