PURPOSE OF THE STUDY.
Previous studies have provided support for the role of low vitamin D levels in the increasing prevalence of asthma. This study examined the relationship between cord-blood levels of vitamin D and respiratory infection, wheezing, and asthma.
Cord blood from study participants (N = 922) was collected as part of a prospective birth cohort of 1105 children recruited by a random sample of midwives in the New Zealand Asthma and Allergy Cohort Study. Questionnaires were administered by study nurses at birth, 3 months, and 15 months and then annually between the ages of 2 and 5 years.
Cord-blood 25-hydroxyvitamin D (25[OH]D) levels were measured and categorized as ≥75, 25 to 75, or <25 nmol/L. The primary outcomes were the incidence of respiratory infection, cumulative wheeze, and incidence of asthma by 5 years of age based on answers to the questionnaires. Multiple confounding covariates were accounted for, including season of birth, ethnicity, and environmental tobacco smoke exposure. The linear regression or the Kruskal-Wallis test for continuous variables and the Wilcoxon-Mann-Whitney test for categorical variables were used to test for trend across vitamin D levels. Multivariable logistic regression models were used to test the association between cord-blood 25(OH)D levels and infection outcomes at 3 months of age.
Data were available for 882 (96%) children at 3 months of age and 823 (89%) children at 5 years of age. The median 25(OH)D cord-blood level was 44 nmol/L. An inverse association was found between cord-blood 25(OH)D levels and risk of respiratory infection by 3 months of age. Newborns with 25(OH)D levels of <25 nmol/L had an increased risk of respiratory infections (odds ratio [OR]: 2.04) and other viral infections (OR: 2.36) compared with those with levels of ≥75 nmol/L. There was an inverse linear association between vitamin D level and cumulative wheezing by 5 years of age but no association with asthma incidence. Every 10 nmol/L increase in cord-blood 25(OH)D level lowered the cumulative risk of wheezing by the age of 5 years (adjusted OR: 0.95).
This birth-cohort study revealed an inverse association between 25(OH)D cord-blood levels and the risk of respiratory and other viral infections by the age of 3 months and cumulative risk of wheezing by the age of 5 years. The 25(OH)D cord-blood levels were not associated with the risk of incident asthma.
The measurement of vitamin D in cord blood, but not at follow-up visits, is a major limitation of this study. Recall bias that resulted from the use of parent questionnaires to detect the outcomes of interest is another limitation. It remains unclear whether low vitamin D levels in utero cause increased respiratory infections and wheezing or if this low level is a marker for likely low vitamin D levels in the future. Further studies are needed to clarify this issue. This study adds to the body of evidence that suggests that low vitamin D levels might play a role in wheezing and respiratory infections in infants and children.
- Copyright © 2011 by the American Academy of Pediatrics