Vitamin D Deficiency Associated With Increased Incidence of Gastrointestinal and Ear Infections in School-age Children

Kathryn A. Thornton, DMD, MPH; Constanza Marín, RD; Mercedes Mora-Plazas, MSc, RD; Eduardo Villamor, MD, DrPH

Disclosures

Pediatr Infect Dis J. 2013;32(5):585-593. 

In This Article

Materials and Methods

Study Population

This study was conducted in the context of the Bogotá School Children Cohort, an ongoing longitudinal investigation of health and nutrition in school-age children. Details on the cohort design[24] and vitamin D substudy[2] have been previously reported. Briefly, in February 2006, we recruited a randomly selected group of 3202 children aged 5–12 years who were enrolled in public primary schools in Bogotá. Because the public school system enrolls a majority of children from low-income and middle-income families in the city,[25] the sample is representative of children from these strata living in Bogotá. Information on sociodemographic characteristics and health habits of the children and their families was elicited from parents through a self-administered questionnaire at the time of enrollment (82% response). Anthropometric measurements and fasting blood samples were obtained from the children by trained research assistants who visited the schools in the following weeks. Using standardized techniques,[26] weight was measured to the nearest 0.1 kg with Tanita HS301 electronic scales (Tanita, Arlington Heights, IL), and height was measured to the nearest 1 mm with wall-mounted Seca 202 stadiometers (Seca, Hanover, MD).

During the academic year after enrollment into the cohort, parents or primary caregivers recorded daily information on the incidence of morbidity episodes using a pictorial diary that was distributed and returned weekly. The diaries have drawings that depict children with symptoms including vomiting, diarrhea, fever, stomach ache, cough and earache/discharge. Caregivers were asked to check each day the child demonstrated these symptoms. Recording in diaries does not require a high level of education, and previous studies suggest that they are adequate to capture incidence of morbidity in developing countries.[27–29]

The parents or primary caregivers of all children gave ritten informed consent prior to enrollment into the study. The study protocol was approved by the Ethics Committee of the National University of Colombia Medical School. The Institutional Review Board at the University of Michigan approved the use of data and samples from the study.

Laboratory Methods

Blood samples were collected by venipuncture in 2816 (88%) children at baseline, during the month of February 2006. This month is considered part of a slightly warmer time of year in Bogotá, but there is no substantial seasonal variation within the samples' collection period. Packed in ice and protected from sunlight, the samples were transported to the National Institute of Health (Bogotá, Colombia) where plasma was separated from an ethylenediaminetetraacetic acid-coated aliquot and cryopreserved at -70°C until transportation to the United States. Quantification of plasma 25-hydroxyvitamin D (25(OH)D), a biomarker of vitamin D status,[30] was completed in 479 randomly selected samples at the Clinical and Epidemiologic Research Laboratory of Children's Hospital Boston (Boston, MA). An enzyme immunoassay (Immunodiagnostics Systems Inc, Scottsdale, AZ) with a competitive binding technique was used to quantify concentrations of plasma 25(OH)D. The assay has a sensitivity of 5 nmol 25(OH)D/L, intraclass coefficient of variation (CV) of 5.3–6.7% and interclass CV of 4.6–8.7%. Samples were analyzed in duplicate.

Data Analysis

Vitamin D status, the main exposure of interest, was categorized according to plasma 25(OH)D concentrations as deficient (<50 nmol/L), insufficient (≥50 and <75 nmol/L) or sufficient (≥75 nmol/L).[31] The primary outcomes were rates of gastrointestinal and respiratory morbidity, including diarrhea, vomiting, diarrhea with vomiting, cough with fever and earache or ear discharge with fever. Report of these symptoms has been related to clinically diagnosed episodes of gastrointestinal and respiratory infections.[32–34] Rates were calculated as the number of days with each symptom or combination of symptoms divided by the number of days the child was under observation.

Four children without morbidity diaries were excluded from the analyses; thus, the final sample size was 475. To identify potential confounders of the association between vitamin D and morbidity, we first compared the distribution of baseline child and maternal characteristics according to vitamin D serostatus with the use of Cochran-Armitage and Wald tests for trend. Children's height-for-age Z scores and body mass index-for-age Z scores were estimated according to the World Health Organization reference.[35] Maternal body mass index was calculated with measured height and weight in 26% of mothers and from self-reported data in the remaining. Household socioeconomic status corresponded to the government's assigned stratum to each household for tax and planning purposes.

Next, we estimated incidence rate ratios (IRRs) and 95% confidence intervals (CIs) for morbidity among children with VDD and insufficiency compared with those who were vitamin D-sufficient using Poisson regression models with the log-link function. Adjusted estimates were obtained from multivariable models that included child's age, sex and household socioeconomic status as covariates. We assessed whether the associations of vitamin D with morbidity differed between girls and boys by testing an interaction term between vitamin D status and sex with use of the likelihood ratio test. All analyses were conducted with Statistical Analysis System software (version 9.2; SAS Institute Inc, Cary, NC).

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