Maintaining optimum vitamin D levels among under-five children is a growing concern given its important role in bone mineralization, remodelling, and immunological functions [1]. Nearly 90% of vitamin D requirement is met through exposure of bare arms, and face to midday sun (between 10 AM and 3 PM) for 10-30 minutes, and 10% through diet [2]. Vitamin D deficiency (VDD) therefore is more likely to be prevalent in overcrowded slums compromising adequate sunlight exposure. Studies have reported VDD prevalence ranging from 34 - 80% among children [3,4]. Very few investigated the determinants of VDD such as sun-exposure among under five children [3]. Hence, a community based study was undertaken to assess prevalence and determinants of VDD among children in the age group of 1-5 years.

A total of 426 apparently healthy children aged 1-5 years were enrolled from a selected urban slum of Mumbai after ethics approval and parental consent over a period of 2 years (1 October, 2015 to 30 September, 2017). There were approximately 20000 households in the study area. Initially the list of 759 children (aged 1-5 years) was obtained from anganwadi workers. A trained social worker visited these households to screen the eligible children. Apparently healthy children aged 1-5 years were included and children with chronic illness, skeletal diseases, and those receiving vitamin D supplementation were excluded. In case the house was locked, it was revisited in next three consecutive days. In case of more than one child in the household, krish grid method of sampling was used to select eligible children. Out of 759 children, 195 children were not eligible, parents of 21 children refused, 8 households were locked, 12 children were in households with more than one child, and parents of 97 children did not consent for phlebotomy.

Details about sociodemographic status, diet (24-hour dietary recall) [5], physical activity and clinical profile were recorded, followed by biochemical investigations. The nutrient composition including calcium and phytate was interpreted as per dietary guidelines for Indian children [5]. Direct sun-exposure was calculated considering average duration and percentage of the exposed body surface area between 10 AM to 3 PM over last 6 months [6]

Serum calcium, phosphorus, alkaline phosphatase, 25-Di-hydroxy vitamin D (25(OH)D) and parathyroid hormone (PTH) were estimated in fasting state using automated blood analyzer and commercially available ELISA based diagnostic kits. VDD was classified as 25(OH)D <20 ng/mL [7].

Association of VDD was assessed with variables such as age, sex, socioeconomic status, physical activity; nutrition intake; growth and clinical parameters and biochemical markers. Chi-square test, Pearson correlation and logistic regression were conducted using SPSS Version 19 (IBM Corp) and P<0.05 was considered as statistically significant.

The mean (SD) age of the children was 34.8 (13) months with 53.8% boys; 76% children belonged to middle socioeconomic group; 84% were in preschools and 7.5% were involved in outdoor activities at school.

The prevalence of VDD among children was 76.8% (95% CI 73.1-80.5). There was no association of VDD status with age and socioeconomic status; though, it was significantly associated with duration of sun-exposure of less than 10 minutes between 10 AM to 3 PM (P=0.01). Despite sun exposure of 10 to 45 minutes in a day, 68.8% children had VDD.

The levels of PTH, alkaline phosphatase and calcium were within normal limits among 83.9%, 92.2% and 87.5% of children, respectively in spite of 25(OH)D <20 ng/mL, with a significant negative correlation (r=-0.12; P=0.02) between PTH and 25(OH)D. Clinical signs of VDD i.e., either genu varum, genu valgum, metaphyseal widening or frontal bossing were evident among 42.7% of children with significant association with frontal bossing (P<0.001), genu varum (P<0.001) and metaphyseal widening (P=0.02). It was more common among children having recurrent upper respiratory tract infections (URTI) (P<0.001).