Vitamin D and preschool children -- predictors of status and relationship with allergic and respiratory diseases in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Nutritional Science at Massey University, Albany, New Zealand

Abstract

Background The role of vitamin D in allergic and respiratory conditions is increasingly being recognised through an immune-modulatory role. The current evidence is inconsistent, with very limited data in preschool children, a target group with high prevalence of early childhood allergic and respiratory disease. There are little data on the vitamin D status and factors associated with vitamin D deficiency in the preschool age group in New Zealand. Knowledge of these factors can assist prediction of preschool children at risk of vitamin D deficiency, improving health outcomes. Aims and Objectives To describe the vitamin D status of a self-selected sample of preschool children and determine predictors of vitamin D deficiency in order to develop a predictive questionnaire to assess vitamin D deficiency in this age group, and to investigate the relationship of vitamin D status and prevalence of allergic diseases - eczema, food allergy, allergic rhinoconjunctivitis and asthma – and respiratory infections. Method A cross-sectional sample of 1329 preschool children aged 2 to <5 years from throughout New Zealand enrolled during late-winter to early-spring in 2012. 25-hydroxyvitamin D (25[OH]D) was analysed from dried blood spots collected using capillary sampling. Caregivers completed a survey describing their child’s demographics, factors known to affect vitamin D status and medical history of allergic and respiratory diseases. Predictors of vitamin D deficiency (25[OH]D <25nmol/L) were identified using multivariable logistic regression in a randomly selected sub-sample (n=929) for development of a predictive questionnaire, which was then validated by receiver operating characteristics (ROC) analysis (n=400). Results Mean (SD) dried blood spot 25(OH)D concentration was 52 (19)nmol/L. Vitamin D deficiency was present in 86 (7%) and vitamin D insufficiency (25[H]D <50nmol/L) in 642 (48%)children. Factors independently associated with the risk of vitamin D deficiency were female gender (OR=1.92, 95%CI 1.17-3.14), children of other non-European ethnicities (not including Maori or Pacific)

(3.51, 1.89-6.50), children whose mothers had less than secondary school qualifications (5.00, 2.44-10.21), who had olive-dark skin colour (4.52, 2.22-9.16), who did not take vitamin D supplements (2.56, 1.06-6.18) and who lived in more deprived households (1.27, 1.06-1.53). There were no children who drank toddler milk with 25(OH)D concentrations <25nmol/L thus these children had a zero risk of vitamin D deficiency. The predictive questionnaire had low sensitivity for the identification of children at risk of vitamin D deficiency (sensitivity 42%, specificity 97%). Children with 25(OH)D concentrations =75nmol/L had a two-fold increased risk for parent reported, doctor diagnosed food allergy (OR=2.21, 95%CI 1.33-3.68). No association was present between 25(OH)D concentration and prevalence of eczema, allergic rhinoconjunctivitis, asthma or respiratory infection. Conclusion Dried blood spot methods facilitated the measurement of 25(OH)D concentrations in a large sample of preschool children from throughout New Zealand. Prevalence of deficiency in winter was low (7%). The predictors of deficiency were consistent with those in previous studies of other age groups in New Zealand. The predictive questionnaire identified less than half of the children with vitamin D deficiency, so has limited diagnostic ability. In this sample of preschool children, vitamin D deficiency was not associated with allergic diseases or respiratory infections. In contrast, high vitamin D concentrations were associated with a two-fold increased risk of food allergy. This relationship between vitamin D status and allergic diseases is complex, and needs to be further investigated in the preschool age group.