Longitudinal analysis of vitamin D levels considering sunshine duration and suggestion for a standardised approach for vitamin D supplementation in children and adolescents with obesity

Background Children with obesity have low 25 hydroxy-vitamin D (25-OH-D-3) levels compared to lean children. Recommendations on when to start vitamin D supplementation differ largely between countries. Longitudinal data on 25-OH-D-3 levels to guide treatment decisions are scarce since they are largely influenced by solar radiation and are difficult to compare. Methods We carried out a retrospective analysis of multiple 25-OH-D-3 and parathyroid hormone (PTH) measurements in a cohort of 543 patients without vitamin D supplementation. All measurements were taken at the local paediatric obesity clinic as documented in the German-Austrian-Swiss APV (Prospective Documentation of Overweight Children and Adolescents) registry from 2009 to 2019. Serial 25-OH-D-3 and PTH levels were adjusted for sunshine duration over the last 30 days to account for seasonal variation, as well as for sex and body mass index (BMI). We further performed an exploratory analysis of the association of sunshine duration, sex, BMI SDS (standard deviation score), abnormal lipid levels or dysglycemia with the 25-OH-D-3 trend. Results 229 obese patients (mean BMI SDS: 2,58 (+/- 0,56), 53% females, mean age: 12 (+/- 3) years, range: 2-21 years) with two, 115 with three and 96 with four repeated 25-OH-D-3 measurements were identified. Mean adjusted 25-OH-D-3 (48.2 nmol/l) and PTH (34.9 ng/l) levels remained stable over 120 weeks. 5% of the patients had an elevated PTH > 65 ng/l. High total cholesterol >= 200 mg/dl and high triglycerides >= 130 mg/dl were associated with higher 25-OH-D-3 levels. Conclusion We propose a simple method to include sunshine duration in the analysis of 25-OH-D-3 levels to minimise the bias of seasonal variation. Based on our data we established the pragmatic strategy of limiting vitamin D supplementation to patients with biochemical signs of mineralisation disorders such as elevated PTH and alkaline phosphatase (AP). In children with normal PTH and AP we recommend adjustment of calcium intake and increase of outdoor activity instead.