Vitamin D and dairy protein in childhood: Vitamin D status and the role of vitamin D and dairy protein in bone mineralisation, growth, and cardiometabolic markers in young children

Research output: Book/ReportPh.D. thesisResearch

  • Nanna Groth Stounbjerg
Background: Children living at northern latitudes are at high risk of inadequate vitamin D status due to low dermal synthesis during winter and low vitamin D intakes, but evidence of the prevalence and determinants among young children is scarce. Vitamin D and high-quality protein, e.g. from dairy products, are considered important for childhood bone health and growth, and may also affect cardiometabolic markers such as blood pressure, blood lipids, glucose homeostasis, and adiposity. However, studies of vitamin D supplementation and higher protein intakes are lacking in children, especially among young and healthy children. Further, results from existing vitamin D supplementation trials are inconsistent, and the specific effects of dairy protein, alone or in combination with vitamin D, has not been investigated.
Objective: The overall objectives of the present thesis were to investigate vitamin D status, its determinants, and its potential association with bone mineralisation, growth, and cardiometabolic markers in young children living in Denmark (55oN), and to investigate potential combined and separate effects of vitamin D supplementation and high dairy protein intake on these outcomes.
Methods: The thesis is based on two studies. Paper I was based on cross-sectional data from the 3-year visit of the SKOT cohorts and included 323, mostly white, children. Paper II and III included data from the 2×2 factorial, randomised 24-week winter trial, D-pro, in which 200 white children aged 6-8 years received blinded tablets with either 20 μg/day vitamin D3 or placebo and substituted 260 g/day of milk products in their usual diet with either high-protein (HP) or normal-protein (NP) yogurt. In both studies, anthropometry, blood pressure, and bone mineral content (BMC), area (BA), and density (BMD) by dual-energy X-ray absorptiometry (DXA) were measured. Vitamin D status was assessed as serum 25-hydroxyvitamin D (s-25(OH)D) by liquid chromatography-tandem mass spectrometry, and blood samples were further analysed for triglycerides and high- (HDL-C) and low-density (LDL-C) lipoprotein cholesterol.
Biomarkers of bone turnover and growth, glucose, insulin, and C-peptide were also measured in D-pro and homeostasis model assessment of insulin resistance (HOMA-IR) calculated. Body composition was measured by bioelectrical impedance in SKOT and by DXA in D-pro.
Results: In Paper I, mean ± SD s-25(OH)D was 69 ± 23 nmol/L across the year and its main determinants were season of examination and use of vitamin D containing supplements. During winter, 38% had inadequate s-25(OH)D (<50 nmol/L), whereof 15% had <30 nmol/L, and these numbers were only 7% and 1%, respectively, during summer. In D-pro, baseline (August-October) s-25(OH)D was 80 ± 17 nmol/L and its main determinants were month of examination,
sunny vacations, and use of vitamin D supplements. Vitamin D status was not associated with bone mineralisation or most of the cardiometabolic markers in either study. Paper II and III included the 184 (92%) children who completed the D-pro trial. The vitamin D supplementation prevented the winter decline in s-25(OH)D (-32 ± 18 nmol/L) and resulting vitamin D inadequacy (57%) observed with placebo. It also increased lumbar spine BMD and whole-body BMC (Paper II) and decreased LDL-C (Paper III) compared to placebo, but did not affect whole-body BMD, linear growth, adiposity, or blood pressure. The protein intake at baseline was 15 ± 2 energy percentage (E%), which increased to 18 ± 3 E% with HP due to increased dairy protein intake. The HP groups had smaller increments in lumbar spine BMD, whole-body BMC and BA, and osteocalcin, but not in whole-body BMD or linear growth, compared to the NP groups (Paper II). Further, the HP yogurts reduced fat mass index and tended to lower insulin,
C-peptide, and HOMA-IR dose-dependently, whereas there was no effect on blood pressure or blood lipids (Paper III). No vitamin D-yogurt interaction was found except on fasting glucose, which was increased by intake of regular yogurt combined with vitamin D (Paper III).
Conclusion: Vitamin D status in young children living at northern latitudes was mainly influenced by season and use of vitamin D supplements, whereas other factors had little or no impact. The findings support recommendations of supplements during winter for children in general and year-round for toddlers, and although vitamin D status was not associated with bone mineralisation or cardiometabolic markers cross-sectionally, the findings of the trial support a
vitamin D intake around 20 μd/day during winter for improved bone mineralisation and blood lipid profile. In contrast, they do not support recommending dairy products with high protein contents for optimising bone mineralisation in well-nourished children. Yet, a higher dairy protein intake may benefit fat mass and insulin markers. The present thesis adds to the limited and inconsistent available evidence concerning vitamin D and dairy protein in relation to bone mineralisation, growth, and cardiometabolic markers in young, healthy children, but more highquality, randomised trials are needed, especially in young children in which long-term implications are investigated, before firm conclusions can be drawn.
Original languageEnglish
PublisherDepartment of Nutrition, Exercise and Sports, Faculty of Science, University of Copenhagen
Number of pages163
Publication statusPublished - 2022

ID: 310428583