Lean Mass Accretion Increases During Summer and Positively Associates With Vitamin D Status in Healthy Children 2–8 y

Population health guidelines for vitamin D intakes and status were designed to support bone health during periods of minimal ultraviolet beta radiation (UVB) exposure. It is less understood how summer UVB exposure influences vitamin D status and lean mass accretion. Thus, the objective of this study was to explore vitamin D status in relation to lean mass outcomes over 12 mo in children 2–8 y. This was a secondary analysis of data from 2 clinical trials (clinicaltrials.gov: NCT02097160, NCT02387892) in Montreal. Children 2–8 y consumed their normal vitamin D intake for 6 mo (Apr–Oct 2014, n=39) and a subgroup (n=21) continuing their normal intake until 12 mo (Apr 2015) with winter and spring study visits in Jan and Apr. At all 4 time-points, serum vitamin D status (total serum 25(OH)D: Liaison, Diasorin and LC-MS/MS) was measured followed by standardized anthropometry, skin colour (forehead, forearm and lower leg; spectrophotometer: CM-700d/600d, Konica Minolta), demographics plus activity and dietary questionnaires. Whole body lean mass and fat mass were measured at baseline, 6 and 12 mo using dual-energy x-ray absorptiometry (Hologic Discovery, APEX software version 13.3) and at 12 mo, muscle area and density at the 66% non-dominant lower leg were measured using peripheral quantitative computed tomography (XCT-2000; Stratec). Spearman correlations, linear regression and a mixed model ANOVA were used. In Apr 2014, children were 5.1 ± 1.9 y (range 1.9–8.2 y), 54% (21/39) male, with BMI Z-score of 0.72 ± 0.60. Only 1 child at 1 time-point reached the vitamin D EAR of 400 IU/d and vitamin D intake (222 ± 89 IU/d) did not significantly change across the study. Serum 25(OH)D increased (p=0.01) from 0–6 mo (Apr: 62.0 ± 14.1 nmol/L, Oct: 73.5 ± 13.4 nmol/L). Both vitamin D intake and summer change of skin colour (r=−0.08, p=0.61), did not correlate with 25(OH)D change even though there was significant tanning of skin over summer (individual typological angle 0–6 mo change: −12.0 ± 5.5°). In a linear regression model that included age, sex and height velocity, the summer increase in 25(OH)D was 3.3 nmol/L less for every 10 nmol/L increment in baseline 25(OH)D (r2=0.60, p=0.01). In the subgroup, 25(OH)D (Figure 1A) decreased (p=0.01) from 6–12 mo (Oct–Apr). The winter 25(OH)D change was not related to vitamin D intake (r=−0.10, p=0.32). The summer % change in lean mass positively correlated with the Apr 25(OH)D (r= 0.37, p=0.02) (Figure 1B). Also, the % change in lean mass was greater in summer than winter (p=0.04) (Figure 1C). The 12 mo % change in lean mass was higher by 1.5% for every 10 nmol/L increment in Oct 25(OH)D (r2=0.66). At 12 mo, age had the greatest contribution to explaining 66% lower leg muscle area and density. For every 5% increment in 12 mo % change in lean mass, muscle area and muscle density were higher by 300 mm2 (r2 = 0.72) and 7 mg/cm3 (r2 = 0.70) respectively. There was no relationship between ethnicity and 25(OH)D or lean mass. Our results suggest that 25(OH)D may be an important factor for lean mass accrual in young children. Lean mass outcomes may be important to consider when deriving future vitamin D recommendations for children. Support or Funding Information Funding from Dairy Farmers of Canada, The Canada Foundation for Innovation and Canada Research Chairs 25(OH)D of the subgroup of children at all 4 time-points (Panel A). Spearman correlations between summer % Δ in lean mass and baseline 25(OH)D (Panel B). Lean mass % Δ (Panel C) during summer and winter in the subgroup. a,b Different superscripts depict significant differences (p< 0.05) using a mixed model ANOVA. Panel A, C: Apr, Oct, Jan n=21, Apr 2015 n=19. Panel B: n=36 (3 children did not have DXA). Data are mean (SD) as a group and individual participant data over time.

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