Early life vitamin D depletion alters the postnatal response to skeletal loading in growing and mature bone

Abstract

There is increasing evidence of persistent effects of early life vitamin D exposure on later skeletal health; linking low levels in early life to smaller bone size in childhood as well as increased fracture risk later in adulthood, independently of later vitamin D status. A major determinant of bone mass acquisition across all ages is mechanical loading. We tested the hypothesis in an animal model system that early life vitamin D depletion results in abrogation of the response to mechanical loading, with consequent reduction in bone size, mass and strength during both childhood and adulthood. A murine model was created in which pregnant dams were either vitamin D deficient or replete, and their offspring moved to a vitamin D replete diet at weaning. Tibias of the offspring were mechanically loaded and bone structure, extrinsic strength and growth measured both during growth and after skeletal maturity. Offspring of vitamin D deplete mice demonstrated lower bone mass in the non loaded limb and reduced bone mass accrual in response to loading in both the growing skeleton and after skeletal maturity. Early life vitamin D depletion led to reduced bone strength and altered bone biomechanical properties. These findings suggest early life vitamin D status may, in part, determine the propensity to osteoporosis and fracture that blights later life in many individuals.

Fig 1. Experimental design showing the creation of murine model of dietary vitamin D deficiency up to culling post mechanical loading.

Fig 2. 3D reconstructed images of μCT scanned mouse tibia–representative image from each group shown.

(a) trabecular region of loaded and non-loaded tibias of early life vitamin D deficient and replete mice aged 10 weeks. (b) mid cortical region of loaded and non-loaded tibias of early life vitamin D deficient and deplete mice aged 10 weeks. (c) trabecular region of loaded and non-loaded tibias of early life vitamin D deficient and replete mice aged 18 weeks. (d) mid cortical region of loaded and non-loaded tibias of early life vitamin D deficient and deplete mice aged 18 weeks.

Fig 3

The effect of diet and mechanical loading on the cortical (a) total volume (TV), (b) bone volume (BV), (c) bone fraction (BV/TV), (d) cortical thickness, (e) cortical porosity and (f) marrow area, (g) trabecular bone fraction, (h) trabecular number, (i) trabecular thickness and (j) trabecular spacing at age 10 weeks age. Data are mean ± sd. *P<0.05, ** p<0.01, *** p<0.001 by Student t-test (n = 6 mice per group for all measured parameters).

Fig 4

The effect of diet and mechanical loading on cortical (a) bone fraction, (b) bone volume and (c) and marrow area at 18 weeks age. Data are mean ± sd. *P<0.05, ** p<0.01, *** p<0.001 by Student t-test (n = 6 mice per group for all measured parameters).

Fig 5. False colour images for whole bone demonstrating the maximum and minimum principal strain at 10 and 18 weeks in previously in vivo loaded and non-loaded conditions under an identical load of 0.83N (n = 1 mouse per group).

Fig 6. Early life vitamin D replete or deficient comparison of percentage of volume distribution of maximum principal strain distribution at 10 and 18 weeks for loaded and nonloaded bones (n = 1 mouse per group).