Bone structure is largely unchanged in growing male CD-1 mice fed lower levels of vitamin D and calcium than in the AIN-93G diet

Background

Calcium (Ca) and vitamin D (vit D) in the AIN-93G diet may be higher than required for healthy bone development, and mask the potential benefit of a dietary intervention.

Objective

The objective was to determine if lower levels of Ca and vit D than is present in the AIN-93G diet supports bone development in growing male CD-1 mice.

Methods

Weanling male CD-1 mice were randomized to modified AIN-93G diets containing either 100 (Trial 1) or 400 (Trial 2) IU vit D/kg diet within one of two or three Ca levels (0.35, 0.30, or 0.25% Ca diet in Trial 1 or 0.35% or 0.25% in Trial 2) or the AIN-93G diet (1000 IU/kg vit D and 0.5% Ca) from weaning to 4 months of age (n = 13–15/group). At 2 and 4 months of age, BMD and structural properties of the tibia were analyzed in vivo. Structure of lumbar vertebra 4 (L4) and mandible, and femur strength were assessed ex vivo at age 4 months.

Results

There were no differences in tibia, L4, and mandible structure between the AIN-93G diet and the 0.35% Ca groups at either vit D level. A few structure outcomes were compromised with the 0.25 and/or 0.3% Ca diets but there were no differences in femur biomechanical strength compared to AIN-93G group in either Trial.

Conclusion

At 400 or 100 IU vit D/kg diet, Ca can be lowered to 0.35% without detriment to BMD or bone structure while bone strength is not altered at lower Ca (0.25%) compared to CD-1 mice fed AIN-93G diet. Because of genetic variation in CD-1 mice among different breeding facilities, results in CD-1 mice from other facilities may differ from the present study.

Bone development in growing female mice fed calcium and vitamin D at lower levels than is present in the AIN-93G reference diet

Background

The AIN-93G reference (REF) diet is used to allow the comparison within and between studies of different research groups but its levels of vitamin D (vit D) and calcium (Ca) may be higher than required for healthy bone structure and bone mineral density (BMD).

Objective

To determine if lower dietary levels of Ca (3.5, 3 or 2.5 g Ca/kg diet) at 1 of 2 levels of vit D (100 or 400 IU/kg diet) supports similar development of bone structure and BMD compared to AIN-93G reference (REF) diet in female CD-1 mice at 2 and 4 months of age.

Methods

Within a trial, weanling female mice (n = 12–15/group) were randomized to 1 of 4 diets until necropsy at 4 months of age: Trial 1: 100 IU vit D/kg + 3.5, 3 or 2.5 g Ca/kg diet or 1000 IU vit D/kg + 5 g Ca/kg diet (REF); and Trial 2: 400 IU vit D/kg + 3.5, 3 or 2.5 g Ca/kg diet or 1000 IU vit D/kg + 5 g/kg diet (REF). At age 2 and 4 months, in vivo bone structure and BMD were assessed using micro-computed tomography (μCT) at the proximal and midpoint tibia. At age 4 months, lumbar vertebra 4 (L4) and mandible structure were analyzed ex vivo, femur strength at midpoint and neck was assessed and serum 25(OH)D₃ and PTH were quantified.

Results

For Trial 1 (100 IU vit D/kg), there were no differences in tibia structure at age 2 and 4 months nor L4 or mandible structure or femur strength at the midpoint or neck at 4 months of age despite lower serum 25(OH)D₃ among all groups compared to REF. For Trial 2 (400 IU vit D/kg), mice fed 2.5 g Ca/kg diet had lower (p < 0.05) Ct.Ar/Tt.Ar and Ct.Th at the tibia midpoint compared to REF. Furthermore, Ct.Th. was greater in REF and 3.5 g Ca/kg diet compared to 2.5 g Ca/kg diet at age 2 but not 4 months of age. At L4, BV/TV was lower (p < 0.05) in the 3 g Ca/kg diet group compared to REF at age 4 months. There were no differences among groups for serum 25(OH)D₃ or femur strength at the midpoint or neck. Serum PTH was not elevated compared to REF in either Trial.

Conclusion

Lowering both dietary vit D (100 IU/kg) and Ca (2.5 g/kg) in AIN-93G diet did not result in differences in bone development of female CD-1 mice at early adulthood. Translational relevance of bone studies conducted using the AIN-93G diet may be affected by its high vit D and Ca content.