Bipedal Loading During Recovery from Lactation Increases Femoral Skeletal Mass in Rats

Abstract

0423 During lactation the maternal skeleton can lose as much as 4.8% BMD in the femoral neck and 5.1% in the lumbar spine in humans (Sowers et al., 1993). Bone lost during lactation can partially recover (Bowman & Miller, 1999). Mechanical loads to the skeleton play a major role in the accumulation and maintenance of skeletal mass and strength. PURPOSE: To determine the effects of bipedal loading during recovery from lactation on femoral skeletal mass. A secondary aim was to determine if bipedal loading during lactation could mitigate bone mass loss at the femur. METHODS: Nulliparous rats (N = 32) were mated and were permitted to go through two reproductive cycles. The day after the second parturition rats were randomized into four groups: Lactation (Lac; n = 8), Lactation, Raised Cage (Lac RC, n = 8), Lactation Recovery (Lac-Rec, n = 8), and Lactation Recovery, Raised Cage (Lac-Rec RC, n = 8). Lac and Lac RC groups were sacrificed after 21 days of lactation and the recovery groups continued on for an additional 6 weeks to assess loading during the recovery period. Femurs were excised at necropsy and were dehydrated, defatted, and weighed to obtain dry weight (g). The bones were ashed in a muffle furnace and weighed again to determine ash weight (g). RESULTS: There were no significant differences between Lac and Lac RC on femoral dry weight (0.652 ± 0.018 vs. 0.623 ± 0.017 g, p = 0.26) or ash weight (0.394 ± 0.011 vs. 0.374 ± 0.011 g, p = 0.22). Lac-Rec RC had greater femoral dry weight (0.724 ± 0.012 vs. 0.681 ± 0.013, p = 0.03) and femoral ash weight (0.450 ± 0.008 vs. 0.423 ± 0.009, p = 0.04) than Lac-Rec. CONCLUSIONS: Bipedal loading during recovery from lactation induced increases in skeletal mass. Bipedal loading during lactation had no effect, possibly because energy needs during lactation are too great. Supported by NIH grant AR044806.