| Summary: | Moderate energy restriction in young, exercising women attenuates the positive
effects of exercise on bone density. Studies have shown that in the absence of adequate
levels of circulating estrogen, there may not be enough functional estrogen receptor-a
(ER-a) to respond adequately to loading. The experiment described in this document is
significant because this model has not been explored under conditions of energy
restriction (EnR) which are known to reduce circulating estrogen levels; it has been
tested only in ovariectomized animals. The central hypothesis of this research is that
reductions in estrogen due to EnR limit the ability of bone to respond to mechanical
loading (LOAD) through a down-regulation of ER-a.
Study one determined which nutrient’s (calcium or energy) restriction (-40 percent)
had the greatest negative effects on the skeletal integrity of exercising female rats and
whether exercise (EX) could mitigate these deleterious changes. EnR caused
detrimental effects in many of the structural properties of bone; however EX attenuated
losses in cancellous bone.
Study two ascertained whether EX maintained cancellous bone mass in female
rats subjected to graded EnR (-20 or -40 percent) and whether changes in endocrine factors
were related. EX preserved cancellous bone volume and osteoblast activity under both
levels of EnR, in addition to total body lean mass and bone mineral content. A similar
maintenance of serum insulin-like growth factor and estradiol occurred in the
EX EnR(40 percent) group suggesting that these changes may be related to the protective
effects of EX.
Study three determined the effects of 40 percent EnR on bone formation rate to LOAD
in young adult female rats and tracked alterations in ovarian function (estradiol).
Although higher than non-loaded animals, the response of bone to LOAD in EnR
animals was dampened in comparison to energy-replete animals.
The experiments described in this document are significant because these are the
first experiments to explore the relationship between EnR and estrogen levels on
cancellous bone response to LOAD. This is particularly important for physically active,
energy restricted women because cancellous bone in these women will not experience
the same effects of loading which can increase their risk for developing osteoporosis.
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