Objective: The effects of varied calcium bioavailability on bone development (young animal) and bone maintenance (aged animal) were evaluated by the application of bone composition and histologic/histomorphometric techniques.
Methods: Groups of male young (3-4 weeks old) and aged (24-month-old) rats were fed nutritionally-complete but calcium-restricted (0.15%) diets in which calcium was derived from spinach or nonfat dry milk (NFDM). A 0.5% calcium casein-based diet group was included as a comparison at the National Research Council's requirement level. All animals were pair-fed to the spinach-fed group. At the end of 60 days, lumbar vertebrae, tibia and femurs were harvested for bone histomorphometric and bone composition analyses.
Results: Histological analysis of the fifth lumbar vertebral body of the young spinach-fed group showed retarded development and mineralization of trabecular bone. Histomorphometry revealed that the spinach-fed group seemed to respond to the lower bioavailable calcium source by reducing the mineralization of bone matrix and stimulating bone resorption with a resultant decrease in bone volume (0.027, 0.059, 0.068 mm3 in spinach, NFDM and 0.5% calcium casein-based diet groups, respectively). The aged animals, however, appeared to respond to the reduction in available dietary calcium by mobilizing skeletal reserves, with a subsequent decrease in bone volume (0.057, 0.060, 0.073 mm3 in spinach, NFDM and 0.5% calcium casein-based diet groups, respectively).
Conclusion: This study demonstrates that a low bioavailable calcium source, spinach, suppresses calcium deposition in bones of young animals resulting in a reduction in both quality and quantity of bone. Moreover, despite the stable structure of the mature bone, a low bioavailable calcium source may also compromise the skeletal integrity of the aged animal.