THE EFFECTS OF DIETARY BORON ON BONE STRENGTH IN RATS

Previous studies from our laboratory found that when boric acid (BA) w as administered in the diet to rats, boron levels in bone were approxi mately fourfold greater than serum levels. The current studies were un dertaken to determine if these elevations produced adverse effects on several bone-related measures, including serum electrolyte levels, bon e structure, and bone strength. Data from two studies are presented: i n the first study, young adult male rats consumed a powdered diet cont aining 0, 3000, 4500, 6000, or 9000 ppm BA for 9 weeks. Endpoints were serum calcium, phosphorous, potassium, and chloride, as well as blood and bone boron concentrations ([B]) measured weekly during the 9-week exposure period, and at 8, 16, 24, and 32 weeks after the end of expo sure. In the second study, the male and female young adult rats diet c ontained 0, 200, 1000, 3000, or 9000 ppm BA for 12 weeks; endpoints me asured weekly were serum levels of calcium, phosphorous, and magnesium , bone [B], and bone structure (humerus) and strength (tibia, femur, a nd lumbar vertebrae). In treated rats, calcium was reduced in the firs t study but not the second. Serum phosphorous was reduced in both stud ies; potassium was unchanged, chloride was increased by 1%, and magnes ium was reduced in all BA-exposed groups in the second study, to a max imal 19% reduction. Bone [B] was consistently increased in all treated groups, to concentrations approximately fourfold those of serum. Afte r cessation of exposure, serum and urinary boron concentrations droppe d to within control values within a week. However, even 32 weeks after the end of exposure, bone [B] remained threefold greater than control s. Male tibia and femur resistance to bending was unchanged. However, vertebral strength in compression was significantly increased by 5-10% in all dose groups (200 to 9000 ppm). The pattern was substantially s imilar in females. Only the humerus was examined by light microscopy a nd was found to be unchanged at any level of BA consumption. These dat a show that, despite a reduction in some serum electrolyte levels, BA consumption increased vertebral resistance to crush force, without det ectably altering the microscopic structure of the humerus or the resis tance of femur and tibia to a bending load. This increase in compressi on resistance occurred at exposure levels substantially below those th at were previously reported to be reproductively toxic.