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ARS Home » Plains Area » Grand Forks, North Dakota » Grand Forks Human Nutrition Research Center » Healthy Body Weight Research » Research » Publications at this Location » Publication #224331
Title: Calcium requirements of growing rats based on bone mass, structure, or biomechanical strength are similar

Author
item Hunt, Janet
item Hunt, Curtiss
item Zito, Carol
item Idso, Joseph
item JOHNSON, LUANN - UNIV OF NORTH DAKOTA

Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2008
Publication Date: 7/1/2008
Citation: Hunt, J.R., Hunt, C., Zito, C.A., Idso, J.P., Johnson, L.K. 2008. Calcium requirements of growing rats based on bone mass, structure, or biomechanical strength are similar. Journal of Nutrition. 138:1462-1468.

Interpretive Summary: Although calcium supplementation increases bone density, the increase is small and the impact on bone strength and fracture risk is uncertain. We investigated how bone density, structure, and strength are affected by increasing dietary Ca concentrations, from deficient to more than generally considered adequate. The long bones (tibias and femurs) of female rats (8/group) were assessed after 13 wk consuming seven different concentrations of calcium, with constant amounts of phosphorous and vitamin D at recommended concentrations. The assessment included calcium and phosphorous mineralization, bone density, breaking strength assessed by a three-point flexure test, and structural properties by micro-CT X-ray scanning and 3-dimensional imaging. Food intake, weight gain, kidney and muscle Ca content, and a marker of bone protein were unaffected by dietary treatment. All bone parameters measured were significantly impaired by the most severe Ca deficiency, but all stabilized at apparent threshold values once calcium requirements were met. The results suggest that a threshold response in bone Ca retention or bone mass is associated with similar threshold responses in bone breaking strength and flexibility as well as bone structure. There was no evidence of improved or impaired bone strength and structure as Ca intakes increased beyond those needed to maximize bone density. It is important to determine adequate Ca intakes for humans because additional calcium likely does not improve bone health and may increase the risk of kidney stones.

Technical Abstract: Although calcium (Ca) supplementation increases bone density, the increase is small and the impact on bone strength and fracture risk is uncertain. To investigate if bone mass, morphology, and biomechanical properties are affected by deficient to copious dietary Ca concentrations, the long bones (tibias and femurs) of growing female Sprague-Dawley rats (8/group) were assessed after 13 wk consuming 1, 2, 3, 4, 5, 6, or 7 g Ca/kg of a modified AIN-93G diet. Dietary phosphorous (P) and vitamin D remained constant at recommended concentrations. The assessment included mineralization, density, biomechanical properties of breaking by a three-point flexure test, and morphological properties by micro-CT scanning of trabecular bone of the proximal tibia metaphysis. Food intake, weight gain, renal and muscle Ca content, and bone hydroxyproline, were unaffected by dietary treatment. All bone parameters measured were significantly impaired by Ca deficiency when diets contained 1 g Ca/kg. Modest impairments occurred with some parameters (bone density, biomechanical bending moment, modulus of elasticity, and stress) at 2 g Ca/kg, but all parameters stabilized between 2 and 3 g/kg diet, with no differences observed between 3 and 7 g/kg. The results suggest that a threshold response in bone Ca retention or bone mass at approximately 2.5 g Ca/kg diet is associated with similar threshold responses in bone breaking strength and related biomechanics as well as trabecular structural properties. There was no evidence of a relative P deficiency or of improved or impaired bone strength and structure as Ca intakes increased beyond those needed to maximize bone density.