Effect of a Chronic Acid Load as Sulfate or Sulfur Amino Acids on Bone Metabolism in Adult Rats

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Abstract
Diets containing an acid load as either sulfur amino acids (SAA) or inorganic sulfate were fed to 45Ca-labeled adult male rats for 10 months. Radioisotope excretion and bone composition data (femur, tibia, mandibles) were compared with those for rats fed a control (15% soy protein) diet. Rats fed the SAA supplement (1.28% cystine plus 0.19% methionine) exhibited a significant reduction in femoral weight and A:R ratio and a tendency toward lower specific gravity, dry weight, fat-free weight and calcium content. Femoral radioautographs indicated a reduction in metaphyseal bone density in six of eight animals. We have postulated that the osteopenia produced by feeding excess free SAA may be due to decreased bone formation caused by a reaction between homocysteine and the aldehyde groups of collagen, as in genetic homocystinuric osteoporosis. Sulfate feeding (1.42% of the diet) produced a significant increase in 45Ca excretion, indicative of enhanced bone resorption, lasting about 2 months. There was a tendency for bone mass measurements to be lower than controls after 10 months, but the differences were not significant. Two of eight sulfate-fed rats showed radiographic evidence of osteopenia. No evidence of osteopenia was seen in the controls or in rats previously fed a high protein diet containing the same concentration of SAA.