Response surface analysis of rat bone composition changes by dietary calcium and silicon

Abstract

Silicon (Si) apparently is involved in bone calcification; however, its affect appears to be related to dietary calcium. The objective of this experiment was to determine the effects of varying concentrations of dietary silicon on bone mineralization when there were incremental increases in dietary calcium. A second objective was to determine if pharmacological effects on bone would be evident at very high levels of dietary silicon (350 to 870 µg Si/g diet). Sixty weanling male Sprague-Dawley rats were randomly assigned to one of nine treatment groups in a two factor, central composite, response surface design. Silicon as sodium meta-silicate and calcium as calcium carbonate were added to a casein-based diet (containing 0.6 µg Si/g diet) at concentrations between 0 and 870 µg Si/g diet and between 0.15% and 0.85% Ca. The rats were fed their respective diets for nine weeks. DNA and hydroxyproline from the humerus bone and alkaline phosphatase from the plasma were determined by colorimetric methods. The femur, L4 vertebra, skull and one incisor tooth were ashed using the lithium-boron fusion technique and their mineral concentrations were determined by inductively coupled argon plasma atomic emission spectrometry. Response surface and regression analyses were utilized to analyze the data. There were no differences in mean body weight between the groups. Increasing dietary silicon increased calcium, phosphorus and magnesium concentrations in the vertebra and skull. The highest skull concentration of calcium, phosphorus and magnesium occurred at the predicted values of 36, 45, 39 µg Si/g diet, respectively. Silicon impacted the copper concentration in the vertebra and femur, with the minimum copper content occurring at the predicted values of 25 and 28 µg Si/g diet, respectively; with increases in copper occurring with both silicon deficiency and silicon excess. Changes in copper were opposite those of the hydroxyproline concentrations of the humeri, where both low and high dietary silicon decreased the hydroxyproline or collagen content, with the maximum collagen production in humeri bones occurring at the predicted value of 59 µg Si/g diet. Dietary silicon impacts bone mineralization in ways that can be generally regarded as positive, because it increases calcium, phosphorus, and magnesium concentrations. These effects may be the result of alterations in the organic matrix, reflected by the changes in copper and hydroxyproline concentrations. Regardless of the mechanism, silicon has such a strong impact on bone maturation and mineralization that is should be considered a mineral of concern in human nutrition. Research was conducted and funded by the USDA, ARS, Grand Forks Human Nutrition Research Center, Grand Forks, North Dakota. Interpretation of the response surface analysis data was conducted at the University of Wisconsin-Stout.