Human Bone Collagen Synthesis Is a Rapid, Nutritionally Modulated Process

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Abstract
We developed a direct assay of human bone collagen synthesis using [13C] or [15N] proline and applied it to determine the effects of feeding in young healthy men. Surprisingly, postabsorptive bone collagen synthesis is not sluggish, being ∼0.07%/h more rapid than that of muscle protein, and capable of being stimulated within 4 h of intravenous feeding by 66 ± 13%. Introduction: All current methods for estimation of bone collagen turnover are indirect, depending on the assay of collagen “markers.” Our aim was to develop a direct method for human bone collagen synthesis to be used to study its physiology and pathology, and specifically, in the first instance, the effect of feeding. Materials and Methods: We applied, over 2 h, flooding doses of [13C] and [15N] proline to label iliac crest bone collagen in eight young healthy men. The rate of collagen synthesis was determined as the rate of labeling of collagen hydroxyproline (assayed by gas chromatography-combustion-isotope ratio mass spectrometry in collagen extracted by differential solubility) compared with plasma proline labeling (assayed by gas chromatography-mass spectrometry). We also determined (in a second group of eight young healthy men) the effect of intravenous nutrition (glucose, lipid emulsion, and amino acids (in the ratio of 55%:30%:15% energy, respectively). Results: Free bone proline labeling was 92 ± 6% of that of plasma proline, supporting the flooding dose assumption. Human iliac crest bone collagen is heterogeneous, with NaCl-EDTA, 0.5 M acetic acid, pepsin-acetic acid, and hot water-extractable pools being responsible for ∼1%, 3%, 8%, and 81% of content, respectively. The synthetic rates were 0.58 ± 0.1, 0.24 ± 0.05, 0.07 ± 0.02, and 0.06 ± 0.01%/h, respectively, giving an average rate of ∼0.066%/h. [13C] and [15N] proline gave identical results. Intravenous nutrition caused the disappearance of proline label from the procollagen pool and its increased appearance in the less extractable pools, suggesting nutritional stimulation of collagen processing. Conclusion: The results show (1) that iliac crest bone collagen synthesis is faster than generally assumed and of the same order as muscle protein turnover and (2) that feeding increases synthesis by ∼66%. Given its ability to detect physiologically meaningful responses, the method should provide a new approach to studying the regulation of bone collagen turnover.