Pigs were raised on six isotopically controlled diets to examine the dietary macronutrients used in the synthesis of bulk bone biochemical components (apatite, collagen and lipids) and individual compounds (bone fatty acids, cholesterol and amino acids from collagen). δ 13 C values of apatite and bulk bone lipids reflected those of the whole diet, with 13 C apatite-whole diet = 10.2 ± 1.3‰ and 13 C bone lipids-whole diet = −2.4 ± 0.7‰. A wide variation observed in the 13 C collagen-whole diet values (0.5 to 6.1‰) was hypothesized to reflect the relative importance of (i) the direct incorporation of essential amino acids, and (ii) the balance between direct incorporation and de novo synthesis of non-essential amino acids. Linear regression (n = 6) was used to assess the relationship between the δ 13 C values of whole diet and bulk bone components and individual compounds. Whole diet δ 13 C values showed a strong correlation with those of bone cholesterol (R 2 = 0.81) and non-essential fatty acids (0.97 ≤ R 2 ≤ 0.99). Not surprisingly, bone linoleic acid δ 13 C values correlated well with dietary linoleic acid (R 2 = 0.95). Mass balance calculations using the δ 13 C values of single amino acids accurately predicted the δ 13 C value of whole collagen. The δ 13 C values of whole diet were well correlated with those of the non-essential amino acids, alanine (R 2 = 0.85) and glutamate (R 2 = 0.96) in collagen. The essential amino acids leucine ( 13 C collagen leu-diet leu = 0.5 ± 1.2‰) and phenylalanine ( 13 C collagen phe-diet phe = −0.6 ± 0.6‰) showed little isotopic fractionation between diet and bone collagen.

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The diets of laboratory rats were isotopically and nutritionally manipulated using purified C 3 and/or C 4 macronutrients to investigate the routing of dietary carbon to bone collagen biosynthesis. Diets were formulated with purified proteins, carbohydrates and lipids of defined composition and natural abundance stable isotope ratios. Bulk protein and constituent amino acid d 13C values determined for whole diet and bone collagen provided the basis for assessing isotopic fractionation and estimating the degree of routing versus synthesis de novo of essential, non-essential and conditionally indispensable amino acids. Essential and conditionally indispensable amino acids were shown to be routed from diet to collagen with little isotopic fractionation whereas non-essential amino acids differed by up to 20‰. Mathematical modelling of the relationships between macronutrient and tissue d 13 C values provided qualitative and quantitative insights into the metabolic and energetic controls on bone collagen biosynthesis. Essential amino acids comprise 21·7 % of the carbon in collagen, defining the minimum amount of dietary carbon routing. Estimates of 42 and 28 % routing were shown for the non-essential amino acids, glycine and aspartate, respectively. In total, the routing of non-essential and conditionally indispensable amino acids was estimated to equal 29·6 % of the carbon in collagen. When the contribution of carbon from the essential amino acids is also considered, we arrive at an overall minimum estimate of 51·3 % routing of dietary amino acid carbon into bone collagen.

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