Expression of the Dietary Isotope Signal in the Compound-specific δ13C Values of Pig Bone Lipids and Amino Acids

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). δ13C values of apatite and bulk bone lipids reflected those of the whole diet, with Capatite-whole diet = 10.2 ± 1.3‰ and Cbone lipids-whole diet = −2.4 ± 0.7‰. A wide variation observed in the Ccollagen-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 δ13C values of whole diet and bulk bone components and individual compounds. Whole diet δ13C values showed a strong correlation with those of bone cholesterol (R2 = 0.81) and non-essential fatty acids (0.97 ≤ R2 ≤ 0.99). Not surprisingly, bone linoleic acid δ13C values correlated well with dietary linoleic acid (R2 = 0.95). Mass balance calculations using the δ13C values of single amino acids accurately predicted the δ13C value of whole collagen. The δ13C values of whole diet were well correlated with those of the non-essential amino acids, alanine (R2 = 0.85) and glutamate (R2 = 0.96) in collagen. The essential amino acids leucine ( Ccollagen leu-diet leu = 0.5 ± 1.2‰) and phenylalanine ( Ccollagen phe-diet phe = −0.6 ± 0.6‰) showed little isotopic fractionation between diet and bone collagen. Copyright  2003 John Wiley & Sons, Ltd.