Natural abundance stable carbon isotope evidence for the routing and de novo synthesis of bone FA and cholesterol

This research reported in this paper investigated the relationship between diet and bone FA and cholesterol in rats raised on a variety of isotopically controlled diets comprising 20% C₃ or C₄ protein (casein) and C₃ and/or C₄ nonprotein or energy (sucrose, starch, and oil) macronutrients. Compound-specific stable carbon isotope analysis (δ¹³C) was performed on the FA (16:0, 18:0, 18:1, and 18:2) and cholesterol isolated from the diet (n = 4) and bone (n = 8) of these animals. The dietary signals reflected by the bone lipids were investigated using linear regression analysis. δ¹³C values of bone cholesterol and stearic (18:0) acid were shown to reflect whole-diet δ¹³C values, whereas the δ¹³C values of bone palmitic (16:0), oleic (18:1), and linoleic (18:2) acids reflected dietary FA δ¹³C values. Dietary signal differences are a result of the balance between direct incorporation (or routing) and de novo synthesis of each of these bone lipids. Estimates of the degree of routing of these bone lipids gleaned from correlations between Δ¹³C_dlipid-wdiet (= δ¹³C_{diet lipid} - δ¹³C_{whole diet}) spacings and Δ¹³C_lipid-wdiet (= Δ¹³C_{bone lipid} - Δ¹³C_{whole diet}) fractionations demonstrated that the extent of routing, where 18:2 > 16:0 > 18:1 > 18:0 > cholesterol, reflected the relative abundances of these lipids in the diet. These findings provide the basis for more accurate insights into diet when the δ¹³C analysis of bone fatty FA or cholesterol is employed.