Feeding frequency and type of isotope tracer do not affect direct estimates of lysine oxidation in growing pigs

Oxidation contributes to the inefficiency of lysine utilization for protein deposition. The influences of feeding frequency and type of isotope tracer on estimated lysine oxidation were studied in growing pigs fed lysine-limiting diets. Yorkshire gilts (n = 11) weighing 40-45 kg were fitted with venous catheters. They were fed, in 3 or 8 equal meals daily, a purified diet based on casein and cornstarch. Lysine intake limited the pigs' protein deposition to 70% of their potential. After a 5-d N-balance period, lysine oxidation was estimated by a primed, constant 26-h infusion of [1-14C]L-lysine and [6-3H]L-lysine. Feeding frequency and type of tracer did not affect lysine oxidation (P > 0.1). Increasing feeding frequency from 3 to 8 times daily reduced the variance and fluctuation of lysine oxidation by 46 and 30%, respectively. The mean lysine oxidation, as a fraction of the true ileal digestible lysine intake, was 9.2% based on the free lysine specific radioactivity (SRA) in plasma, 20.1% based on free lysine SRA in liver and 21.8% calculated from N-balance data. On the basis of liver free lysine SRA, tracer dilution methods and N-balance data give similar quantitative estimates of lysine oxidation (P > 0.10). Isotope tracer studies that cover one or more complete feeding cycles, i.e., feeding-to-feeding periods, can be used to obtain valid daily lysine oxidation values.