The influence of feeding frequency on nitrogen turnover and whole body protein synthesis in adult rats (estimation using [14C]- and [15N]-labelled leucine)

Male Wistar rats (17 wks. old, body weight similar to 400 g), fitted with an intra gastric cannula and with a catheter in the vena jugulars were divided into 3 groups and given a marginal ration of the feeding solution Nutrison Standard (1 g protein and 350 kJ ME per day). Group 1 had ad lib. access to the drinking bottle, the groups 2 and 3 were pair fed by gastric infusion, splitted up into 2 greater meals for group 2 respectively into 6 smaller meals for group 3. After adaptation all animals get an i.p. injection of doubly labelled tracer solution (200 mu l) containing 2.5 mg L-[N-15]leucine (72 atom% N-15) combined with either [1-C-14]- or [U-C-14] leucine (37 kBq). The course of (CO2)-C-14 expiration was estimated by breath test over 4 h in intervals of 15 min and the course of urinary N-15 excretion over 24 h in intervals of 45 resp. 90 min. An infusion of saline (0.9 %, 5 ml/h) into the vena jugularis was used to provoke sustained urine production of the animals during the experiment. From the parameters of the excretion curves of breath (CO2)-C-14 resp. urine N-15 (cumulative end value) and from the N balance the portions of leucine-C and leucine-N used for protein synthesis, transamination, decarboxylation and total oxidation as well as the kinetic parameters for whole body protein metabolism were computed. The following conclusions were drawn: 6 x Feeding regime produces a small but measurable amino acid economy effect in comparison to 2 x feeding regime. Protein gain for 2 x feeding group was significant smaller than for 6 x feeding group, though protein synthesis rate was higher, but was overcompensated by a greater increase of protein breakdown rate for the 2 x feeding group. Energy storage in form of fat and glycogen built From decarboxylation was unaffected by feeding frequency. The amount of leucine oxidized for heat production was 4% higher for the 6 x feeding group. Transamination rare For leucine was estimated to 8-15 %. Absolute values for protein flux, protein synthesis and protein breakdown may be overestimated or underestimated because the metabolism of [N-15]leucine does not exactly agree with that of total N; but the proportions of them and therefore also the conclusions will be true. Better results for absolute values will be obtained using a mixture of N-15 labelled AA, N-15 labelled protein or hydrolysate of N-15 labelled protein (yeast) as the tracer source.