The effect of varying the quality of dietary protein and energy on food intake and growth in the Zucker rat

Abstract

I . Food intake and rates of protein, lipid and energy deposition were measured for lean and obese (fatty) Zucker rats offered to appetite from 34 d of age to slaughter at 66 d of age, one of sixteen semi-synthetic diets. Measurements were also made of the digestibility of dietary protein and the metabolizability of dietary energy. Total carcasses were analysed for protein and lipid, and body energy was calculated thereby. Changes in body constituents were calculated by the comparative-slaughter technique.2. In Expt I , four rats of each phenotype and sex were offered one of four diets, each of which contained either I 50 or 300 g casein ( I 50 C and 300 C respectively)/kg and either I 50 or 300 g cellulose ( I 50 CELL and 300 CELL respectively)/kg (diets I 50 C/I 50 CELL, I 50 c/300 CELL, 300 c/ I 50 CELL and 300 C/300 CELL. As expected, males ate more and had higher rates of protein deposition than female animals of the same phenotype on all diets. These sex differences were greater for the lean phenotype. The results for animals in this experiment are presented with, and discussed in relation to, those obtained previously for animals of both sexes fed on cellulose-free diets having these two levels of casein. 3. In Expt 2 , four female animals of each phenotype were fed one of twelve semi-synthetic diets, each of which contained casein, gluten or zein at one of the following levels (g crude protein (nitrogen x 625)/kg diet): 93, 132, 267 or 627. On all diets containing zein both fatty and lean rats had similar, low food intakes and failed to grow. Fatty rats fed on diets containing casein or gluten had higher rates of food intake, weight gain, lipid and energy deposition than lean rats, but similar rates of protein deposition. Rats fed on diets having the two lower levels of casein ate more and grew better than animals of the same phenotype fed on the two corresponding diets containing gluten but at higher protein levels differences in food intake and growth attributable to differences in protein quality disappeared and furthermore, the rate of protein deposition became similar and maximal for both phenotypes.4. The results from both experiments are discussed in relation to previous work on appetite control in the Zucker rat. It appears that fatty and lean rats eat during growth to attain the maximal rate of protein deposition of which they are capable. The rate of lipid deposition would appear to be of no importance in the food intake regulation of animals depositing protein maximally.5. Rats given diets that fail to support maximal rates of protein deposition appear to regulate their intake of digestible energy rather than that of digestible protein. They do not overeat protein-deficient diets in order to acquire sufficient protein for maximal growth although the factors that induce satiety in these animals are unknown.In previous work, congenitally-obese (fatty) and lean Zucker rats of both sexes were given free access during growth to a range of diets whose protein content was varied by altering the propo...

The postprandial rates of glycogen and lipid synthesis of lean and obese female Zucker rats

“…In contrast, the rate of lipogenesis in adipose tissue was signi®cantly lower in both the obese groups than in the lean controls when the data are expressed per g tissue. We did not measure adipose tissue mass, but it is known to be about four times greater in obese Zucker rats of this age than in their lean littermates, 16 so that it is likely that adipose tissue lipogenesis per whole rat would be greater in the obese than the lean rats.…”

Energy partitioning, tissue growth and appetite control

“…The evidence that first drew me to this dangerously simple concept came from the studies of Radcliffe & Webster (1978, 1979 who offered female, lean and congenitally obese, 'fatty', Zucker rats ad lib. access to a range of diets varying widlely in protein concentration, protein quality, fat and fibre.…”

Energy/protein ratio in natural ingredient diets for weanling rats

“…Others (Radcliffe & Webster, 1979) found that compensating mechanisms were effective up to 30% dilution when CP level was about 135 g/kg of food, whereas with levels of about 270 g/kg, dilutions of only 15% were possible without negative effects. Data in the literature discussed above was obtained using synthetic diets, in comparison to this trial where natural ingredients were used.…”