Bones, Muscles and Visceral Organs of Protein-Malnourished Rats (Rattus norvegicus) Grow More Slowly but for Longer Durations to Reach Normal Final Size

Starting at weaning (22 d), Sprague-Dawley rats were fed either a control diet high in protein (CT, 24% protein) or an isocaloric low protein diet (LPT, 4% protein) to determine how protein malnutrition alters the rate and timing of limb bone growth. Length and width measurements were taken from longitudinal radiographs to provide complete growth trajectories of both treatments. Data collection continued until rats reached adult size, which varied among diet-sex groups. The rats were then killed and five muscles and eight organs were weighed. A nonlinear Gompertz model was then fit to each trajectory for 13 skeletal measurements, producing parameters that described the rate and timing of growth for each rat, the unit of analysis. Parameter differences due to diet, sex and litter were tested by using a mixed-model, three-way ANOVA. For most measurements, the LPT rats were not significantly smaller than the CT rats, for the model's prediction of final size. Bone length was significantly less affected than width. The instantaneous initial growth rate, maximum rate of growth and rate of growth decay were significantly higher in the control rats for all measurements. The rats fed the low protein diet grew for significantly longer periods of time. For all muscles and most organs relative to body size, there was no difference between rats fed the two diets. The exceptions, eyes and brains, were proportionally larger in the LPT rats, suggesting that these organs receive nutritional priority during growth. For the systems in this study, structures that grow or have the potential for extended growth are less affected by the nutritional insult.