Body growth and food intake in moderately and severely malnourished rats

A longitudinal study with eighty-six rats (Rattus norvegicus albinus var. Wistar) of both sexes was made. The aim was to express adaptability to malnutrition in terms of Age-of-Fitness Point (AFP) and Trend-to-Fitness Stage (TFS). The AFP means the age in which both control and malnourished animals show the same Relative Food Intake (RFI) value, expressed in mg of food intake per g of body weight. The TFS means the percent of RFI reached by the malnourished animals at a given age stage. A TFS=70 for example, means that the animals have to gain 30% of RFI to reach its AFP. Growing animals were submitted to one of the following treatments: control (fed on a stock diet ad libitum), moderate malnutrition (fed on 1/2 of the daily stock diet of a control of the same age and sex) and severe malnutrition (fed on 1/3 of the mentioned diet). Body weight and food intake of all of the animals were registered every ten days, when they were from 20 to 80 days old. Relative Food Intake (RFI) was calculated dividing the food intake (mg) of each animal/day by its respective body weight (g). The RFI data series belonging to each sex and treatment was obtained. The significance of the three independent variables (age, sex, and nutrition) was tested by multiple regressions. Covariation and degree of linearity between RFI and age were tested by simple correlation and simple regression tests. The intercepts of the regressions were all significant. For both sexes they were greater in controls than in the moderately malnourished group, in the latter being greater than in the severely malnourished group. The slopes were significant and negative in all of the groups. While, in males and females, control and moderately malnourished rats showed little differences, the severely malnourished slopes were, respectively, 3.2 and 2.4 times lower than in controls. In controls there were sexual differences for RFI correlated with age which were not present in the malnourished groups. The AFP was calculated by extrapolation: 80-90 days of age in males, with RFI of 80 mg/g, and 100-110 days of age in females, with RFI of 70 mg/g. In males TFS was 29.5% and 88.6 % in moderate and severe malnutrition, respectively. In females these values were 9.1% and 63.2%, respectively. Sexual dimorphism in TFS was significant in the moderately malnourished animals and nonsignificant in the severely malnourished ones.