Metabolic Rate and Body Composition in Rats Nutritionally Deprived before or after Weaning

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Abstract
Summary: In order to manipulate cell size and cell number, rats were subjected to nutrient restriction either prenatally and until weaning at 3 weeks (group I), or from 3 until 10 weeks (group II). Body weights of group I rats were 30% of normal at the height of deprivation at 3 weeks. By 16 weeks of age maximum weights were reached, which remained subnormal in the majority; by 26 weeks only one of six rats had attained a normal weight. Body weights of group II were 27% of normal at the height of deprivation at 10 weeks; subsequently, only one of six did not recover. The weight, protein, and DNA content of liver, kidney, and heart were significantly decreased. The combined weight and protein content of the three organs was 30% of normal in group I and 25% of normal in group II; the DNA content of the three organs was 57% and 38% of normal in group I and group II, respectively. The greater deficit in weight than DNA content, and the mean protein/DNA ratios reflect a decrease in cell size (or increase in the cell density) of the three organs, which was greater in group I (60%) than group II (71%). Muscle cell density was increased in group I, but not in group II. At 16 weeks of age all parameters were similar in the two experimental groups (ranging from 68% to 82% or normal). Organ weight, protein, and DNA content remained deficient only in group I; these values were 74%, 70%, and 77% of normal, respectively, at 32 weeks. The deficit was proportionate to the deficit in body weight. The basal metabolic rate was measured 17–20 hr after food removal (BMR) on the day the animals were killed. In group I and II correlations between the BMR and four parameters of body composition were linear during a 4-period at the height of deprivation. Comparison of means from the regressions showed no difference between group I and II, or between these groups and normal controls with regard to the BMR per body weight, organ weight, and organ protein. The BMR per mg DNA was lower in group I and II than in normal control rats (P < 0.05). The BMR of normal rats age 3–32 weeks showed a curvilinear correlation with body weight (BWt), BMR = 1.24 BWt0.583; organ weight (OWt), BMR = 4.30 OWt0.766; and organ protein (OPr), BMR = 4.30 OPr0.604. By contrast, the regression on DNA was linear (BMR = 7.97 + 0.449), although marked changes in body composition occurred between 3 and 32 weeks. Speculation: Future analyses of body composition of infants with small-for- gestational age dwarfism may identify a type which is characterized by a decrease in cell number per unit of weight and height. This study indicates that such a decrease in total body cell density can be diagnosed by measuring the BMR, which thus may prove to be a practical tool in the identification of infants with a poor growth potential because of prenatal influences and genetic factors.