Energy balance through age-related diurnal body weight variations in genetic obese and nonobese rats.

Indirect evidence of energy balance in laboratory rats is provided through the study of diurnal body weight variations in two inbred lines: obese beta and nonobese alpha, from birth to 200-300 days of age, with different feeding patterns from 25 to 75 days of age. Nocturnal weight gain (NWG) was the gain recorded after the dark phase, in direct relation to the acquisition of exogenous calories in excess of the current metabolic expenditure at nighttime. Daytime weight variation was either weight gain during lactation (DWG) or weight loss from weaning onwards (DWL), recorded after the light phase. DWL is in direct relation to daytime energy output, when metabolic expenditure exceeds the low rate of acquisition of exogenous calories. The correlation between averaged individual DWL and NWG absolute values was highly significant at every age studied. An increase in absolute DWL values with age was observed and at adulthood DWL was compensated for equivalent NWG. This increasing energy output with age during daytime, is most likely related to the maintenance of increasing biomass and consequently, to progressive reduced growth energy availability. The existence of energy homeostasis and ponderostat with set points genetically prescribed in adults, is suggested. Significant differences between lines found before adulthood give indirect evidence of higher fat accretion in the obese line in those periods of intense growth, known as the active phase of obesity.