The leptin-fat ratio is constant, and leptin may be part of two feedback mechanisms for maintaining the body fat set point in non-obese male Fischer 344 rats

While plasma leptin and adiposity have been found to be strongly related, t he specific nature of this relationship has yet to be clarified. Hence, pla sma leptin and three indicators of adiposity were measured in adult male Fi scher 344 rats on three different long-term diets: continuous ad libitum fe eding; ad libitum feeding until early adulthood, then continuous 60% calori c restriction; and ad libitum feeding until early adulthood, then 60% calor ic restriction until 16 months, then ad libitum feeding for 5 months. Body fat was found to be a good linear correlate of plasma leptin, with a zero Y -intercept, and a constant plasma leptin-body fat ratio. The number of adip ocytes per rat and % body fat were strong quadratic correlates of plasma le ptin. This study is the first to find a zero Y-intercept and constant plasm a leptin-body fat ratio, probably because it is the first to simultaneously measure both plasma leptin and body fat accurately, and to account for con founders such as gender, genetic background, age, physical activity, and po ssibly obesity. The study also explored the effect of switching calorically -restricted rats to ad libitum feeding. This led to a rapid rise, and then synchronized up-down cycles in average daily food intake and body weight, w ith a steady upward trend toward a new stable body-weight set point. It is hypothesized that this pattern resulted from two simultaneous feedback mech anisms, possibly involving leptin. In conclusion, this study suggests that, under controlled conditions, the plasma leptin-body fat ratio is a constan t for a particular mammalian strain, independent of dietary history.