While plasma leptin and adiposity have been found to be strongly related, the specific
nature of this relationship has yet to be clarified. Hence, plasma leptin and three
indicators of adiposity were measured in adult male Fischer 344 rats on three different
long-term diets: continuous ad libitum feeding; ad libitum feeding until early adulthood, then continuous 60% caloric restriction; and ad libitum feeding until early adulthood, then 60% caloric 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 adipocytes per rat and % body fat were strong quadratic correlates of plasma
leptin. This study is the first to find a zero Y-intercept and constant plasma leptin-body
fat ratio, probably because it is the first to simultaneously measure both plasma
leptin and body fat accurately, and to account for confounders such as gender, genetic
background, age, physical activity, and possibly 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, with a steady upward trend toward a new stable
body-weight set point. It is hypothesized that this pattern resulted from two simultaneous
feedback mechanisms, possibly involving leptin. In conclusion, this study suggests
that, under controlled conditions, the plasma leptinbody fat ratio is a constant for
a particular mammalian strain, independent of dietary history.
Key words
Ob Gene Product - Adiposity - Bodyweight Regulation - Caloric Restriction - Plasma
Leptin
