Increased adiposity programmed by catch-up growth: requirement for leptin signals?

Low birth weight (LBW) followed by accelerated growth during infancy and childhood are associated with increased risk of obesity, insulin resistance, and cardiovascular disease in adulthood (1–4). Recent efforts to characterize the mechanism underlying this phenomenon of metabolic programming have largely focused on the effects of leptin treatment during the period of catch-up growth. A report in this issue of Endocrinology is the first to examine whether leptin signals are required for programming of increased adiposity due to accelerated catch-up growth (5). Rodent models have provided a useful means to parse the respective contributions of LBW vs. increased growth rates in the postnatal period to program adverse metabolic outcomes. Since the 1950s, manipulations of litter size have been used to control nutrient intake and growth rates in neonates (6). Animals that receive more nutrients as a consequence of lactation in small litters are heavier at weaning and gain more weight as adults; conversely, those reared in large litters are smaller at weaning and gain less weight as adults (6–8). Persistent effects of neonatal overnutrition on adiposity are concomitant with decreased sensitivity to leptin (9). The observations that blunting catch-up growth during the lactation period or delaying it to the postweaning period can prevent adverse metabolic outcomes support the idea that both the rate and time window of catch-up growth are critical determinants of the degree of programming in LBW models (10). Studies to identify molecular mechanisms underlying metabolic programming due to catch-up growth initially focused on the potential role of hyperinsulinemia (11, 12); however, two seminal discoveries shifted the focus to leptin. First, Ahima et al. (13) characterized a surge in plasma leptin levels in the first 2 wk of life, a period when leptin does not influence food intake or body weight (13–16). Second, the demonstration that leptin can influence the establishment of patterns of connectivity in hypothalamic circuits regulating energy balance during the neonatal period (17) led many to investigate leptin’s role in programming increased adiposity during the neonatal period. It has been difficult to evaluate the role of neonatal leptin in mediating persistent effects of catch-up growth on metabolic phenotypes. Differences in the sex and background strain of rodent models, as well as in the mode of leptin administration, result in a wide variation in the severity (and sometimes direction) of reported metabolic outcomes.Whereasperipheral leptinadministration inneonates often leads to leptin resistance and increased susceptibility to high-fat diet (HFD)-induced obesity in male mice (18–20), the degree of effects vary, as increased adiposity on chow has been observed in some studies (20) and not others (18). Both acute and persistent effects of neonatal leptin treatment are sexually dimorphic. In contrast to its effects in males, neonatal leptin treatment is reported to acutely reduce body weight as well as to protect against programming effects of small litter size in females (9, 21, 22). Several labs are using rodent models to examine the role of leptin in the context of catch-up growth associated with LBW. Intrauterine growth restriction (IUGR), achieved by reductions in either caloric intake or protein in the maternal diet, is followed by catch-up growth in the postnatal period (23). The nutrient status of the dam during lactation (restricted vs. unrestricted) and litter size influence the rate of catch-up growth achieved under different experimental paradigms, such that the body weight and adiposity of weanlings reflect the nutrient availability during lactation (23, 24). A premature leptin surge has been reported