Antecedent protein restriction and high-fat feeding interactively sensitise the leptin response to elevated insulin

Using a rat model of moderate (8 vs. 20% protein) isocaloric protein restriction initiated in early life (low protein, LP), we examined the possible basis for the association between impaired early growth and elevated leptin levels in later life in man by examining the acute leptin response to insulin and its relationship with glucose utilisation. We placed subsets of LP rats on a high-saturated-fat (HF) diet containing 20% protein for 4 weeks (LP-4HF) or 8 weeks (LP-8HF), making comparison with age-matched control (C) groups (C, C-4HF, C-8HF). At ambient insulin concentrations, LP was not associated with altered leptinaemia compared with C, despite a more active lipolytic programme as inferred from increased adipocyte sensitivity to norepinephrine. HF feeding led to insulin resistance with respect to whole-body glucose disposal (R(d)) (measured using [3-(3)H] glucose at steady state) in both LP and C in vivo and impaired suppression of agonist-stimulated lipolysis by insulin in LP but not C in vitro. Whereas insulin infusion for 2 h (while maintaining euglycaemia) only modestly increased plasma leptin levels in vivo in C, C-4HF, C-8HF and LP groups, the leptin response to insulin was greatly enhanced in the HF-fed LP groups. A close positive correlation (r = 0.96) existed between plasma leptin levels and R(d) in the C groups (viz. C, C-4HF, C-8HF) whereas a close inverse correlation (r = 0.95) existed between plasma leptin levels and insulin-stimulated R(d) in the LP groups (viz. LP, LP-4HF, LP-8HF). Glucose utilisation (estimated from 2-deoxy-D-[1-(3)H] glucose 6-phosphate accumulation) in vivo in two intra-abdominal and two superficial adipose-tissue depots was consistently higher in the LP group. After HF feeding, glucose utilisation by the superficial adipose-tissue depots was threefold higher in the LP than in the C group. We conclude that protein restriction from conception to adulthood followed by high-fat feeding sensitizes the acute leptin response to insulin, an adaptation associated with enhanced glucose utilisation by adipose tissue. This effect is observed despite impaired insulin sensitivity, both at the level of whole-body glucose disposal and adipocyte anti-lipolysis, and increased lipolytic activity (although the latter is not in itself sufficient to influence the leptin response). We propose that associations between a low birthweight and elevated leptin concentrations in later life may reflect long-term modulation of adipocyte glucose handling.