Obesity and metabolic syndrome result from excess calorie intake and genetic predisposition and are mechanistically linked to type II diabetes and accelerated body aging; abnormal nutrient and insulin signaling participate in this pathologic process, yet the underlying molecular mechanisms are incompletely understood. Mice lacking the p66 kDa isoform of the Shc adaptor molecule live longer and are leaner than wild-type animals, suggesting that this molecule may have a role in metabolic derangement and premature senescence by overnutrition. We found that p66 deficiency exerts a modest but significant protective effect on fat accumulation and premature death in lepOb/Ob mice, an established genetic model of obesity and insulin resistance; strikingly, however, p66 inactivation improved glucose tolerance in these animals, without affecting (hyper)insulinaemia and independent of body weight. Protection from insulin resistance was cell autonomous, because isolated p66KO preadipocytes were relatively resistant to insulin desensitization by free fatty acids in vitro. Biochemical studies revealed that p66shc promotes the signal-inhibitory phosphorylation of the major insulin transducer IRS-1, by bridging IRS-1 and the mTOR effector p70S6 kinase, a molecule previously linked to obesity-induced insulin resistance. Importantly, IRS-1 was strongly up-regulated in the adipose tissue of p66KO lepOb/Ob mice, confirming that effects of p66 on tissue responsiveness to insulin are largely mediated by this molecule. Taken together, these findings identify p66shc as a major mediator of insulin resistance by excess nutrients, and by extension, as a potential molecular target against the spreading epidemic of obesity and type II diabetes.