Insulin resistance doesn't induce weight gain per se; it is a correlate of being overweight (in about 20% of those who are overweight; however, 80% of people with type 2 diabetes are overweight). Patients who are insulin resistant are usually hyperinsulinemic and hyperglycemic, and tend also to have other endocrine compensations, e.g., changes in glucagon, cortisol, etc., ... ultimately leading to a NET conversion of fuels for storage (glycogen, lipid).
It is more likely that you have got that backwards; weight gain induces insulin resistance. Increasing or decreasing adiposity has been shown to increase or decrease insulin resistance respectively. However, experimental induction of insulin resistance does not necessarily lead to weight gain. Further, a good deal of lean people are in fact insulin resistant. The mechanism by which obesity leads to insulin resistance is much debated. The three leading proposals are 1) adipocytes are less insulin sensitivity than muscle cells, thus changing the ratio of muscle to fat will decrease insulin sensitivity, 2) lipids inhibit glucose metabolism and thus insulin action, and 3) intermediates of triglyceride storage in muscle and liver interfere with the insulin signaling cascade (lipotoxicity hypothesis).
However, it is possible that insulin resistance leads to weight gain. For example, assume that one develops insulin resistance at the muscle and/or liver. This leads to mild hyperglycemia which stimulates increased insulin secretion. Now keep in mind that the adipocytes maintain normal insulin action and are now being exposed to excessive insulin levels. Thus while muscle and liver are receiving relatively normal insulin action (decreased sensitivity offset by increased insulin levels) the adipocytes are being excessively stimulated and thus excessively storing lipids.
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