Human Models of Selective Insulin Resistance: Pancreatic Clamp

This is a single-center, prospective, randomized, controlled (crossover) clinical study designed to investigate the impact of lowering insulin levels on hepatic glucose production (HGP) vs de novo lipogenesis (DNL) in people with insulin resistance. The investigators will recruit participants with a history of overweight/obesity and evidence of insulin resistance (i.e., fasting hyperinsulinemia plus prediabetes and/or impaired fasting glucose and/or Homeostasis Model Assessment of Insulin Resistance \[HOMA-IR\] score \>=2.73), and with evidence of metabolic dysfunction-associated steatotic liver disease (MASLD). Participants will undergo two pancreatic clamp procedures -- one in which serum insulin levels are maintained near hyperinsulinemic baseline (Maintenance Hyperinsulinemia or "MH" Protocol) and the other in which serum insulin levels are lowered by 50% (Reduction toward Euinsulinemia or "RE" Protocol). In both clamps the investigators will use stable-isotope tracers to monitor hepatic glucose and triglyceride metabolism. The primary outcome will be the impact of steady-state clamp insulinemia on HGP vs DNL.

Although high blood sugar and risk of heart disease are the most well-known health effects of type 2 diabetes (T2DM), metabolic dysfunction-associated steatotic liver disease (MASLD), in which too much fat accumulates in the liver, has come to be recognized as another important complication. Unchecked, MASLD can progress to severe liver inflammation, liver failure, and even liver cancer. The investigators suspect that high levels of the blood sugar-lowering hormone insulin leads to excessive fat production by the liver, and so lowering insulin levels might help to improve MASLD. In order to answer this question, the investigators will recruit people with MASLD at risk for T2DM to perform a "pancreatic clamp" - a procedure in which the body's production of insulin is temporarily shut off and then replaced at the same or lower levels. Again, the investigators expect that lowering insulin levels will lower fat production ("de novo lipogenesis" or DNL). Research participants in this prospective, randomized, controlled (crossover) study will therefore undergo two pancreatic clamps in random order: one roughly maintaining their own internal ("basal") insulin level ("MH Protocol") and one in which the investigators lower that basal insulin level by 50% ("RE Protocol"). In each case, the investigators will observe the absolute and relative changes in the liver's production of glucose (hepatic glucose production, HGP) and of triglycerides (de novo lipogenesis, DNL) using stable-isotope tracers.