1. We have investigated the possibility that the effect of insulin on triglyceride metabolism is related to the individual's responsiveness to insulin-mediated glucose utilization. Changes in plasma triglyceride levels were determined during 2 h infusions of insulin with glucose that maintained euglycaemia in 17 subjects, some of whom were overweight and/or hypertrigly-ceridaemic.

2. Plasma triglyceride concentrations fell in most subjects (mean ± sd: 19.9 ± 13.0%). The percentage fall in plasma triglyceride was inversely related to body mass index (r = −0.64, P < 0.01) and to basal triglyceride concentration (r = −0.69, P < 0.005), but directly to insulin sensitivity (r = +0.48, P < 0.05), and was unrelated to plasma free fatty acid concentration.

3. Since insulin sensitivity was also related to body mass index and basal triglyceride level stepwise regression analysis was carried out to determine the influence of these three variables on insulin-mediated lowering of plasma triglyceride. The percentage fall in plasma triglyceride remained independently related to insulin sensitivity (P < 0.05) and to body mass index (P < 0.05), and these two variables accounted for 44% of the fall in triglyceride.

4. Resistance to insulin (in terms of glucose utilization) may therefore be one significant, independent factor determining the plasma triglyceride concentration.

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