['A high-fat diet has been recommended for correction of biochemical abnormalities and muscle energy state in patients with complex I (NADH dehydrogenase) deficiency (CID).', 'This study evaluated the effects of intravenous infusion of isoenergetic amounts of triacylglycerol or glucose on substrate oxidation, glycolytic carbohydrate metabolism, and energy state in patients with CID.', 'Four CID patients and 15 matched control subjects were infused with triacylglycerol (1.85 mg x kg(-1) x min(-1)) or glucose (5 mg x kg(-1) x min(-1)) while at rest. Respiratory calorimetry was used to evaluate mitochondrial substrate oxidation. Metabolism of glycolytic carbohydrate was determined on the basis of the rates of appearance and concentrations of plasma lactate from dilution of [1-(13)C]lactate measurements. In addition, high-energy phosphate metabolism was measured in forearm muscle by (31)P magnetic resonance spectroscopy.', 'Whole-body oxygen consumption rates were higher in the patients than in the control subjects (P < 0.05). Oxygen consumption and high-energy phosphate metabolism in forearm muscle were not significantly different between the 2 infusion groups. The rates of appearance and concentrations of plasma lactate were higher in each of the 4 patients than in the control subjects (P < 0.05) and were lower during the triacylglycerol infusion than during the glucose infusion (P < 0.05); the differences were comparable in the patients and control subjects.'] Does triacylglycerol infusion improve hyperlactemia in resting patients with mitochondrial myopathy due to complex I deficiency?