['Both training and normal body mass index are associated with high insulin sensitivity, but the mechanism may be different.', 'The aim of the study was to examine whether lean trained humans may be protected from acute free fatty acid (FFA)-induced insulin resistance compared with lean sedentary humans.', 'We conducted an interventional trial using either a 6-h lipid (20% Intralipid at 90 ml/h) or glycerol (2.25 g/100 ml at 90 ml/h) infusion along with a concurrent hyperinsulinemic-euglycemic clamp and serial muscle biopsies (0, 120, 360 min) at a clinical research unit at the University of Minnesota.', 'The study included lean endurance-trained (n = 14) and sedentary (n = 14) individuals matched for age, gender, and body mass index.', 'We measured the decline in glucose infusion rate (GIR) during the hyperinsulinemic-euglycemic clamp.', 'The trained group had higher baseline mitochondrial DNA copy number, mRNA of cytochrome C oxidase subunit 3, and insulin sensitivity (as measured by GIR) compared with the sedentary group. When FFA was acutely elevated to the upper physiological range (0.6-0.7 mEq/liter) by lipid infusion, the GIR in both activity groups declined similarly compared with their respective glycerol controls, although insulin signaling, as measured by Ser 473 pAKT/AKT, remained comparable. Specific to the trained group, the stimulatory effect of hyperinsulinemia on mitochondrial mRNA levels during the glycerol infusion was absent during the lipid infusion.'] Does acute free fatty acid elevation eliminate endurance training effect on insulin sensitivity?