Twelve weeks of endurance training increases mitochondrial density and percent IMCL touching mitochondria and alters IMCL storage distribution

Intramyocellular lipids (IMCL) are elevated in obesity and are correlated with insulin resistance (IR); however, elite athletes have elevated IMCL but are highly insulin sensitive. We postulate that the IMCL‐mitochondria physical relationship delineates whether IMCL are metabolically active. Methods: Muscle biopsies from 24 women (12 lean; 12 obese) were analyzed for IMCL and mitochondria ultrastructure using electron microscopy prior to and following a 12‐week progressive endurance training protocol. Results: In obese women, there was a strong trend for higher IMCL area density in the intermyofibrillar (IMF) region (P = 0.053) vs. lean. Training increased IMF IMCL size (P = 0.008), total fiber mitochondrial size (P = 0.006 and number (P = 0.009), subsarcolemmal (SS) (P = 0.01) and total fiber (P < 0.0001) mitochondrial area density, and the % IMCL touching mitochondria in the SS (P = 0.04), IMF (P < 0.0001) and total fiber (P < 0.0001), whereas it decreased SS #IMCL/area (P = 0.001). Following training, IMCL area density decreased in SS (P = 0.009), increased in IMF (P = 0.07), and did not change in the total fiber. HOMA‐IR was higher in obese women (P = 0.002), with no training effect. Discussion: The increase in IMCL‐mitochondrial proximity and mitochondrial density suggests an increased capacity to utilize IMCL following endurance training. Training‐induced IMCL mobilization from SS into IMF may be more favorable, as SS IMCL could prevent signal transduction and GLUT‐4 mediated glucose uptake. (Supported by CIHR).