Effect of glycogen loading on skeletal muscle cross-sectional area and T2 relaxation time.

This study was performed to investigate if glycogen loading of skeletal muscles, by binding water, would effect the cross-sectional area (CSA) and if an altered water content would alter the transverse relaxation time (T2) measured by magnetic resonance imaging (MRI). Five healthy volunteers participated in a programme with 4 days of extremely carbohydrate-restricted meals followed by 4 days of extremely high carbohydrate intake. The CSA and T2 of thigh and calf muscles were related to the intramuscular glycogen content evaluated at days 4 and 8. An increase in glycogen content from 281 to 634 mmol kg(-1) dry wt increased the CSA of the vastus muscles by 3.5% from 78 +/- 11 to 80 +/- 12 cm2 and the thigh circumference by 2.5% from 146 +/- 20 to 150 23 cm2. Calf circumference increased non-significantly by 4% from 78 +/- 15 to 82 +/- 19 cm2. Mono-exponential T2 decreased in m. tibialis anterior from 27.8 +/- 1.2 to 26.9 +/- 1.7 ms, did not change in m. vastus lateralis 26.5 +/- 1.9 ms/26.6 +/- 1.3 ms or in m. gastrocnemius 29.5 +/- 1.0 ms/29.8 +/- 1.9 ms. Glycogen loading increased the signal intensity mainly at different echo times (TE) 15 and 30 ms. The study shows that increased glycogen filling in the muscles increases muscle CSA and that this can be detected by MRI. The signal intensity increased the most at shorter TEs suggesting a more tight intracellular binding of water in glycogen loaded muscles.