A new article, published in The Journal of Physiology (September 2017) by Danish Diabetes Academy PhD fellow Han-Chow Koh, shows that in insulin sensitive individuals, such as well-trained athletes, fat (as lipid droplets) accumulation and utilization in skeletal muscles is differentially regulated across limbs, muscle fiber types and within muscle fibers.
OUR UNDERSTANDING OF THE METABOLISM OF LIPID DROPLETS ARE INCOMPLETE
It is known that fat is an important fuel for muscular contractions during exercise and sports activities. Fat storage in the form of lipid droplets can be found in human skeletal muscle. However, our understanding of the metabolism of these lipid droplets remains incomplete. Interestingly, the amount and location of the lipid droplets found in the muscle has been associated with the sensitivity of the muscle to insulin and this has relevance to type 2 diabetes.
The objective of this study was to use transmission electron microscopy to examine how the lipid droplets are distributed in the arms and legs of well-trained cross-country skiers before and after an intensive bout of exercise. These findings will be compared with subsequent studies on type 2 diabetic patients.
GREATER FAT STORAGE IN MUSCLE FIBRES
Two distinct pools of lipid droplets could be observed in human skeletal muscle fibers – one beneath the plasma membrane and the other between the contracting apparatus of fibers. "By estimating lipid droplet content per unit volume of fiber space in resting muscles, we could find higher lipid droplet content in leg than in arm muscle fibers, and higher content in slow-twitch than in fast-twitch muscle fibers", Han-Show Koh states. Slow twitch muscle fibers are predominantly used during endurance exercise, while fast-twitch muscle fibers are more involved during strength and intensive exercises. They also observed that during exhaustive exercise, lipid droplets between the contracting apparatus, but not those beneath the plasma membrane, were utilised by both slow- and fast-twitch muscle fibers.
In summary, there appears to be greater fat storage in muscle fibers, which are frequently used during endurance exercise, and the fat stores in close vicinity to the contracting apparatus of muscles are preferentially used during exercise.
AUTHORS AND AFFILIATIONS
Koh HE1, Nielsen J1,2, Saltin B3, Holmberg HC4, Ørtenblad N1.
1Department of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC), University of Southern Denmark, Odense M, Denmark
2Department of Pathology, SDU Muscle Research Cluster (SMRC), Odense University Hospital, Odense C, Denmark
3Copenhagen Muscle Research Centre, University of Copenhagen, Copenhagen, Denmark
4School of Sport Sciences, UiT Arctic University of Norway, Tromsø, Norway
Source: The Journal of Physiology