Fat content in individual muscle fibers of lean and obese subjects

To examine skeletal muscle intracellular triglyceride concentration in different fiber types in relation to obesity.

Skeletal muscle fiber type distribution and intracellular lipid content were measured in vastus lateralis samples obtained by needle biopsy from lean and obese individuals.

Seven lean controls (body mass index (BMI) 23.0+/-3.3 kg/m(2); mean+/-s.d.) and 14 obese (BMI 33.7+/-2.7 kg/m(2)) individuals; both groups included comparable proportions of men and women.

Samples were histochemically stained for the identification of muscle fiber types (myosin ATPase) and intracellular lipid aggregates (oil red O dye). The number and size of fat aggregates as well as their concentration within type I, IIA and IIB muscle fiber types were measured. The cellular distribution of the lipid aggregates was also examined.

The size of fat aggregates was not affected by obesity but the number of lipid droplets within muscle fibers was twice as abundant in obese compared to lean individuals. This was seen in type I (298+/-135 vs 129+/-75; obese vs lean, P<0.05), IIA (132+/-67 vs 79+/-29; P<0.05), and IIB (103+/-63 vs 51+/-13; P<0.05) muscle fibers. A more central distribution of lipid droplets was observed in muscle fibers of obese compared to lean subjects (27.2+/-5.7 vs 19.7+/-6.4%; P<0.05).

The higher number of lipid aggregates and the disposition to a greater central distribution in all fiber types in obesity indicate important changes in lipid metabolism and/or storage that are fiber type-independent.