Objective: Insulin resistance is closely associated with two disparate aspe
cts of lipid storage: the intracellular lipid content of skeletal muscle an
d the magnitude of central adipose beds. Our aim was to determine their rel
ative contribution to impaired insulin action.
Research Methods and Procedures: Eighteen older (56 to 75 years of age) men
were studied before elective knee surgery. Insulin sensitivity (M/DeltaI)
was determined by hyperinsulinemic-euglycemic clamp. Central abdominal fat
(CF) was assessed by DXA. Skeletal muscle was excised at surgery and assaye
d for content of metabolically active long-chain acyl-CoA esters (LCAC).
Results: Significant inverse relationships were observed between LCAC and M
/DeltaI (R-2 = 0.34, p = 0.01) and between CIF and M/DeltaI (R-2 = 0.38, p
= 0.006), but not between CF and LCAC (R-2 = 0.0005, p = 0.93). In a multip
le regression model (R-2 = 0.71, p < 0.0001), both CF (p = 0.0006) and LCAC
(p 0.0009) were independent statistical predictors of M/DeltaI. Leptin lev
els correlated inversely with M/DeltaI (R-2 = 0.60, p = 0.0002) and positiv
ely with central (R-2 = 0.41, p = 0.006) and total body fat (R-2 = 0.63, p
= 0.0001).
Discussion: The mechanisms by which altered lipid metabolism in skeletal mu
scle influences insulin action may not be related directly to those linking
central fat and insulin sensitivity. In particular, it is unlikely that mu
scle accumulation of lipids directly derived from labile central fat depots
is a principal contributor to peripheral insulin resistance. Instead, our
results imply that circulating factors, other than nonesterified fatty acid
s or triglyceride, mediate between central fat depots and skeletal muscle t
issue. Leptin was not exclusively associated with central fat, but other fa
ctors, secreted specifically from central fat cells, could modulate muscle
insulin sensitivity.