and other potentially active products of fatty acids. In vivo capacity
to oxidize fatty acids is reduced in insulin-resistant individuals
(35). This deficiency may be more evident during exercise
(41). It is likely that this deficiency in fatty acid oxidation is
due to a decline in mitochondrial oxidative function (42). There
are many potential causes of decreased mitochondrial oxidative
capacity, including genetics; a lack of physical activity is most
likely a major factor in patients with type 2 diabetes. Mitochondrial
oxidative capacity is decreased by inactivity (43), and as
little as a single bout of exercise [thereby stimulating triacylglycerol
deposition in muscle intramuscular triacylglycerol (IMTG)
oxidation] can transiently reverse insulin resistance (44).
It is possible thatIMTGdoes not exert a direct effect on insulin
sensitivity, but that accumulation of IMTG represents a dysregulation
of normal tissue lipid metabolism and that other intracellular
lipids or lipid products actually induce insulin insensitivity.
Fatty acids entering the cell are converted to their corresponding
fatty acyl CoAs before being transported across mitochondrial
membranes for oxidation. Fatty acyl CoAs that do not enter
mitochondria are substrates for the synthesis of triacylglycerol
and phospholipids. Diacylglycerol is a second messenger product
in the pathway of triacylglycerol synthesis and can also induce
insulin resistance by impairing the intracellular insulin signaling
cascade (45). There are other potentially active products
of fatty acyl CoAs. Thus, palmitoyl CoA is rate-limiting in the de
novo synthesis of ceramide (46). Ceramide can also induce insulin
resistance in vitro (45), although in vivo data are not yet
available.
The exact mechanisms by which disruptions in intramuscular
trafficking of fatty acids in muscle are linked to impaired insulin
signaling are under current investigation. One proposal is that
elevated intracellular concentrations of diacylglycerol activate
protein kinase C, which in turn is an inhibitor of insulin signaling
transduction (40). Regardless of the specific intracellular mechanisms
at the molecular level, it is clear that insulin resistance is
not simply the result of increased fat mass and release of FFAs
into plasma at an accelerated rate, with the muscle responding to
elevated plasma FAA concentrations. Rather, alterations in the
metabolic function of muscle are central to the development of
insulin resistance and ultimately diabetes.