Type 2 diabetes develops in stages. The onset of the process
involves a decreased ability of insulin to stimulate muscle to clear
glucose from the blood. So-called “insulin resistance” of muscle
is a hallmark of the metabolic syndrome, which is considered to
be a precursor of frank diabetes (30). Insulin secretion is amplified
in the initial phase of insulin resistance to enable muscle to
clear glucose from plasma adequately to maintain normal glucose
concentrations. As the metabolic syndrome progresses to
diabetes, increased insulin secretion is unable to effectively
counterbalance the ineffectiveness of insulin to stimulate muscle
glucose uptake, and glucose intolerance ensues. Only in the later
stage of diabetes does the pancreas lose the ability to secrete extra
insulin in response to hyperglycemia. Disruption of the normal
rate of muscle glucose uptake by muscle is thus central to the
onset and progression of diabetes (31).
A relative increase in body fat is an appealing explanation for
the decline in insulin sensitivity in both obese and elderly individuals.
A higher percentage of body fat generally translates to a
higher rate of appearance of free fatty acids (FFAs) in plasma
(32), and a relation between an elevated availability of FFAs and
insulin resistance has been recognized since the “glucose–fatty
acid cycle” was proposed by Randle et al (33) in 1963. However,
over the past few years it has become evident that changes in the
metabolic function of muscle itself plays a more direct role in the
genesis of insulin resistance than previously appreciated. The
central thesis of the glucose–fatty acid cycle is that elevated
plasma FFA concentrations limit glucose uptake in muscle by
inhibiting the oxidation of glucose (33). Thus, according to this
theory, the genesis of insulin resistance lay entirely with the
increased availability of FFAs, and the muscle responded normally
to that signal to limit glucose uptake and oxidation. However,
research done in our laboratory (34), as well as in others
(35), has shown that the glucose–fatty acid cycle was inadequate
to explain regulation of muscle glucose uptake in a physiologic
setting. Rather, alterations in metabolic function within the muscle
are more likely at the heart of the genesis of insulin resistance.
Recent studies that used new applications of magnetic resonance
spectroscopy to quantify triacylglycerol deposition in
muscle have revised thinking about possible mechanisms by
which alterations in lipid metabolism may affect insulin sensitivity
in muscle. Triacylglycerol deposition in muscle has been
found to be associated with insulin resistance in a variety of
circumstances (36 –39), whereas obesity without insulin resistance
is not associated with increased triacylglycerol deposition
in muscle. Increased triacylglycerol deposition in muscle has
been interpreted to be an indicator of dysfunctional muscle lipid
metabolism that is likely related to insulin resistance by mechanisms
independent of total body fat mass (40). An accumulation
of intracellular triacylglycerol results from an imbalance between
tissue fatty acid uptake and fatty acid disposal. Fatty acid
uptake by muscle is directly proportional to delivery in a wide
variety of circumstances (27). Although fatty acid delivery to
muscle is generally elevated in obesity (because of a large fat
mass), triacylglycerol deposition in muscle is not elevated in
obese subjects who are not insulin resistant (35). It is becoming
clear that, rather than an increased delivery of FFAs to muscle, it
is more likely that impaired disposal via oxidation is the principal
basis for accumulation of triacylglycerol deposition in muscle