[IP] How Does Exercise Use Insulin w/o Glucose?
The Effects of Physical Activity on Metabolism
During physical activity, exercising muscles significantly increase their use
of oxygen and of substrates such as muscle and liver glycogen, muscle
triglycerides, and free fatty acids (6). These substrates are mobilized by the
action of epinephrine, glucagon, cortisol, and growth hormone. In a healthy
person, blood glucose concentration continues to be regulated by these hormones
and by insulin so that it remains within an appropriate range (7).
The brain requires a minimum concentration of plasma glucose for maintaining
normal functions, including surveillance of the space surrounding the athlete,
judgment, and consciousness. Because of this requirement, hypoglycemia is
severely maladaptive during exercise. At the other extreme, the kidneys may not
be able to maintain adequate circulating plasma volume if hyperglycemia and
glycosuria diminish the ability to deliver oxygen to exercising muscles.
Additionally, the metabolism of fatty acids for energy results in the generation
of ketones that, in excessive concentrations, depress plasma pH and interfere
with the normal function of nerves and muscles, including the heart. In short,
in the healthy person, the concerted action of several hormones allows for as
much as a 20-fold increase in oxygen consumption without departure of several
key substances from a narrow range of plasma concentrations needed for normal
Type 1 diabetes may involve normal hormonal responses mediating the release of
fuels or degrees of deficiency of glucagon and/or epinephrine response (8). The
availability of insulin to counter excess elevation of plasma glucose is not
under the exquisite control seen in a healthy individual. To increase glucose
absorption and insulin levels in a diabetic person, capillary blood flow
increases in exercising muscle to enhance the accessibility of insulin receptors
to the insulin in circulation. Also, blood flow to the insulin injection sites
increases. With the loss of the tight normal control of circulating insulin and
glucose, dangerously disabling hypoglycemia may occur during exercise. Another
risk is delayed hypoglycemia occurring several hours after exercise, possibly
Conversely, if preexisting mild ketosis or significant hyperglycemia is present
at the start of vigorous physical activity, ketoacidosis may be precipitated.
This occurs when the so-called counter-insulin hormones raise the level of
glucose and free fatty acids more rapidly than the enhanced insulin uptake by
muscle receptors and the exercise-induced increase of insulin concentration can
act to suppress lipolysis and ketogenesis. By careful assessment of metabolic
control, athletes with type 1 diabetes must learn to regulate the injection of
correctly adjusted doses of insulin and ingest appropriate nutrients to
duplicate the system that runs automatically and without conscious effort in
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