As
a result of several research studies published over the last few
years, exercise has been shown to play a critical role in the
management of blood glucose levels among individuals with type
2 diabetes. Specifically,
the acute effects of exercise have been found to cause a decrease
in plasma glucose levels, which without proper planning may cause
hypoglycemia. Hypoglycemia
is the most common risk associated with exercise for individuals
with diabetes who exercise and are treated with insulin or oral
secretagogues.
A
2001 study published in the Medicine and Science in Sports and
Exercise (Poirier,P. et. al, 2001) examined the effects of dietary
status on changes in blood glucose concentrations during an acute
bout of exercise in subjects with type 2 diabetes. Subjects were
treated with oral hypoglycemic agents where none of the subjects
used exogenous insulin, six subjects used Glyburide alone, whereas
four were treated with a combination of Glyburide and Metformin.
All subjects were previously accustomed to mild exercise,
but none had engaged in a regular exercise program for 3 months.
In addition, subjects were instructed to NOT change their dietary
habits. No medication
was taken the morning of the exercise sessions in both conditions.
The exercise session consisted of cycling for 60 minutes
at a workload corresponding to 60% of individual VO2peak.
Each subject performed the exercise protocol in a randomized
order at a 7-day interval, either in the fasted state or in the
fed state.
Results
showed that although exercise began with moderately elevated plasma
glucose levels in both situations, the plasma glucose decline
was rather small (-9%) when subjects exercise while fasted, whereas
an important decrease (-40%) occurred when exercise was performed
1-2 hours after breakfast. It is especially important to note
that blood glucose levels after exercise remained lower throughout
the day in the fed state compared to the fasting state. The researchers
concluded that breakfast-induced relative hyperinsulinemia potentiates
the blood glucose lowering effect of a 60 minute aerobic exercise
session.
This
information is of particular importance as many individuals with
diabetes who are exercising or planning to initiate exercise as
part of their diabetes management need education regarding the
optimal time to exercise.
In addition, many individuals initiate exercise without
considering when their last meal was or of what it consisted.
The blood glucose response to exercise is affected by the
type, amount, and intensity of exercise as well as time of previous
meal and current fitness level. Taking all those factors as well
as the risk of hypoglycemia into consideration, it is best to
initiate exercise 30-40 minutes post meal.
When exercise is initiated after a meal, individuals have
“fuel” on board to maintain the exercise session.
Checking blood sugars pre and post exercise serves as a
motivator as a lower post blood sugar should be observed, moreover
the pre/post measurement serves as a guide to help the exerciser
determine how much “fuel” they must have on board to sustain the
given activity.
During
unplanned exercise/activity, occurring more than 2-3 hours since
the last meal, carbohydrate replacement/snacks may be necessary
to prevent hypoglycemia. This is especially important when insulin
adjustments are not made or when exercise is of long duration.
Exercise of mild to moderate intensity lasting less than 30 minutes
may not need CHO replacement. Exercise of moderate intensity lasting
between 30-60 minutes will need 15g of carbohydrate snack. It
is important to note that exercise of moderate intensity increases
glucose uptake by the muscle 2-3 mg/kg of body weight/min above
resting levels. In addition, exercise of high intensity lasting
more than 60 minutes will need 30-50g of carbohydrate replacement.
High intensity exercise may effect the rate of glucose utilization
by the muscle as much as 5-6 mL/kg/min or an additional 350 to
420 mg of glucose for every minute of exercise. High intensity
exercise is not an appropriate starting point for sedentary individuals
with diabetes as other co-morbidities such as hypertension are
commonly observed. It is possible, however that the exerciser
will progress to participating in higher intensity activities.
For
individuals who are trying to lose weight, snacking may not be
the best option. If no other option is available and exercise
must be performed around the 3-5pm time frame, the exerciser needs
to check pre exercise blood glucose level and assess the duration,
intensity and when the next meal will be consumed. In doing so,
the exerciser will be able to determine whether a snack is necessary.
When deciding to consume a snack, the optimal time is approximately
15-30 minutes before exercise of no longer than 45 minutes.
For
individuals who take insulin and are starting an exercise program,
it is always best to check blood sugars pre and post exercise
to help determine the effects of exercise.
A decrease in the amount of insulin is almost always necessary.
If exercise is planned for after breakfast or any meal, reduction
in short acting insulin is permissible.
If exercise is planned for later in the afternoon between
meals, a reduction in the long acting insulin taken before breakfast
is essential. More often than not, the long acting insulin will be peaking
at that time. Exercise should not be performed during peak insulin
time unless adjustments are made. Generally, a two-unit decrease
of insulin has been found to be adequate. Keep in mind, however
that as duration increases, less insulin is needed as contracting
muscles act like insulin. The pre/post blood glucose measurement
will serve as a good guide. Also of importance, is post exercise
hypoglycemia, blood sugars will continue to drop post exercise
as muscles and liver are replenishing glycogen stores.
Because of this mechanism, check blood sugars randomly
throughout the post exercise window (i.e. 2hrs post and before
next meal) to help establish patterns.
For
individuals taking oral secretagogues (sulfonylureas, meglitinide,
nateglinide) who experience repeated low blood sugars post exercise
(i.e. less than 80) despite planning exercise after meals, it
may be best to add one more carbohydrate choice to the meal prior
to exercise. This
may prevent snacking after exercise and therefore increasing the
potential for weight loss. If low blood sugars are still occurring despite the additional
CHO choice then medicine changes may be warranted and the physician
managing the diabetes needs to be notified.
Typically,
hypoglycemia is not of concern for individuals with type 2 DM
who take metformin. What needs to be of concern is the possible
activation of “leaky
liver.” Exercisers should check BG levels pre and post exercise
when it has been more than 3 hours since last meal. If a rise
in BG levels is occurring, then the individual has disrupted the
balance between the rate of glucose disposal via liver and glucose
uptake via muscle. Glucose uptake via muscle cannot maintain adequate
utilization of glucose for energy at the rate in which the liver
is disposing of stored glycogen as glucose, therefore higher glucose
levels are present in the blood stream. By ingesting a carbohydrates
pre exercise, blood glucose levels should drop post exercise because
excess demands are not placed on the hepatic system in terms of
supplying additional glucose. Rather, the exerciser is drawing
glucose from the meal/snack that was just consumed. Over time the hepatic functioning should improve along with
increased insulin sensitivity.
Hypoglycemia
is the most common risk of exercise for individuals with diabetes.
This phenomenon is less likely if individuals plan their
exercise around meals. It is important to always wear identification
while exercising and have quick glucose supplies around all the
time. Glucose tablets are a good example as well as juice. Remember
to treat low blood sugar with the rule of 15. Consume 15g of total
carbohydrate and wait 15 minutes, check blood sugar and assess
how long it will take to get home if away or when the next full
meal will be consumed.
Kristina
Sandstedt, MS received her Masters of Science degree from the
University of Montana-Missoula. She is certified as an Exercise
Specialist through the American College of Sports Medicine.
She is currently working as a Clinical Exercise Physiologist
and Diabetes Educator for the Early Outpatient Phase II Cardiovascular
Rehabilitation unit and the outpatient Diabetes Self-Management
classes as well as individual consultations at Boone Hospital
Center in Columbia, Missouri.
