Basic HTML Version
VOLUME 9 NUMBER 2 • JUNE 2012
67
SA JOURNAL OF DIABETES & VASCULAR DISEASE
REPORT
response, blood glucose levels may remain elevated for several
hours post exercise.
11
Research over the last five years has suggested that intermittent-
type activities such as basketball, hockey and football, where periods
of high-intensity sprints are interspersed with periods of moderate-
intensity jogging, may have a more favourable blood glucose
response during and after exercise. In theory, the adrenal response
to the high-intensity sprints may prevent the hypoglycaemic effect
of the moderate-intensity jogging.
11
Improved insulin sensitivity
Numerous mechanisms may explain why exercise may improve insulin
sensitivity, including increased muscle blood flow (capillary blood
flow in particular), increased insulin binding by muscle receptors
and increased insulin-regulating glucose transporters. However, the
primary theory and benefit of exercise occurs during the 12 to 24
hours post exercise, during which time glycogen is replenished.
During the course of exercise, liver and muscle glycogen stores
are utilised as an energy source. Post exercise, the liver and muscle
cells draw glucose from the blood to replenish the stores utilised
during exercise. This process requires little insulin and is enzyme
initiated. When the glycogen stores are low, glycogen synthase
levels increase, resulting in glucose uptake from the circulation.
Regular training
Regular, sustained exercise or training has a different overall
effect on blood glucose compared with a single exercise session.
The literature suggests that patients with type 1 diabetes should
exercise daily or at least on alternate days to improve and then
maintain their insulin sensitivity. Insulin sensitivity is enhanced with
regular exercise/training due to enhanced insulin-mediated glucose
disposal throughout the whole body. Furthermore, muscle levels of
GLUT 4 and the activity of glycogen synthase increase significantly
with training. However, with cessation of regular activity, insulin
sensitivity is rapidly lost within days.
14
Pharmacokinetics of insulin
Factors influencing insulin action
The type of insulin injected, in particular the time of peak action,
influences absorption of insulin. Furthermore, the circulatory effects
of certain types of exercise may influence the absorption of insulin.
Endurance-type activities may increase total body circulation,
while resistance exercise may attract blood flow primarily to the
exercising muscle group. Because of these circulatory differences,
hypoglycaemic episodes increase with endurance-type activities.
13
The muscle mass or number of muscle fibres recruited during
swimming may be higher than cycling because one is exercising
both upper and lower limbs. This also increases the frequency of
hypoglycaemia. Jogging may also lead to more hypoglycaemia
than cycling due to the increased recruitment of muscle fibres with
weight-bearing exercise.
13
Ambient temperature
A warm environment may lead to more vasodilatation and
better absorption and circulation of insulin. Exercise in a warm
environment also increases the stress on the cardiovascular system
and thus increases energy expenditure. This combination may lead
to more hypoglycaemia.
13
A cold environment at rest may lead to vasoconstriction and
lower levels of circulating insulin. However, when exercising in a cold
environment, suchas swimming, theremaybe increasedglucoseuptake
to heat the muscles and this may lead to more hypoglycaemia.
11
Injection site and injection depth
One should always try to exercise the muscles distal to the injection
site. A cyclist should inject in the abdomen as opposed to the
leg.
13
In addition, the latest research on insulin needles suggests
that shorter 4- and 5-mm needles may be just as effective as 8-
to 12-mm needles. Their use may prevent deeper intramuscular
injections and lower the frequency of hypoglycaemia.
Hydration levels and smoking
If a person is well hydrated, he/she may absorb more insulin and
circulate it more efficiently. Smoking decreases capillary blood flow
and consequently absorption.
If we combine the blood glucose effects related to duration and
intensity of exercise with the lack of neuro-endocrine precision
found in type 1 diabetes, and add the variability of insulin and
the various factors influencing insulin action mentioned above, it
is difficult to ascertain how exercise could reliably benefit blood
glucose control. To compound this problem, one of the primary
factors preventing good glycaemic control may be hypoglycaemia
or the threat of hypoglycaemia.
Hypoglycaemia
Fear of hypoglycaemia is the primary factor affecting the attitude
of patients towards exercise. According to Rabasa-Lohret, those
individuals who best understood how insulin functioned in
their body were shown to be less fearful of physical activity and
hypoglycaemia. Furthermore, those individuals with the greatest
fear of physical activity had the poorest control of their diabetes.
15
In McMahon’s euglycaemic clamp studies in 2007, patients
performing endurance-type activities at 16:30 had a biphasic need
for glucose infusion.
16
Glucose infusion was required during the
45 minutes of exercise and for 45 minutes post exercise. Further
glucose infusion was required seven to 11 hours post exercise, which
equated to a potential hypoglycaemic episode between 02:00 and
04:00. Dealing with glucose requirements during activity is not too
difficult. However, dealing with late hypoglycaemia, occurring in
the late evening and early morning has always been a challenge.
16
According to Riddel, exercise late in the day may lead to
nocturnal hypoglycaemia, which may be unnoticed during sleep
in the majority of individuals.
12
The incidence of hypoglycaemia
may be as high as 26% in adolescents and children on the night
of exercise. This may be due to a child or adolescent’s inadequate
glycogen-replenishment strategies, low glycogen-storage capacity
and blunted counter-regulatory responses overnight.
Strategies to prevent hypoglycaemia
The literature suggests that while taking multiple daily injections,
the most favourable times to exercise are in the pre-prandial periods
before breakfast and before dinner. The reason for this suggestion
is that the active patient is less likely to become hypoglycaemic
at these times due to increased insulin resistance.
17
Tansey
et al.
explained that performing low/moderate-intensity endurance-type
exercise postprandially resulted in an 86% chance of developing
hypoglycaemia if the patient’s glucose level was below 7.0 mmol/l
before the start of the exercise.
18