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66
VOLUME 9 NUMBER 2 • JUNE 2012
REPORT
SA JOURNAL OF DIABETES & VASCULAR DISEASE
Correspondence to: Andrew Heilbrunn
Biokinetics Department, Centre for Diabetes and
Endocrinology, Houghton, Johannesburg
e-mail: andrew@cdecentre.co.za
S Afr J Diabetes Vasc Dis
2012;
9
: 66–69.
Does exercise improve or impair blood glucose control
in type 1 diabetes?
ANDREW HEILBRUNN
O
ver the last four decades, large landmark studies such as the
Harvard study and the Cooper Centre Longitudinal Study
(CCLS) clearly demonstrated that regular physical activity
reduces the risk of coronary heart disease, stroke, osteoporosis, and
colon and breast cancer in the general population.
1-3
There is also
evidence that physical activity reduces obesity, osteoarthritis, lower
back pain and clinical depression.
4
With regard to type 2 diabetes, randomised, controlled trials
have demonstrated that physical activity can delay the progression
of impaired glucose tolerance to type 2 diabetes when combined
with dietary changes.
5
In patients with established type 2 diabetes,
regular physical activity significantly improves glycaemic control,
reduces cardiovascular risk factors and may reduce chronic
medication dosages.
6
Cardiovascular disease
Incidence of cardiovascular disease and mortality are increased in
both type 1 and type 2 diabetes and cardiovascular disease remains
the most common cause of death.
7
With respect to type 1diabetes,
the Pittsburgh IDDM Morbidity and Mortality study demonstrated
that at 25 years’ duration of diabetes, men who had participated
in team sports during high school were three times less likely to
report macrovascular disease and had mortality rates three times
lower than those who did not.
8
A cohort study of patients with type
1 diabetes found that the seven-year mortality was 50% lower in
those reporting more than 2 000 kcal of weekly exercise (equivalent
of ≥ seven hours of brisk walking per week) compared to those
reporting less than 1 000 kcal of physical activity per week.
9
How much exercise is necessary to bring about physiological
adaptations to decrease cardiovascular disease and increase
longevity?
Findings from the Harvard study suggest 75 minutes of physical
activity per day. The CCLS findings suggest 30 to 60 minutes of
physical activity per day, at a moderate intensity.
1,2
In light of the above benefits, diabetes associations strongly
support the role for physical activity in the management of diabetes.
For type 2 diabetes, the latest American Diabetes Association
(ADA) and Society for Endocrinology, Metabolism and Diabetes
of South Africa (SEMDSA) guidelines recommend a minimum of
150 minutes’ exercise per week, combining moderate-intensity
aerobic exercise and strength/resistance training.
10
Many of the guidelines applicable to patients with type 1
diabetes are based on understandings gained from small cohort
studies on individuals with type 1 diabetes, people without diabetes
or people with type 2 diabetes.
9
One could postulate that the type 2
diabetes guidelines would be suitable for the population with type
1 diabetes; however, the few randomised, controlled trials reported
for type 1 diabetes have not shown the same glycaemic benefits.
In the absence of comprehensive exercise guidelines for type 1
diabetes patients, a large segment of primarily young patients with
type 1 diabetes participate in marathons, high-intensity ball sports
and body building. Some may challenge themselves with recreational
climbing and diving. However, a large percentage of these patients do
not understand the complex relationship that exists between exercise
and their diabetes. Once again, this raises the question; Does regular
exercise improve or impair blood glucose control in type 1 diabetes?
Exercise physiology
Exercise physiology and in particular, that of fuel sources and delivery,
is critical to understanding the challenges faced by individuals with
type 1 diabetes. Exercise increases both oxygen demands (which
are met by the cardio-pulmonary system) and fuel demands (met
by the neuro-endocrine system).
11
Autonomic and endocrine regulation
In the non-diabetic person, insulin levels decrease with the onset
of exercise; this allows for an increase in counter-regulatory
hormones, in particular glucagon. This response leads to hepatic
glucose production, and the subsequent increase in blood glucose
is balanced by glucose uptake into the muscles. Due to this precise
neuro-endocrine regulation, blood glucose levels remain stable
under most exercise conditions.
12,13
In type 1 diabetes, the pancreas cannot regulate insulin levels in
response to exercise and there may be impaired glucose counter-
regulation, making normal fuel regulation nearly impossible.
12,13
Therefore, the patient with type 1 diabetes is at risk of becoming
hypoglycaemic or hyperglycaemic, dependant largely on the levels of
circulating insulin and the duration and intensity of the exercise.
11
Duration and intensity of exercise
Exercises performed at a low-moderate intensity, (40–80% of one’s
heart rate maximum) for a long period of time, such as jogging,
cycling and long-distance walking, may cause more hypoglycaemia
than higher-intensity exercise. This acute complication is because
this mode of exercise circulates exogenous insulin more efficiently
and the counter-regulatory response may be blunted.
11
High-intensity exercise such as sprinting or playing squash may
cause hyperglycaemia due to an excessive adrenal response. This
phenomenon may lead to ‘hepatic dumping’, which results in large
amounts of glucose entering the circulation. Due to this adrenal