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EXERCISE AND DIABETES - a patient's guide
Dr Michael Crooke - Biochemistry Pathologist
EXERCISE AND TYPE 1 DIABETES
The choice to exercise or to play sport can be difficult
for those with type 1 diabetes. A common disincentive is
that control may become erratic, ranging from hyper to hypoglycaemia
over short time periods. However, the benefits of exercise
usually outweigh the risks and knowledge of the interactions
of insulin and exercise will minimise any uncertainties.
Important benefits of exercise are improved general fitness,
better blood lipids, maintenance of ideal weight, lower
blood pressure and requirement for overall lower doses of
insulin. Especially for children with diabetes, there is
better self esteem/peer acceptance. Within wide limits,
having to take insulin should not restrict participation
in exercise or sports even those of a vigorous or competitive
nature.
Those who have established complications of diabetes should
check with their medical advisers, as some activities may
be more risky and similarly those who are about to newly
begin vigorous exercise in middle age should have a check-up
by their GP or specialist.
Exercise and body function
To help cope with possible swings in blood glucose during
and after exercise it is necessary to have some understanding
of what happens to body metabolism during exercise when
diabetes is not present. Resting muscles mainly use fats
(fatty acids) for producing energy but as exercise begins,
the muscles begin to use glucose as well. This glucose comes
from both the blood and stores of glucose in muscle (muscle
glycogen).
The more intense the exercise, the faster glucose is used.
As exercise continues the muscles use a mixture of fatty
acid and glucose, but eventually the glucose from stored
muscle glycogen runs out. If extra glucose from stored muscle
glycogen runs out, if extra glucose isn't found blood glucose
would drop and hypo would occur, even in people without
diabetes, after only about 30 minutes of moderate exercise.
The extra glucose to continue exercise comes from the
liver, by two different mechanisms. Firstly, the liver can
break down its own stores of glycogen and release the resulting
glucose to the blood so that it can be used by the exercising
muscle. Secondly, the liver can make new glucose from protein
and fat. To allow these two processes to work efficiently
it is necessary for the body to make some changes in hormone
levels. These are to decrease levels of insulin and to increase
levels of glucagon and adrenalin. These changes happen automatically
as blood glucose begins to fall and the overall affect is
that blood glucose remains constant over hours of exercise,
even including marathons.
In type 1 diabetes it is not possible for the automatic
changes in insulin to happen and so exercise may be complicated
by too much or too little insulin being present, depending
on the time of the last injection and the type(s) of insulin
used.
Too much insulin causes exercising muscle to use glucose
and prevents the liver from releasing glucose from glycogen
and from making new glucose. Thus body glucose levels can
fall quickly, possibly leading to a hypo.
The main risk of hypoglycaemia occurs when exercise is
taken at or near a peak of insulin action, e.g. 2-3 hours
after an injection of fast acting insulin. Just taking extra
carbohydrate may not be enough to prevent this risk because
regular exercise causes the same dose of insulin to work
better, using up glucose faster. A further factor, which
increases the risk of hypoglycaemia, is that insulin may
be absorbed faster during exercise, no matter which injection
site is used. Thus it is usually necessary to reduce insulin
before exercise.
If there is too little insulin present (a long time since
last injection, or insulin reduced too much) blood glucose
may actually increase during the exercise period. This happens
because low insulin prevents muscle using glucose efficiently
but also because the low insulin allows glucagon and adrenalins
to very rapidly break down liver glycogen and to make new
glucose, thus releasing excessive amounts into the blood.
On a lesser scale this is just like what happens in diabetic
ketoacidosis and, in fact, some ketones may appear if vigorous
exercise is undertaken when insulin is too low.
Basic guidelines to minimise risks of hyper or hypoglycaemia
related to exercise:
Avoid exercise if glucose is over 14mmol/L and there are
urine ketones
Or if glucose is over 16mmol/L without ketones (adults)
Or if over 20 without ketones (children)
Or if glucose is under 6 and no snack is available prior
to exercise
Measure blood glucose before intended exercise. Avoid
exercise near peaks of insulin, especially fast -acting
insulins. Often this isn't possible to arrange so careful
reduction of insulin is the only option.
Don't exercise alone and do consider a medic alert bracelet.
The abdominal site is probably preferable for injection,
as absorption of insulin during exercise appears to be more
consistent than from the thigh.
Measure blood glucose after 30-60 minutes of moderate-vigorous
exercise. The usual signs of hypoglycaemia are often not
easy to pick during exercise.
Carry a carbohydrate snack. This will usually be required
after 30-60 minutes of moderate-vigorous exercise as such
exercise can use 20-50 grams of carbohydrate per hour. The
amount required may be more or less, depending on the starting
blood glucose.
If possible, exercise at the same time each day. This
makes it easier to observe each individual's response to
exercise and to adjust the food/insulin as required.
Further fine tuning revolves around knowing your own responses
and adjusting food and insulin. It is useful to record details
of what happened at each exercise period.
Adjustment of food and exercise
It is difficult to give anything but general guidelines.
Individuals vary greatly and need to work out their own
responses and requirements. On average, moderate exercise
in an adult (enough to make you puff) uses an extra 10-15
grams of carbohydrate each hour and vigorous exercise may
use 2-3 times this amount. This needs to be replaced, starting
during the exercise. A carbohydrate drink is suitable at
this stage.
Not all the carbohydrate used needs to be replaced during
the exercise but it is essential not to be misled by a normal
or acceptable glucose just after the exercise. The muscle
and liver glycogen that has been used up needs to be replaced
and this takes place over the 12 hours after exercise. Thus
snacks are often needed after the exercise to prevent late
hypoglycaemia, more especially when exercise has been vigorous.
It may be prudent to have a slightly higher glucose target
before bed on exercise days and to have extra supper if
this is not achieved.
Children are complicated
Often it has not been possible to decrease insulin in
anticipation of exercise. Extra snacks may be required for
hours.
It is virtually impossible to calculate energy output
and thus carbohydrate requirements in spontaneous play/exercise.
Some children may need up to 30-50 percent more carbohydrate
on exercise days than on non-exercise days, as well as 30-50
percent less insulin.
Adjustment of insulin and exercise
Individuals vary even more greatly in their response to
insulin than to food so no detailed plan is available to
suit everyone.
If exercise is planned it is usually necessary to reduce
insulin unless the exercise is very mild and less than 20
minutes.
Reduce the insulin that will be acting most during the
exercise period. 30 - 50 percent reductions may be required
for moderate vigorous exercise and some individuals have
been known to require 80 percent reductions, especially
of fast acting insulin.
Ensure that peak action times of long acting insulins
are taken into account as well as those of fast insulins.
The long acting insulin may also need to be reduced to avoid
late hypoglycaemia.
Even if exercise is planned and insulin is reduced, carbohydrate
supplements will be needed for any more than very mild exercise.
Regular exercise can decrease the overall dose of insulin
required, even on non-exercise days. This is because exercise
improves the sensitivity of muscle to insulin and the same
glucose uptake is achieved at lower levels of insulin.
It should be remembered that although exercise does have
health benefits the main reason for exercise should be because
you enjoy it. It should not be a penance punctuated by hypo
or hyperglycaemia.
EXERCISE AND TYPE 2 DIABETES
The benefits of exercise may well be greater in type 2
than in type 1 diabetes.
One of the major factors involved in causing type 2 diabetes
is insulin resistance, e.g. normal or even high levels of
insulin are present, but it does not work efficiently. The
exact cause of insulin resistance is still the subject of
much research and drugs are becoming available to decrease
resistance but the most sensible approach is to exercise.
Regular exercise, even walking, decreases insulin resistance,
allowing the person's own insulin to lower glucose better.
Exercise is especially beneficial in those who are overweight,
as the weight loss associated with exercise also decreases
insulin resistance. Because of this double effect, just
a moderate amount of mild exercise, even with only a small
weight loss, can have a large effect on blood glucose. It
is not necessary to run marathons, often just 30 minutes
of walking daily will be enough.
Exercise in type 2 diabetes is also generally beneficial
to cardiovascular fitness and in helping reduce cardiovascular
risk factors at a time of life when such reductions are
most beneficial (most of those with type 2 diabetes are
diagnosed over the age of 45). Exercise lowers blood pressure
and improves the blood lipid profile. The lipid lowering
effect is most marked for triglycerides, which are often
raised in type 2 diabetes. Recent research suggests that
it is especially important to lower triglycerides in type
2 diabetes because they tend to be associated with a higher
risk of disease of the heart and blood vessels than is shown
by the more well known cholesterol level.
As in type 1 diabetes it is important to be aware that
there may be risks associated wit exercise in type 2 diabetes
and that the risks will be greater in some individuals than
others. In general, walking if able, is suitable for everyone.
For higher levels of exercise it would be wise to have a
check up, especially if type 2 diabetes has been present
for more than five years. As in type 1 diabetes, some forms
of exercise will be unsuitable if there is evidence of heart/blood
vessel disease or complications of diabetes.
For example, jogging or high impact aerobics is unwise
if neuropathy is present and gym work with heavy weights
should be avoided if significant retinopathy is present.
The same problems with hypoglycaemia and exercise as in
type 1 diabetes exist for those with type 2 diabetes who
are on insulin and the precautions and solutions are the
same. For those being treated with diet or diet plus tablets
there are fewer problems. The ability for insulin levels
to decrease during exercise remains, so hypoglycaemia is
automatically guarded against. Those who are on diet or
diet plus metformin will not become hypoglycaemic from exercise.
There is a risk of hypoglycaemia from the sulphonylurea
group of drugs, but if this occurs, the problem is usually
simply resolved by reducing the dose of the medication.
With proper adjustment of the sulphonylureas it should not
be necessary to take extra carbohydrate before, during or
after exercise in those with type 2 diabetes. The common
sulphonylureas are glibenclamide, glipizde, gliclazide and
tolbutamide.
* Reprinted courtesy of Wellington Diabetes News, and
Dr Michael Crooke.
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