Family doctor



EXERCISE AND DIABETES - a patient's guide


Controlling diabetes and exercising can be a balancing act. This article gives helpful advice on how people with diabetes can exercise safely.


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.


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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