Benefits of exercise for diabetic patients
Regularly supervised exercise programmes ± dietary advice has been shown in international studies to reduce the risk of developing type 2 diabetes in people with impaired glucose tolerance: NICE Public Health
Guidance 38; Lancet 2006: 368:1673–9;
doi:10.1016/S0140-6736(06)69701-8. Lancet 2009; 374:1677–86;
This effect is as good as, or better than, giving metformin. Successful participants required frequent (e.g. every 1–2 ths) individualized advice and encouragement from physical trainers ± dietitians and other healthcare professionals.
Exercise is good for people with diabetes
• increases insulin sensitivity.
• improves glucose tolerance.
• aids weight reduction.
• reduces the risk of coronary heart disease.
• makes many people feel good.
Insulin and/or muscle contraction stimulates the movement of GLUT4 to the cell surface.
GLUT4 then ‘opens the door’ into the cell and glucose is taken up passively from the bloodstream as needed, e.g. during exercise for energy and after exercise to replenish glycogen stores.
As the blood glucose concentration falls, pancreatic insulin release is reduced, glucagon rises, and glucose is released from the liver glycogen stores to ‘top up’ the blood glucose concentration.
If CHO has been eaten, this will be absorbed into the bloodstream and insulin will be released, if necessary, to store it in the liver and/or facilitate its use by the exercising muscles.
Liver glycogenolysis will cease while the glucose derived from the meal is distributed.
However, if exercise continues and the blood glucose level falls, insulin release will fall and liver glycogenolysis will again release glucose into the circulation.
With prolonged exercise, adrenaline and glucagon levels rise, glucose and insulin levels fall, and fatty acids are released as fuel.
This process can still occur in a person with diabetes treated by diet alone, and to a large extent in patients being treated with metformin.
However, as soon as sulfonylureas or, particularly, insulin injections are introduced, the fine-tuning of glucose balance in exercise is disturbed.
The effects on blood glucose and other biochemistry, such as lipids, in a person with insulin-treated diabetes who exercises depend on the amount of circulating insulin and how much food has been eaten . The crucial difference between the diabetic and non-diabetic athlete is that there is no fine on–off control of insulin release.
Someone who is insulin-deficient is likely to have a high blood glucose. Exercise will further increase the blood glucose as the stress hormone response releases glucose from the liver.
As exercise continues, the muscles take up glucose from the bloodstream (much of this uptake is insulin-independent although insulin will move more GLUT4 transporters to the muscle cell surfaces).
However, this effect is unlikely to outweigh that of hepatic glucose release. Fat breakdown, lipolysis, which occurs in prolonged exercise to provide free fatty acids as an additional fuel, may be followed by ketone formation in insulin deficiency.
Any food eaten will merely serve to exacerbate the hyperglycaemia, thus exercise may worsen hyperglycaemia and promote ketosis in an insulin-deficient person.
Hypoglycaemia may ensue in someone who has a large subcutaneous reservoir of injected insulin, especially if they increase the circulation to the injection site by the exercise.
As before, the exercising muscles will use their stored glycogen. The presence of insulin ensures good glucose uptake by the exercising muscles. However, high plasma insulin concentrations inhibit glucose release from the liver, further reducing the blood glucose concentration. Hypoglycaemia rapidly ensues.
This situation can be prevented by eating CHO which will be absorbed and top up the blood glucose level as exercise proceeds. Glucose absorption from the gut is not dependent on insulin.
In patients taking sulfonylureas, the drugs enhance pancreatic insulin release, as well as improving tissue glucose uptake. Thus they may produce hyperinsulinaemia and can cause exercise-induced hypoglycaemia.
Which form of exercise and how much?
To derive full benefit from exercise it should be regular—at least 20–30 min on at least 5 days each week.
The aim should be to keep the heart rate within the training zone, i.e. between 60 % and 85 % of the maximum heart rate (ADA advises at least 50–70 %.) The heart rate should not exceed the maximum, which is calculated by subtracting the age from 220.
Two or three 10-minute episodes are nearly as good as one longer one. Obese patients who are not reducing calorie intake may have to do double this exercise requirement to lose weight.
All exercise programmes should start gradually, building on existing exercise capacity and capability. Long-term success is improved by continued contact or support after the initial programme.
Exercise does not have to be weight-lifting or marathon running. A brisk walk will maintain the pulse rate within the training zone.
Furthermore, exercise which does not reach the training zone, such as gardening or gentle swimming, can be helpful in improving well-being and maintaining a full range of joint movement.
Exercise may be anaerobic (e.g. sprint), aerobic (e.g. running, swimming, dancing), or resistance (e.g. weight-training) or a combination of these.
All forms of exercise have been shown to improve glucose levels, but aerobic physical exercise is likely to reduce the HbA1c more.
Exercise is often added to a weight-reducing diet. Note that exercise has beneficial effects on its
own (e.g. improved glucose control).
Helping people with diabetes to exercise safely
Use common sense. Always start gradually. The exercise activity(ies) chosen must be practical and fit in with everyday life in all seasons. Patients should seek expert advice (e.g. a trainer or join a club/gym) for activities that are out of the ordinary for them. If it hurts, stop.
Exercise fot Diet-treated diabetes
No special measures need to be taken regarding blood glucose balance if the glucose is well controlled on diet alone.
Exercise for Non-insulin-treated diabetes
Metformin alone usually presents no problem, although if insulin sensitivity increases and weight is lost, the metformin dose may be reduced.
Unexpected or vigorous exercise in patients taking sulfonylureas occa-sionally causes hypoglycaemia, which may be prolonged. In this case, the person should check his/her blood glucose during or after exertion and eat some CHO if necessary.
If hypoglycaemia ensues, the person should eat a series of small snacks until he/she is sure that the blood glucose has stabilized.
The blood glucose must be checked regularly for at least 24 hrs. If the exercise is planned, it is better to reduce the dose of sulfonylurea or meglitinide before the exertion so that hypoglycaemia is prevented.
If too much CHO is eaten to cover the exertion, excess insulin will be released and this may compound any late exercise-induced hypoglycaemia.
If the exercise is regular, a long-term reduction in sulfonylurea dosage may be possible.
Incretin-effect enhancing drugs are relatively new agents and experience of vigorous exercise when on them is limited. These drugs suppress gluca-gon which is needed for glucose release from the liver. However, a study in
non-diabetics given exenatide found that glucose levels rose during exercise and fell later.
Many patients will also be taking other drugs that could cause hypoglycaemia so the combination needs to be looked at. The other drugs may need to be reduced before exercise.
Exercise for Insulin-treated diabetes
Exercise is a common cause of hypoglycaemia. It is preferable to reduce the insulin acting during the exertion (and afterwards if exercise is vigorous or prolonged exercise). Eating extra can result in an increase in weight, especially in type 2 diabetes.
In general, advise 15–30 g glucose before exercise. If the exercise is very vigorous, unusual, or hazardous, take about 15–45 g every 30–60 mins and afterwards.
Tailor advice to the individual patient and situation. Encourage blood glucose testing as the symptoms of hypoglycaemia can be hidden by the sweating, tachycardia, and breathlessness of exercise.
For prolonged exertion it is theoretically helpful to eat some high-fibre CHO as well.
For planned exercise, the insulin acting during the time of exertion should be reduced beforehand. If the extent of the exertion is unknown (as in learning a new sport) it is better to reduce the insulin by about 20–40 % for the first few occasions.
Inject the insulin away from any exercising muscle. Take care to avoid the risk of hypoglycaemia while swimming or driving home.
The next meal should contain extra high-fibre CHO to prevent subsequent hypoglycaemia. The next dose of insulin may also need to be reduced after vigorous or endurance exercise.
Hypoglycaemia may occur up to 24 hrs after exercise as the body replenishes its glycogen stores.
There is no simple calculation for the amount of insulin dose reduction and the amount of extra CHO. Each person has to work it out for him/herself. The key is SMBG. This should be done four times a day (before each meal and before bed) and also immediately before and after the exercise until it has become familiar.
As people train regularly, they will need less extra food for exercise and less insulin reduction.
Glucose control in dangerous activities
This applies mainly to people on insulin injections. In some sports (e.g. suba-qua diving, hang-gliding) a person could die if he/she becomes confused or comatose.
Other sports involve taking responsibility for others, as either coach or leader, or in sharing safety (e.g. belaying a climber).
There is little or no margin for error and the individual must ensure that hypoglycaemia will not occur.
It is safer to have a buddy who knows about the diabetes and what to do if hypoglycaemia occurs.
Reduce the insulin dose which acts during the activity by 20 % (50 % if hypoglycaemia prone or no warning of hypoglycaemia).
The last meal before the activity should contain more CHO than usual. Eat an appropriate double snack if there has been a long preparation time for the activity, especially if this has involved exercise (e.g. rowing out to a diving point,
walking to the base of a climbing route).
Check SMBG before starting the hazardous activity. If it is < 6 mmol/l eat an additional snack and recheck
SMBG after 15–30 mins. Immediately before starting the activity (e.g. just before putting foot to rock) eat 15 g glucose.
The aim is for the activity to take place on rising glucose from gut absorption which is independent of insulin concentration.
The same principles can be applied for situations in which hypoglycaemia could let the person down (e.g. in competition) or let others down (e.g. team sports).
The difficulty lies in balancing freedom from hypoglycaemia and safety vs impairment of performance by hyperglycaemia.
Each sport-sperson has to spend some time experimenting for themselves. Start sugary and then refine.
There are few activities which are unsuitable for people with diabetes. Only patients who are competent and confident in glucose self-management and who have informed support from a diabetologist should undertake
high-risk activities such as scuba diving or ice climbing.
Unplanned ‘one-off ’ holiday activities are particularly dangerous as equipment and supervision vary, food may be unfamiliar, and communication may be difficult.
Both patients and professionals may find helpful advice on RunSweet, a site about diabetes and sport: M <http://www.runsweet.com/index.html>
Are you Fit to exercise?
Doctors and nurses are frequently asked to confirm fitness to exercise. In general, if the patient can walk briskly without problems he/she is usually fit to start an exercise programme. Areas to consider are:
• hypoglycaemia awareness or not (see advice about glucose)
• autonomic neuropathy
• No symptoms. Diabetes may modify cardiac symptoms so their lack cannot guide the degree of exercise that can be undertaken.
At the time of writing routine cardiac screening before exercise is not recommended. In patients with macro- or microvasculopathy, seek symptoms of cardiac disease and consider cardiology assessment before
they start an exercise programme.
• Known cardiac disease. Ask a cardiologist’s advice. Appropriate exercise is good for people with coronary heart disease, but only with the advice of a cardiologist after any treatment required has been instituted.
• Look at the feet—are they at risk?
• All patients should have appropriate footwear for their chosen activity. Ask the podiatrist if advice is needed.
• Neuropaths must be careful to avoid injuries, rubs, and blisters, and increased callus formation on pressure areas.
• Charcot feet—avoid weight-bearing exercise. Risk of multiple fractures.
• Foot ulcers—avoid weight-bearing exercise.
• Patients with peripheral arterial disease should keep their feet warm in cold weather, and always protect them.
• Seek and treat athletes’ foot.
Unstable proliferative retinopathy is a contraindication to exercise (especially resistance) as it could cause blindness from vitreous haemorrhage.
Seek ophthalmologist’s advice after treatment.
• Cardiac autonomic neuropathy can cause sudden death. ADA 2014 states ‘individuals with diabetic autonomic neuropathy should undergo cardiac investigation before beginning physical activity more intense than
that to which they are accustomed.’ M <http://care.diabetesjournals.org/content/37/Supplement_1/S14.full.pdf>
Autonomic neuropathy can also cause postural hypotension and hypoglycaemic unawareness
What to tell a trainer or exercise supervisor
Patients should tell anyone supervising their exercise that they have diabetes. If they are on most non-insulin treatments there are unlikely to be any significant issues. Insulin-, sulfonylurea-, or meglitinide-treated patients
should explain that they have adjusted their diet and treatment for the exercise and that problems are unlikely.