Figures 1, 2 and 3 contain guidelines using a traffic light approach to assist exercise specialists with clinical decision-making regarding people with diabetes starting or continuing exercise based on glucose levels and other factors. These Action Plans will be individually guided by the exercise specialist, in consultation with the individual. Figures 4 and 5 contain simplified flow charts summarizing the guidelines. These resources refer to a Diabetes Healthcare Professional who is a clinician with appropriate qualifications to understand the interactions between medications, glucose levels, carbohydrate intake and co-morbidities. For example, in Australia, this includes doctors, nurse practitioners, credentialed diabetes educators, accredited exercise physiologists, physiotherapists and accredited practicing dietitians.
Type 1 Diabetes
For people with type 1 diabetes, hypoglycaemia during and for up to 24 h following exercise are usually the main risks. The following recommendations are from a recent international consensus statement on type 1 diabetes and exercise [24].
Exercise is contraindicated if glucose has been < 2.9 mmol/L or if a hypoglycaemic event that required assistance from another person to treat the event within the previous 24 h. These situations significantly increase the risk of a more serious hypoglycaemic episode occurring during exercise. If glucose is between 2.9 and 3.9 mmol/L, exercise should not commence until the hypoglycaemia is treated. However, even after treatment, if starting glucose was 2.9–3.9 mmol/L, exercise should be avoided if alone, or the type of exercise is potentially unsafe (e.g. swimming, skiing, surfing, rock climbing etc.). The Action Plan (Fig. 1) and flow chart (Fig. 4) provide more guidance for various scenarios.
Initial hypoglycaemia treatment involves consuming one serve (15 g) of fast-acting carbohydrate and re-checking glucose after 15 min. Another serve of fast-acting carbohydrate should be administered each 15 min if glucose remains < 4.0 mmol/L. After initial treatment, monitoring is advised for clinical features of hypoglycaemia such as abnormal sweating, trembling, anxiety, hunger, weakness, dizziness, inability to think straight and tingling sensations in the mouth and/or fingers. If still wishing to exercise, ensure glucose is ≥ 5.0 mmol/L before beginning the exercise and follow up with one serve of slow-acting carbohydrate. Closely monitor glucose levels by re-testing every 15 min. If not wishing to exercise, ensure glucose is ≥ 5.0 mmol/L and follow up with slow-acting carbohydrate if the next meal is not within 30 min. Figure 1 provides examples of foods that are classified as fast- and slow-acting carbohydrate.
A number of additional factors can increase the risk of hypoglycaemia during or after exercise including increased circulating insulin from the release of residual injected insulin, inadequate glucose production from the liver, individual fitness level, glycogen recovery, the mode, duration and intensity of exercise, the environment and the person’s hydration status [33, 38]. Figure 1 provides specific guidelines for monitoring glucose after exercise. An increased risk of night-time hypoglycaemia due to afternoon/evening exercise or changes in exercise (e.g. increased intensity/duration) should lead to more glucose surveillance. Measuring glucose before bed and setting an alarm to wake up and check blood glucose (e.g. 2.00 am) or using a continual glucose monitor with an alarm is recommended. If the glucose level is < 7 mmol/L before bed, additional carbohydrates should be consumed.
As mentioned previously, it is recommended that the glucose trend be established prior to exercising with two to three glucose measures. The glucose target for the start of exercise for a person with type 1 diabetes should be individualized based on the intended type, duration and intensity of exercise, when medications were used and food consumed, the trend in glucose and exercise experience. As a general guideline for most people intending to complete any type of exercise for around 1 h, a starting glucose between 7.0 and 10.0 mmol/L is recommended. Figures 1 and 4 provide advice for when the starting glucose is outside this range and for carbohydrate consumption during exercise.
Hyperglycaemia during or following exercise may be associated with ketosis (due to absolute or relative insulin insufficiency). A glucose level > 15.0 mmol/L is used as a threshold to investigate further. This includes assessing whether food has been consumed in the previous 90 min, if the person has had their usual insulin dose, whether they are feeling well and for the presence of ketones. If small to moderate levels of blood ketones are present (0.6–1.5 mmol/L) or if ketones can not be measured then the need for a reduced corrective insulin dose should be assessed. If this is needed, then glucose should be checked after 30 min and if it is decreasing and the person is feeling well then low-intensity, short duration (< 30 min) exercise can be started with caution. More substantial ketosis is an absolute contraindication and may require medical attention. The Action Plan provides specific recommendations for the possible scenarios based on these measures. A glucose level > 15.0 mmol/L should trigger extra surveillance of the person’s general feeling of wellness. Dehydration can result from frequent urination due to hyperglycaemia and may lead to symptoms of heat illness, especially when exercising. Remaining hydrated is especially important when the glucose level is high.
Elevations in blood glucose are more likely following high-intensity or resistance exercise [39, 40]. This is likely due a number of mechanisms including an increased stress response leading to hormones such as catecholamines inducing gluconeogenesis and glycogenolysis [41]. A prolonged aerobic cool down has been recommended to minimize glycaemic excursions [24].
Type 2 Diabetes
The following recommendations for people with type 2 diabetes are consistent with a recent position statement from the American Diabetes Association. [4]
Type 2 Diabetes Treated with Insulin and/or Sulphonylureas
Hypoglycaemia during exercise, or for up to 12 h after exercise, is the main risk for individuals with type 2 diabetes taking insulin and/or sulfonylurea medication. Exercise is contraindicated if a person has had a hypoglycaemic event that required assistance from another person to treat the event within the previous 24 h, if feeling unwell or glucose is < 4.0 mmol/L and the intended exercise is being done alone or is potentially unsafe. The Action Plan (Fig. 2) and flow chart (Fig. 5) provide more guidance for various scenarios.
Glucose between 4.0 and 5.4 mmol/L may herald impending hypoglycaemia during exercise and warrants one to two serves of fast-acting carbohydrate. For individuals aware of their own response to exercise with this starting glucose level, this may be sufficient. For those new to exercise, with glucose between 4.0 and 5.4 mmol/L, exercise should not start and glucose monitoring should occur 15 min later. Once glucose is ≥ 5.5 mmol/L and the individual has no symptoms of feeling unwell, then exercise can start. Sulphonylureas are insulin secretagogues that increase the risk of hypoglycaemia during moderate to high-intensity exercise. Being aware of each person’s insulin/sulphonylurea action profile is important. People taking short/rapid/intermediate-acting insulin should avoid exercising when blood insulin is peaking.
As mentioned previously, it is recommended that the glucose trend be established prior to exercising with two to three glucose measures. If the glucose level is falling and it has been greater than 90 min since eating, then one serve of a slow-acting carbohydrate should be considered. This will be dependent on the duration and intensity of exercise, the carbohydrate intake and the glucose level prior to the start of exercise.
To prevent hypoglycaemia, the timing of exercise and/or medication administration and/or dose should be considered. If night-time hypoglycaemia is likely, check the glucose level before sleep, once during the night (e.g. 2:00 am) and immediately upon waking. If the glucose level is < 7 mmol/L before bed, additional carbohydrates should be consumed. If the glucose level is frequently within the red area of the Action Plan, a Diabetes Healthcare Professional should be consulted to review the factors that may be causing the sub-optimal glucose control.
The glucose target for the start of exercise for a person with type 2 diabetes treated with Insulin and/or Sulphonylureas is between 5.5 and 15.0 mmol/L. If exercise is longer than 1 h, additional carbohydrates are likely to be needed. If an individual has a glucose level > 15.0 mmol/L, assess whether it is due to inadequate insulin treatment, acute illness or infection or food intake. If the glucose level is measured within 2 h of eating or the previous food had a high glycaemic index, exercise may be beneficial in lowering the glucose. Extra fluid intake is advised if exercising with high glucose. If the high glucose is due to missed medications, exercise at a low intensity and ensure that the person catches up on the missed dose as soon as possible. If the high glucose level is due to acute illness or infection, postpone exercise.
Type 2 Diabetes (Lifestyle Controlled or Treated with Diabetes Medications Other than Insulin or Sulphonylureas)
The interaction of exercise with diabetes medications other than insulin and sulphonylureas has not been well studied [14, 25]. Drugs such as biguanides (e.g. metformin), thiazolidinediones (e.g. rosiglitazone), alpha-glucosidase inhibitors (e.g. acarbose), sodium-glucose transporter-2 (SGLT) inhibitors (e.g. dapagliflozin, empagliflozin) and glucagon-like peptide 1 (GLP-1) agonists (e.g. exenatide) are thought to have a minimal effect on increasing the risk of exercise-induced hypoglycaemia when used alone. However, these drugs can potentiate the hypoglycaemia effects of insulin and sulphonylureas. It is recommended that regular glucose monitoring is only necessary in individuals taking any of these medications when starting or changing an exercise program. When these medications are combined with a sulphonylurea and/or insulin, additional monitoring as per the insulin/sulphonylurea guidelines should be conducted.
To prevent hypoglycaemia, the timing of exercise and/or medication administration and/or dose may need to be considered. A doctor, nurse practitioner or diabetes educator should be consulted prior to changing medication dose. If the glucose level is frequently within the red area of the Action Plan, a Diabetes Healthcare Professional should be consulted to review the factors that may be causing the sub-optimal glucose control. The Action Plan (Fig. 3) and flow chart (Fig. 5) provide more guidance for various scenarios.