EXERCISE AND INSULIN

• TYPES OF EXERCISE

• Aerobic exercise - exercise that involves continuous, repeated movements of large muscle groups. Examples include walking, cycling, jogging, and swimming. During aerobic exercise, muscles utilize energy pathways that require oxygen. If energy requirements exceed oxygen supply, anaerobic pathways are then utilized.
• Anaerobic exercise - exercise that involves alternating between short bursts of vigorous activity and periods of rest. Examples include weightlifting, sprints, and interval training. Anaerobic exercise utilizes energy pathways that do not require oxygen (e.g. muscle phosphagens, muscle glycogen). These pathways may become depleted quickly. If anaerobic activity is prolonged (e.g. greater 30 seconds), aerobic sources of energy become involved.
• Mixed exercise - exercise that is a combination of aerobic and anaerobic exercise. Examples include team sports like basketball and soccer. [1,2]

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• EFFECTS OF EXERCISE ON GLUCOSE AND INSULIN LEVELS

• Overview
• The effect of exercise on blood glucose and insulin levels is dependent on a number of variables
• Insulin dose, carbohydrate consumption, exercise type, exercise duration, and individual conditioning all have an effect on glucose and insulin regulation during exercise
• The physiologic responses discussed below are given in general terms and may vary by individual


• Aerobic exercise - during aerobic exercise, muscles increase their uptake of glucose by as much as fifty times. This effect occurs independent of insulin. In order to keep blood glucose levels steady (70 - 110 mg/dl), insulin levels decrease, and counter-regulatory hormones (e.g. glucagon, cortisol, growth hormone, norepinephrine) increase. In diabetics who use insulin, insulin levels may not fall appropriately during exercise because exogenous insulin levels are not controlled by counter-regulatory hormones. This may lead to hypoglycemia if carbohydrates are not consumed or if insulin doses are not reduced.


• Anaerobic exercise - during anaerobic exercise, muscles utilize muscle phosphagens and muscle glycogen for energy. Insulin levels do not decrease as much compared to aerobic exercise and glucose levels tend to rise secondary to counter-regulatory hormones. In diabetics who use insulin, anaerobic exercise typically causes an increase in blood glucose levels.


• Mixed exercise - during mixed exercise, the effects of anaerobic and aerobic exercise offset each other and blood glucose levels tend to stay steady


• Post-exercise - after exercise, insulin-independent glucose uptake by muscles remains elevated for approximately two hours. Insulin sensitivity also increases after exercise and remains elevated for about 24 hours in most individuals. Prolonged, strenuous exercise may increase insulin sensitivity for up to 48 hours. Diabetics who use insulin are at increased risk for hypoglycemia after exercise if insulin doses are not adjusted. The greatest risk is during the night that follows afternoon exercise (nocturnal hypoglycemia). [1,2]

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• GLUCOSE/CARBOHYDRATE ADJUSTMENTS FOR EXERCISE

• Overview
• The table below presents recommendations for carbohydrate consumption based on glucose levels checked before exercise
• The recommendations are derived from the American Diabetes Association and expert opinion
• Because a number of variables affect glucose response to exercise, individual needs may vary from the recommendations below

• References [1,2]

Carbohydrate adjustments for exercise
Pre-exercise glucose: < 90 mg/dl (< 5 mmol/L) Aerobic exercise (> 30 minutes) Ingest 15 - 30 grams of a high glycemic index carbohydrate Start exercise after blood glucose rises to > 90 mg/dl Consume 0.5 - 1 gram of carbohydrate per kg body weight per hour of exercise (0.5 - 1 gram/kg/hour of exercise) Anaerobic exercise or short-duration aerobic (< 30 minutes) Ingest 15 - 30 grams of a high glycemic index carbohydrate Start exercise after blood glucose rises to > 90 mg/dl Additional carbohydrates may not be necessary depending on prior insulin intake
Pre-exercise glucose: 90 - 150 mg/dl (5 - 8.3 mmol/L) Aerobic exercise (> 30 minutes) Consume 0.5 - 1 gram of carbohydrate per kg body weight per hour of exercise (0.5 - 1 gram/kg/hour of exercise) Anaerobic exercise or short-duration aerobic (< 30 minutes) Additional carbohydrates may not be necessary depending on prior insulin intake
Pre-exercise glucose: 150 - 250 mg/dl (8.3 - 13.9 mmol/L) Aerobic exercise (> 30 minutes) Initiate exercise and delay carbohydrate intake until glucose levels are < 150 mg/dl Consume 0.5 - 1 gram of carbohydrate per kg body weight per hour of exercise (0.5 - 1 gram/kg/hour of exercise) Anaerobic exercise or short-duration aerobic (< 30 minutes) Additional carbohydrates may not be necessary depending on prior insulin intake
Pre-exercise glucose: 250 - 350 mg/dl (13.9 - 19.4 mmol/L) Check for ketones (blood or urine). See ketone testing below. If ketones are moderate-to-large (blood ≥ 1.5 mmol/L), do not exercise. Initiate appropriate insulin therapy and/or contact health professional. If ketones are negative or small (blood < 1.5 mmol/L), initiate mild-to-moderate exercise. Delay intense exercise until glucose is < 250 mg/dl. Intense exercise may raise glucose levels.
Pre-exercise glucose: > 350 mg/dl (> 19.4 mmol/L) Check for ketones (blood or urine). See ketone testing below. If ketones are moderate-to-large (blood ≥ 1.5 mmol/L), do not exercise. Initiate appropriate insulin therapy and/or contact health professional. If ketones are negative or small (blood < 1.5 mmol/L), consider giving corrective insulin dose at 50% of normal. Initiate mild-to-moderate exercise. Delay intense exercise until glucose is < 250 mg/dl. Intense exercise may raise glucose levels.

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• INSULIN ADJUSTMENTS FOR EXERCISE

• Overview
• Insulin doses may need to be reduced to avoid hypoglycemia, particularly for aerobic exercise
• The amount of reduction necessary to prevent hypoglycemia will depend on the individual and the type and length of exercise
• The table below gives general recommendations for adjusting insulin for exercise. Patients will need to tailor these recommendations based on their individual needs.
• When starting or changing an exercise regimen, diabetics using insulin should closely monitor their blood sugars before, during, and after exercise

• References [1,2]

Suggested reduction in bolus (premeal) insulin if given within 90 minutes prior to exercise
30 minutes of exercise	Bolus insulin dose reduction
Mild aerobic	-25%
Moderate aerobic	-50%
Heavy aerobic	-75%
Intense aerobic/anaerobic	No dose reduction
60 minutes of exercise	Bolus insulin dose reduction
Mild aerobic	-50%
Moderate aerobic	-75%
Heavy aerobic	N/A
Intense aerobic/anaerobic	N/A
Prevention of nocturnal hypoglycemia (particularly for exercise performed in afternoon or evening)
For first meal after exercise (within 90 minutes) Consider consuming 1.0 - 1.2 grams/kg of carbohydrates Consider reducing bolus (premeal) insulin dose by ∼ 50%

• References [1,2]

Suggested reductions in basal insulin for prolonged exercise (> 30 minutes)

General considerations Prolonged anaerobic exercise may not require any insulin adjustments Mixed activity (anaerobic + aerobic) might require smaller reductions in insulin. Performing anaerobic exercise before aerobic exercise may attenuate the hypoglycemia caused by aerobic exercise. For strenuous prolonged aerobic exercise (e.g. marathon running, cycling, cross-country skiing), increased carbohydrate intake as opposed to insulin reduction may help improve performance Patients using multiple injections a day Consider reducing the basal insulin dose before prolonged exercise by 20% If nocturnal hypoglycemia is a concern, consider reducing overnight basal insulin dose by 20% Patients using insulin pumps Option one Consider suspending insulin pump during prolonged exercise. Use caution is suspending for more than an hour. Option two Reduce basal insulin by 50 - 80% up to 90 minutes before prolonged exercise

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• KETONE TESTING

• Overview
• In diabetics, a lack of insulin causes the liver to secrete ketone bodies (β-hydroxybutyric acid, acetoacetate). Ketone bodies are used by cells as an alternative fuel source when insulin is deficient. A buildup of ketone bodies can lead to diabetic ketoacidosis.
• Diabetics can test for ketone bodies in the blood or urine. Blood testing can be done with certain glucose meters that provide ketone testing with special strips. Urine testing can be done with widely-available ketone dipsticks.
• Positive ketone levels are found in normal individuals during fasting states and pregnancy
• The table below provides a general estimate of ketone body levels and how they compare in blood and urine. Cutoff values for urine can vary by manufacturer.

• In type one diabetics, ketone levels > 1.5 mmol/L should be addressed immediately by checking glucose levels and administering corrective insulin

Blood value (mmol/L)	Urine value
< 0.6	Negative
0.6 - 1.5	Small to moderate
1.6 - 3.0	Moderate to large
≥ 3.0	Large

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• GLYCEMIC INDEX AND EXERCISE

• Overview
• The glycemic index (GI) is a measure of how much blood sugar levels tend to rise after the ingestion of various carbohydrate-containing foods (see glycemic index for a full discussion)
• Foods with a high GI (≥ 70) tend to cause quick rises in blood glucose levels that are shorter in duration
• Foods with a low GI (≤ 55) tend to cause slower rises in blood glucose levels that are longer in duration
• Foods with a medium GI (56 - 69) fall somewhere between high and low GI foods
• General considerations for consuming carbohydrates based on their GI are given in the table below

• References [2]

Glycemic index (GI) and exercise
Low GI foods Consumption of low GI foods before exercise may help prevent hypoglycemia during prolonged activity Consumption of a meal or bedtime snack with a low GI after afternoon exercise may help prevent nocturnal hypoglycemia for up to 8 hours Low-to-moderate GI foods may be preferred for prolonged aerobic activity at low-to-moderate workloads
High GI foods High GI foods can be used to prevent/treat hypoglycemia during aerobic activity Consumption of high GI foods after exercise may enhance recovery

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• BIBLIOGRAPHY

• 1 - PMID 27926890 - Physical Activity/Exercise and Diabetes: A Position Statement of the American Diabetes Association, Diabetes Care 2016
• 2 - PMID 28126459 - Exercise management in type 1 diabetes: a consensus statement, Lancet Diabetes Endocrinol 2017