Exercise can be an important tool for a person who has diabetes. However, people who use insulin to control their blood sugar levels may need to take certain precautions when exercising.

Diabetes is a condition that can result in a person having atypical blood glucose levels. Glucose is a type of sugar the body uses for energy. The hormone insulin allows cells to absorb glucose. People with diabetes are either unable to produce enough insulin or unable to use it in the usual way.

This means glucose can build in their bloodstream and cause certain health problems. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) note that exercise can help people with diabetes reduce their blood glucose levels.

If people take insulin to manage their blood sugar, they may need to take certain measures to prevent it from becoming too low during exercise. Read on to learn more about exercising safely when taking insulin.

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The NIDDK states that glucose is the body’s primary source of energy. Insulin enables cells in the body to use glucose as energy. It also helps with the storage of excess glucose within the body.

Insulin sensitivity refers to how sensitive a person’s body is to insulin. People with high insulin sensitivity require less insulin to reduce their blood sugar levels.

The American Diabetes Association (ADA) notes that exercise can increase a person’s insulin sensitivity. This allows the cells in the muscles to absorb glucose more efficiently.

As such, a person who uses insulin to manage their blood sugar levels may be at risk of low blood sugar, also known as hypoglycemia, if they take their usual insulin dose before exercising.

People will need to be careful to adjust their insulin dose before they exercise to prevent harmful effects.

Because people can have different levels of insulin sensitivity, it is best to speak with a doctor about how to appropriately adjust insulin intake.

A position statement from the ADA suggests that additional carbohydrate intake or a reduction in insulin is usually necessary to prevent hypoglycemia in a person performing 30 minutes or more of aerobic exercise. This includes activities that increase a person’s heart rate, such as jogging, swimming, or cycling.

It also states that a person can make a 20% reduction in basal, or long-acting, insulin doses before and after exercise. However, this may not entirely reduce the effect exercise has on blood sugar levels.

If a person uses an insulin pump, they can reduce insulin levels or suspend delivery before exercising. Using this method, a person can reduce their insulin 30–60 minutes before exercising. If a person turns off their pump, they should not have it off for more than an hour.

Additionally, the ADA notes that people planning to exercise 2–3 hours after using bolus insulin, which is short-acting, may need to reduce their dose by 25–75%. This can depend on how long or how intensely they want to exercise.

It is best for people to check their blood glucose levels before they exercise and roughly every 30 minutes during exercise. This allows them to make suitable adjustments to keep their blood sugars within target ranges.

Experts recommend aiming for a blood glucose level of 90–250 milligrams per deciliter (mg/dL) before exercising.

The ADA suggests that a person with a blood sugar level below 100 mg/dL during or after exercise should have 15–20 grams (g) of carbohydrates to raise it. They recommend the following for this purpose:

  • four glucose tablets
  • a 15 g glucose gel tube
  • half a cup of juice or nondiet soda
  • a tablespoon of sugar or honey

After taking carbohydrates, people can wait 15 minutes and recheck their blood sugar levels. If the reading is still below 100 mg/dL, they can take an additional 15 g of carbohydrates and check their levels after another 15 minutes.

A person can repeat this cycle until their levels are over 100 mg/dL. This is known as the “15-15 rule“. People need to ensure their blood sugar levels are over 100 mg/dL before exercising.

Researchers note that resistance exercise, used for muscle building, can initially cause an increase in blood sugar, followed by increased insulin sensitivity. This type of exercise was associated with less blood sugar variability after exercise.

On the other hand, aerobic exercises cause an initial decrease in blood sugar. Researchers found that high intensity interval training, involving bouts of intense exercise followed by rest periods, was less likely to cause hypoglycemia than moderate intensity aerobic exercise.

Other factors that may affect a person’s blood sugar levels alongside exercise include:

  • Time of day: A study from 2023 found people who performed moderate to vigorous intensity exercise in the afternoon or evening had a reduction of insulin resistance of up to 25%.
  • Length and intensity of exercise: The ADA notes that low blood sugar is more likely to occur during or after a period of exercising strenuously or for a long time.
  • Skipping a meal before exercising: This can lead to hypoglycemia.
  • Eating an excess of carbs before exercising: This can result in a person’s blood sugar being higher than their goal range.

Mild hypoglycemia symptoms include:

More serious signs of hypoglycemia include:

How to manage hypoglycemia

A person can use the 15-15 rule to treat mild symptoms of hypoglycemia.

Having blood sugar that is too low for too long can be fatal. People showing serious signs of severe hypoglycemia may require a glucagon injection. Glucagon is a hormone that releases glucose stored in the body into the bloodstream.

If glucagon is not available, a person should seek immediate medical attention.

The ADA notes that blood sugar can continue to drop 24 hours or more after exercise. This is because exercise causes a person to be more sensitive to glucose.

For this reason, people need to monitor their blood sugar levels before and after exercise. This helps them figure out the effect of exercise on their blood sugar and prevent it from becoming too high or too low.

It is also advisable for a person to monitor their blood sugar more frequently than usual for 24 hours after exercise.

Exercise can cause a person to become more sensitive to insulin. It is important to be careful to adjust insulin to prevent hypoglycemia during or after exercise.

People with diabetes can monitor their blood sugar levels before and after exercise and use this information to speak with their doctor about adjusting their insulin intake. They may also consider discussing their exercise regimen with a healthcare professional.