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Diabetes is characterized by the body’s inability to produce or efficiently use insulin, a hormone that regulates glucose (blood sugar). In type 1 diabetes, the beta cells in the pancreas have stopped producing insulin. In type 2 diabetes, the body produces insulin but does not use it efficiently for helping fuel the body.
Diabetes typically causes a buildup of glucose in the bloodstream (hyperglycemia), but in certain situations, diabetes can also result in low glucose levels (hypoglycemia). Both states can have serious medical complications.
Controlling glucose is perhaps the biggest challenge for people with diabetes. Consistent, moderate exercise plays an important role in managing diabetes and preventing medical complications caused by uncontrolled glucose. Short-term consequences of hypoglycemia can include insulin shock and diabetic coma. Short-term consequences of hyperglycemia can include diabetic ketoacidosis, hyperosmolar hyperglycemic nonketotic syndrome (HHNS) and coma.
Long-term complications of hyperglycemia include:
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Kidney damage. Almost a third of all people with diabetes develop diabetic nephropathy (kidney disease), and diabetes is the leading cause of chronic kidney failure in the United States. Additionally, people with diabetes are predisposed to other urogenital problems, such as urinary tract infections and sexual dysfunction.
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Nerve damage. Many people with diabetes experience diabetic neuropathy.
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Vision loss. Diabetic retinopathy, a condition caused by damage to the blood vessels in the retina, is a leading cause of blindness. Hyperglycemia also contributes to glaucoma and cataracts.
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Cardiovascular complications. People with diabetes and uncontrolled glucose are at increased risk of developing cardiovascular conditions, such as angiopathy (a disease that affects the blood vessels), heart attack and stroke.
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Skin disorders and foot problems. If not healed, conditions such as ulcers may lead to the need for amputation.
The effect of exercise on glucose is complex. It varies by individual and depends on a number of factors, including:
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Duration of exercise. Prolonged periods of exercise may cause the body to produce hormones, such as adrenaline, which can counteract insulin’s benefits and cause blood sugar to rise. People with diabetes should speak with their physician about adjusting medications prior to exercise sessions to account for elevated glucose.
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Intensity of exercise. The liver may release more glucose into the bloodstream during and after a vigorous workout. This hyperglycemia can lead to complications such as ketoacidosis.
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Glucose level at the beginning of exercise. Exercise typically lowers blood sugar levels. This can put the patient at risk for developing hypoglycemia during or after exercise.
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Insulin levels at the beginning of exercise. If an individual exercises while insulin levels are elevated, the combination can result in hypoglycemia.
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Amount of insulin resistance (a condition in which the cells do not respond to or efficiently use insulin). Exercise can improve insulin sensitivity. This improved response makes it easier for the body to transfer sugar from the bloodstream into the cells, which lowers glucose levels.
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Personal physiology and metabolism. The muscles are among the major organs that remove glucose from the bloodstream. Therefore, individuals can better regulate their glucose during exercise by increasing their muscle–to–fat ratio.
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Level of hydration. Dehydration and hyperglycemia can trigger HHNS.
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Whether an illness is present. Infections and other conditions can alter glucose levels.
The human body is powered by muscles, which rely on two fuel sources, sugar and free fatty acids (fat), to produce energy during exercise. Some of the sugar and fat is used immediately upon ingestion. The rest, however, is stored in the blood as well as the liver and muscles (in the form of glycogen) for future use.
During the first 15 minutes of exercise, the muscles are propelled by using sugar from the bloodstream and muscle glycogen. After 15 minutes of activity, the muscles gather their energy from the glycogen reserve in the liver. When about 30 minutes has passed, the body turns to its free fatty acids for energy. Put simply, exercise promotes the transfer of sugar from the bloodstream into the muscle cells, where it belongs.
As a result of this transfer, blood sugar levels and glycogen stores generally decrease after exercise. This means that there is less glucose in the bloodstream and muscles after physical activity, which allows people to manage their glucose more efficiently. The body replaces its glycogen stores after each exercise session. This process can take anywhere from four to 24 hours, depending on the duration and intensity of the workout.
In addition to directly affecting glucose levels, regular exercise can also increase insulin sensitivity (how the body’s cells react to the effects of insulin). Exercise reduces body fat, which makes the insulin receptor sites (the place where insulin connects to cell membranes) more functional.
Reduced body fat also appears to increase the number of insulin receptors. The more insulin receptors an individual has, the greater the sensitivity to the effects of insulin. Exercise also increases blood flow, allowing insulin to more readily  transport glucose for fuel. As a result, insulin typically functions more efficiently after exercise, and patients who take insulin or antidiabetic agents may be able to decrease their dosage, as advised by their physician. Improved circulation also helps in treating complications such as foot ulcers and peripheral arterial disease.
Patients experiencing ketosis, a buildup of ketones in the bloodstream, are advised not to exercise. Ketones are waste products produced when the body burns fat instead of glucose for energy, which can happen if there is a severe lack of insulin. Ketosis and hyperglycemia can trigger diabetic ketoacidosis. Ketosis can be detected with a simple ketone test.
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