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Why Can't I Exercise with Ketones?

After you eat, fats are digested in the stomach into their building blocks — fatty acids. These fatty acids are then absorbed into the bloodstream or stored in the body. When you exercise the body uses two sources of fuel — sugar and these stored fatty acids — to generate energy.

It works this way: stimulated by the demand from your exercising muscles your body pours glucose into your bloodstream, which cannot get into your muscles when you do not have enough insulin available to "unlock the doors" to your muscles. Your body then cleverly looks to alternative sources for fuel — the free fatty acids — to help fuel your muscles since it cannot get to the glucose. The use of fat for fuel results in a byproduct called ketones. These ketones accumulate in the blood and also spill over into the urine.

The presence of ketones is called "ketonuria," and further dehydration and ketone build-up can result in ketoacidosis, which is a medical emergency. The bottom line is that the presence of ketones in someone with type 1 diabetes shows a dangerous lack of insulin and the immediate need for more insulin. Exercise, at this time, will only burn more fat and produce more ketones.

Find more information about physical activity and diabetes in Staying Healthy with Diabetes – Physical Activity & Fitness available from the Joslin Online Store.

Page last updated: September 18, 2019