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From São Paulo, Brazil:

Why shouldn't we exercise when the blood sugar level is over 250mg/dl? Why does the blood sugar rise?


First of all I want to thank you for sending this question to the diabetes team. This is a question frequently asked by many individuals, and the answer always deserves repeating.

Before you begin an exercise session, be sure you are in good control of your diabetes. The beneficial effect of exercise on an individual's blood glucose is to lower it. The availability of insulin to muscle cells is one factor that will determine the effect exercise has on blood glucose levels. One of insulin's primary roles is to allow muscle cells to use glucose. With exercise, less insulin is needed for this to happen, but don't forget, you do need some insulin. Therefore, if you begin exercise with an insulin deficiency, blood glucose levels and ketones can rise.

How can this happen? When blood glucose levels are elevated (generally greater than 250 to 300 mg/dl) (meaning there is not enough insulin in your body), when you exercise, your body starts to break down fat and makes even more glucose to feed the hungry muscle cells. The breakdown of fat to ketones exceeds the ability of muscles to use them. The cells can't get the glucose through, because there is not enough insulin. This is why sometimes when you exercise blood glucose may go up instead of down. Ketones can also increase your blood glucose.

The general guidelines for exercise are as follows:

  1. If blood glucose is greater than 250 mg/dl, check for ketones. If ketones are present, do not exercise.

  2. If blood glucose is over 300 mg/dl, do not exercise, whether ketones are present or not.

Lastly, keep your doctor and other diabetes team members aware of your activity level and show them your blood sugar diary. Depending on your usual level of activity, your insulin dose may need to be changed.


Original posting 7 Aug 1998
Posted to Exercise and Sports


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Last Updated: Tuesday April 06, 2010 15:08:58
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