Diabetics Develop Higher Blood Sugar During Exercise

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Nov. 18, 1999 (Santa Cruz, Calif.) -- Insulin-dependent diabetics who take part in sports may need to take more insulin after intense exercise rather than less, contrary to conventional wisdom.

A new study, published in the November issue of the Journal of Clinical Endocrinology and Metabolism, shows that insulin-dependent diabetics, known as type 1 diabetics, are likely to see an increase in their blood sugar, not a decrease, after an intense bout of physical exertion.

It is healthy for blood sugar to rise during exercise because the muscles need the excess fuel to compensate for the increased demand placed upon them. But in most people the body will adjust after exercising and bring the blood sugar levels back to normal. That is not the case for a type 1 diabetic, because their bodies will not circulate enough insulin, which is required to convert the sugar in the blood.

"Anyone who is competitive, who is doing a sprint, playing hockey, [playing] basketball ... is at significant risk of developing high sugar as a consequence of their exercise," study investigator Errol Marliss, MD, tells WebMD. Marliss is professor of medicine and director of the McGill Nutrition and Food Science Center at the Royal Victoria Hospital in Montreal.

"There's a big difference in the blood [sugar] response depending on the intensity of exercise," Marliss says. "The classic teaching about [exercise by a diabetic] who is insulin-treated is, 'Look out, because your blood sugar is going to go down; you may have to either take extra carbohydrate or less insulin.' [But] a lot of very athletic, type 1 diabetic people have been telling their doctors for years, 'Look, Doc, I'm getting [too much blood sugar] when I exercise,'" Marliss says, "and that is a predictable consequence of higher intensities of exercise."

"Intense exercise makes you put out anti-insulin hormones," says Stanley Feld, MD, chairman of the task force of the American Association of Clinical Endocrinologists (AACE), who reviewed the study for WebMD. That's normal, and a healthy person will secrete enough insulin after exercise to lower the excess blood sugar no longer needed by the muscles. That is not the case with a type 1 diabetic.

To show this, the researchers tested six diabetic young men who had been diagnosed and treated for type 1 diabetes since childhood. None displayed any evidence of diabetic complications; all had excellent diabetes control, and all engaged regularly in sports and weight training. The researchers took steps to ensure that the diabetics had normal levels of insulin in their blood before exercising, so that their blood sugar was at a healthy level.

The participants underwent cycling tests to the point where they were unable to continue because of muscle exhaustion. They were then compared to other participants in the study who did not have diabetes. By testing people with type 1 diabetes whose blood sugar was healthy before exercise, the researchers found that after the cycle test their blood sugar remained higher when compared to the healthy control group.

Therefore, after exercise, the normal response is to increase insulin to lower the blood sugar released in exercise, Marliss says. "If you are normal, you can compensate by putting out extra insulin," Feld tells WebMD. Type 1 diabetics do not have the ability to increase their production of insulin to counteract the increase in glucose, according to Feld.

Marliss says that currently there is no therapy for a diabetic's body to self-regulate the increased demand for insulin, so the job falls to the diabetic to closely monitor their own sugar levels and adjust the insulin accordingly. "What one would need to do for a person with diabetes," Marliss says, "is to devise a strategy where one would mimic the normal response when the exercise stops, which is that insulin levels go up. There isn't a really good one at this point; therefore, the bottom line is, anybody with diabetes doing intense exercise needs to monitor their sugars even more rigorously than they would with low-intensity exercise, like walking or jogging."

Vital Information:

  • Contrary to conventional wisdom, type 1 diabetics may need to take more insulin, rather than less, following intense exercise.
  • Exercise causes an increase in blood sugar levels, and a person with type 1 diabetes does not produce enough insulin naturally to bring the levels back down.
  • Diabetics need to monitor their blood sugar rigorously when engaging in intense physical activity, even more so than with low-intensity exercise.
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