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Diabetes and Exercise
How to Avoid Dangerous Drops in Blood Sugar

Exercise and Type 1 Diabetics

There are times when exercise may NOT be appropriate. If you're a regular reader of my column, you know that exercise can help a wide range of medical ailments from arthritis to high cholesterol. But the medical studies showing benefit for people with Type 1 diabetes have been contradictory. Some show benefit, while others have demonstrated possible harm.

The problem seems to come from a condition known as hypoglycemia. Let me take a minute to explain what's going on.

Somebody who has type 1 diabetes cannot produce the hormone insulin. We all need insulin in our bodies to convert sugar, starches and other food into energy. Without insulin, our bodies can experience a dangerous drop in the amount of glucose in our blood. You'll hear it called "low blood sugar" or when it's more serious, "hypoglycemia."

To combat hypoglycemia, type 1 diabetics take regular measurements of their blood sugar levels, often several times a day. When blood sugar levels are low, type 1 diabetics raise it by eating or drinking things higher in sugar or carbohydrates. In extreme cases, they may need to resort to an injection of glucagon.

What does exercise have to do with blood sugar? When we exercise, our muscles use blood sugar for energy throughout the workout. The longer and more intense the workout, the more blood sugar you need to make it through. (That's why we always suggest clients eat 1-2 hours before a training session, so they have the energy or blood sugar to finish.)

In a remarkable study conducted by researchers in America, France, Spain and Switzerland, they split type 1 diabetics into four groups.

Group A didn't modify their regular insulin doses, but they took in 10-20 grams of extra carbohydrates for each hour they exercised.

Group B reduced their daily insulin doses by more than 10% and also took in 10-20 grams of extra carbohydrates for each hour they exercised.

Group C also reduced their daily insulin doses by more than 10%, but they didn't take in any extra carbohydrates.

Group D didn't reduce their insulin OR take in any extra carbohydrates.

When the study was finished, researchers discovered that the two groups (A & B) who took in the extra carbohydrates before, during and after exercises experienced, "significantly less hypoglycaemias than groups C and D where the patients exercised without substantial extra [carbohydrate] replacement during exercise."

In fact, a change in insulin didn't seem to make any difference. Just taking in the extra 10-20 grams of carbs per hour of exercise was all that was needed to stave off hypoglycemia. All the health benefits of a regular exercise program are once again available. All you need are some carbs to help see you through.

A word of caution. Some people with diabetes feel that if they go into a workout with a high starting blood glucose, they're protected. They think they don't need to take carbohydrates during the workout. That feeling is wrong. Here's why.

If your blood glucose level is twice as high as normal, that means your body is holding about 5 grams of glucose in the circulation. When we exercise, we need up to 50 grams of glucose per hour. That 5 grams extra will be depleted quite rapidly.

The best that you can say is if you start exercising with a high blood glucose, you can delay taking in extra carbohydrates for a few more minutes...but that's all. You still need the extra carbohydrates to prevent hypoglycemia.

A special warning. The study authors said that if you have HIGH blood glucose (greater than 14.0 mmol/l) and positive urine ketones, you should NOT exercise because of the metabolic misbalance. Physical exercise, especially intense exercises should be avoided if positive ketonuria is present.

If you want details on the study, the title is, "A new table for prevention of hypoglycaemia during physical activity in type 1 diabetic patients." It was published in Diabetes & Metabolism, Volume 30, Number 5, in November of 2004. (Pages 465-470.) Click Here for the web page.

Click on the image below to download the study in Adobe PDF Format.

Click Here for the Study in Adobe PDF Format

UPDATE: In a small study published in 2015, researchers had 13 obese men and women with type 2 diabetes exercise. When the results came in, the people who exercised before eating had blood glucose levels that were reduced by 18 percent. But the people who exercised after eating had blood glucose levels reduced by 30 percent. More proof of what we said five years ago, that eating 45 minutes to an hour before a workout is the best way to get results.

-Journal of Applied Physiology February 17, 2015

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Updated 6/28/2015
Updated 5/9/2022