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Why How Much Insulin You Have “on Board” during Exercise Matters

Dec 16, 2008

More and more of our patients using insulin are exercising and participating in sports. They often lower their basal rates or decrease their mealtime dose to try and compensate. This can lead to hyperglycemia or hypoglycemia after exercising. Why does that happen and what changes over time. Dr. Sheri Colberg, Ph.D., FACSM explains how that happens and what your patients can do about it in Why How Much Insulin You Have "on Board" during Exercise Matters

Why How Much Insulin You Have “on Board” during Exercise Matters


By Sheri Colberg, Ph.D., FACSM

SheriIn people without diabetes and in most people with type 2 diabetes, insulin levels in the blood fall during exercise, and the rise in glucagon release from the pancreas stimulates the liver to produce more glucose. If you have to inject insulin, however, your body can’t lower your circulating levels when you start to exercise. Having too much insulin under those circumstances is bad news because it stimulates your muscles to take up glucose from your bloodstream. Muscle contractions do the same thing, and the effect is additive, meaning that higher insulin levels can result in double the glucose-lowering effect and rapid-onset lows. Insulin excesses also keep your fat cells from releasing free fatty acids, lowering the amount of fat available as an alternative fuel for muscles. Note that if you exercise with higher blood sugars, you may use slightly more glucose and less glycogen as fuel.

The amount of insulin that is circulating in your bloodstream during exercise is critical in determining how well you perform and whether you fatigue early from hypoglycemia. You need to have some insulin in your body. If you have too little, your body will be missing the normal counterbalance to the rise in your glucose-raising hormones, and you could end up hyperglycemic instead. A fine balance is required because if your insulin levels are too high, they can severely inhibit the release of these hormones and you can end up low. You need some of these hormones because adrenaline mobilizes stored fat and causes muscle glycogen breakdown, and glucagon increases glucose production by your liver. Without these, your muscles can take up more glucose than your liver produces. In one study, intense cycling done with extremely low circulating insulin caused hyperglycemia and exaggerated lipolysis (mobilization of fat), whereas the same exercise with too much insulin resulted in hypoglycemia and reduced release of fat. To perform optimally, you need some insulin in your body to counterbalance the release of glucose-raising hormones, but not so much insulin that your blood sugars drop excessively.

How Training Affects the Fuels That Your Body Uses

Physical training improves the capacity of your body to metabolize fat, which generally results in greater use of it, slower depletion of muscle glycogen, and reduced reliance on blood glucose during an activity after your muscles have adapted. The training effect on fuel utilization is evident when you have diabetes because you’ll find that you either need to take in less carbohydrate for the same activity after several weeks or need to lower your insulin less to compensate.

Some of these training adaptations occur because of a lesser release of your glucose-raising hormones when you’re exercising at moderate or lower intensity. People without diabetes experience the same training effect, but it may be harder for them to see it because their blood glucose levels hardly fluctuate. Insulin release usually goes down during exercise (if you make some or all of your own), but training actually causes it to go down less. As a result, after training, your body uses less glucose and muscle glycogen and slightly more fat when you do the same intensity of exercise—all of which result in more normal (higher) blood sugar levels and reduced risk of getting low.

This change in fuel use explains why you may need more carbohydrate to maintain your blood sugar levels when you first start doing an activity but less after doing the same activity for several weeks. In other words, you may find that after training for several weeks, your blood glucose does not drop as significantly as it did when you first started, even if your insulin levels are the same. But if you work out harder to reach the same relative intensity (e.g., if reaching 80 percent of your maximal after training requires you to do a harder workload than at the start), your carbohydrate use during the activity will likely be just as high as before. In addition, the training effect is sport specific, which means that if you’ve been running and then decide to try a new activity like swimming, your blood sugars will probably drop more during swimming until you’re trained in that sport as well.

This column is excerpted from Diabetic Athlete’s Handbook (available November 6, 2008 from Human Kinetics), which contains essential exercise-related information and examples for type 1 and type 2 diabetic exercisers. Look for it in stores or find links to places to buy it online on www.shericolberg.com, along with additional information.

Courtesy of www.diabetesincontrol.com Dec, 2008