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How Training Affects the Fuels Your Body Uses and Insulin Action

Last week I was on the phone with Rick Philbin, from Animas. Rick is a type one patient who is also active in DESA. A few years ago we crowned Rick the world’s most fit diabetes sales rep. When you check out his insulin use it is almost non-existent. How is that possible? This week Sheri Colberg, Ph.D., FACSM, has the answer and more in How Training Affects the Fuels Your Body Uses and Insulin Action

How Training Affects the Fuels Your Body Uses and Insulin Action
By Sheri Colberg, Ph.D., FACSM

SheriPhysical 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. 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. So, keep in mind that you may find that after training for several weeks, your blood glucose does not drop as significantly as it did when you first started.

When you’re physically trained, you will likely have heightened sensitivity to insulin, which allows your muscles to take up glucose more easily despite having lower levels of insulin. This effect is especially evident in people with type 2 diabetes or anyone else who is more resistant to insulin (such as those with type 1 who have “double diabetes,” or symptoms of both types). In fact, regular physical activity improves blood glucose control by increasing your insulin action, both right afterward for up to a day or two and overall.

Right after you work out, your insulin action increases mainly because you’re taking up glucose to restore the muscle glycogen that you used. You may need to reduce your basal levels of insulin and doses for meals to compensate for this effect and lower your risk of postexercise lows. By measuring your blood sugars, you’re likely to be more aware of changes in your insulin action than anyone without diabetes. You will need less insulin not only during exercise but also afterward, particularly during the window of opportunity for maximal rates of glycogen repletion that occur during the first half hour to two hours after exercise.

Over the long haul, though, training helps with insulin action by increasing your muscle mass, in effect giving you a larger “glucose sink” in which to put excess glucose after meals. Trained athletes generally have low levels of circulating insulin despite being extremely insulin sensitive. Insulin action, though, begins to decline after a period with no exercise, in as little as one to two days, even if you’re normally active. Many athletes report that their total insulin requirements increase after two to three days without their regular exercise (such as when they’re too busy to exercise, injured, or sick). By way of example, Peter Nerothin of San Diego, California, notices that when he hasn’t trained for a few days, he doesn’t get nearly the same blood glucose responses to meals. To adjust for periods of less activity, he has to take more insulin up front for his carbohydrate, along with possibly giving himself some extra units in a dual wave using his insulin pump to cover the three to four hours after eating and waiting longer before starting to eat to lower his postmeal spikes.

Although insulin action can stay higher for 24 to 48 hours (or more) after an activity, a study of type 1 diabetic runners found no change in insulin sensitivity following a marathon. In spite of 50 percent glycogen depletion in these athletes, they were no more insulin sensitive on the day after the marathon than they were on a resting day before it, and they had increased utilization of fat. These findings are similar to those in nondiabetic people after a marathon. The cause is likely muscular damage from long-distance events that reduces a person’s ability to restore glycogen until the muscles are repaired.

As you can see, many factors affect your blood sugar control during and after a workout. Keep in mind that you will tend to lower your blood sugars more when participating in new or unusual activities, but the intensity and duration of your exercise will also affect glucose use. Intense activities may temporarily raise your blood sugar levels but can cause them to fall later when your muscle glycogen is being restored, so be vigilant then to prevent postexercise lows. The reward of exercise training, though, is that you will lower your overall insulin needs with regular workouts of any type.

                                                                                                                                                           

This column is excerpted from Diabetic Athlete’s Handbook (released November 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