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Exercise and Diabetes

There is no doubt that exercise is beneficial to everybody.  Exercise has been shown to improve blood pressure, lower the risk of cardiovascular heart disease, improve lipid profiles (that is, raise the good HDL, and lower the bad LDL and total cholesterols) and improve insulin sensitivity, among many other benefits.  The old school of thought, in the days shortly after the discovery of insulin, was that Type 1 Diabetics should not exercise because of the risk of hypoglycemia, or low blood sugar.  It is now known that with a little monitoring and adjustment to treatment protocols, most diabetics can safely exercise to gain the benefits of regular physical activity.  In fact, there are many Type 1 Diabetics who have not only been able to exercise successfully, but have successfully competed in demanding sporting events such as ice hockey, basketball, baseball, ballet, cross country skiing (at the Olympics), swimming (again at the Olympics), marathons, triathlons, and rowing, to name a few.

The following topics will be discussed below:

Why should I exercise?

Tips for starting an exercise routine

 How can I find people to work out with?

 How can I prevent hypoglycemia during and after exercise?

Links to helpful exercise sites

Why should I exercise

 “I want to be the next Olympic (fill in your sport here) gold medalist . . .” or “I just want to get in better shape . . . but I have diabetes, how do I get started?”

 Well, I’m sure you’ve heard this before, but you should talk to your doctor or at least let him/her know what you are planning on doing, especially if you are not currently active.  If you can, I would also suggest talking to an exercise physiologist, preferably one that is familiar with diabetes, because that person should be able to give you the “inside scoop” as to how exercise affects the body.  After that, it’s really just trial and error.  Trial and Error?!?!

 Yes, I always hated when people told me that, too.  I wanted somebody to tell me, “Doing XXX will work.”  But each body is unique and will react in different ways to different kinds of activities.  The best bet is to gain knowledge, look at guidelines, try different things, do blood tests often especially when starting new routines, keep track of blood glucose, insulin dosage, what works, and what doesn’t, and don’t give up.  It may be helpful to talk to someone who has diabetes and exercises regularly to see what works for them. 

What can exercise do for your health?

 Regular physical activity, or exercise, results in the following changes in everybody:

 While all of these changes are beneficial and important, we are most immediately concerned with the mechanisms that deal with insulin sensitivity and glucose control.  Several studies have shown that exercise is effective in lowering blood glucose and helping people achieve better control over their diabetes.  Because of the increase in insulin sensitivity due to regular physical activity, Type 1 Diabetics also often find that they need less insulin.  However, the immediate affect on blood glucose can be different depending on the intensity and/or duration of a single bout of exercise (that is how hard you are working and/or for how long).  It may help to understand the basics of metabolism in order to figure out why you blood sugar acts the way it does and what you can do to keep it where it should be. 

 Muscles require little “bundles” of energy in order to contract.  These “bundles” are called ATP, (adenosine tri-phosphate).  The body has three ways of supplying the muscles with the ATP they need to contract.  One way is through a system that uses the ATP stored in the muscle, and is called the ATP/Cp cycle.  The body has a very limited store of ATP and will use that up in about 10 seconds and therefore needs a way to create it as well (Colberg ).  In fact, the body has two ways of making ATP.  The first way, which is the second system of supplying ATP to muscles, is called Glycolysis and uses glucose to make ATP.  This glucose comes from either the blood stream (that is your blood glucose) or from glucose which has been stored in the muscle as glycogen.  All of this glucose and glycogen ultimately comes from the carbohydrates you eat.  The third and final system is Beta-oxidation and makes ATP through the use of fatty acids which come from your fat cells (adipose tissue), blood triglycerides, and intramuscular triglycerides (which are very small fat stores within your muscles, and these are good to have); these fats also ultimately come from the food you eat.  All of these systems are running all of the time, but each one dominates at certain levels of intensity of activity. 

 ATP/Cp Cycle

Because of the very limited amount of ATP that is stored and therefore available to the muscles, this system is best for very short yet very intense bouts of activity.  Classic examples are the 100 meter sprint or lifting weights (especially the competitive body building type weights).  Because this system relies on the stored ATP and does not use any other substrates (like carbohydrate or fat) to make it, there is very little appreciable change in blood glucose due to short, intense sprints.  In fact, due to some hormonal releases, there may actually be an increase in blood glucose.

 Glycolysis

There are two subgroups to glycolysis and each has several steps involved, which means that the production and therefore supply of ATP is slower than that of the ATP/Cp cycle.  The first sub-group is the Lactic acid cycle, which uses muscle glycogen and dominates activities lasting from about 20 seconds to 2 minutes, at a very slightly lower intensity than the activities primarily feuled by the ATP/Cp cycle (Colberg ).  This pathway also produces the by-product lactic acid, which is the “burn” you may feel after a short but intense workout.  Classic examples of activities dominated by the Lactic acid cycle are the 400 and 800 meter runs.  These types of activities can actually cause an increase in blood glucose immediately afterward.  This is caused by the release of hormones that function in increasing the blood glucose and can last for several hours after the end of the workout, which may make additional insulin post-exercise necessary.  The increase in blood glucose may also be aided by the lactic acid that was produced because that can be converted back into glucose after the end of exercise.  However, blood glucose may decrease more later on because of the replacement of muscle glycogen, which is used and often depleted during these types of work-outs.  After the activity stops, the body uses the glucose in the blood to restore the stores of glycolysis in the muscles.

 The second glycolytic system is Aerobic glycolysis, which has an extra step in it than the Lactic acid cycle because it uses blood glucose and therefore needs to transport the glucose from the blood into the muscle.  This pathway provides energy for activities that last longer than 2 minutes, such as the mile or the 2 mile run (Colberg ).   This will have more of an effect on blood glucose because this system uses glucose from the blood in order to produce ATP.  In general, the longer the duration of activity, the greater the effect it has on blood glucose.

 Beta-oxidation

This process produces the most ATP, but it has the most steps to complete this process.  For this reason, Beta-oxidation dominates for very long duration and low intensity activity.  In fact, Beta-oxidation is the principal energy cycle while you are at rest or going for a leisurely walk, but it is also dominant for long endurance runs and activities lasting more than 30 minutes (Colberg ).  You may think that because Beta-oxidation uses fat that these activities will not cause a great change in blood glucose.  However, endurance activities will almost always have a lowering affect on blood glucose.  You mustn’t forget that all three systems are always on, some more than others at certain times.  Even though you are mostly using beta-oxidation, you are also using up glucose.  Even if you only use a little glucose at a time, if you work out for a long amount of time the glucose that is being used will add up.  There is another part of this as well.  During glycolysis, the preferred source of glucose is the stored muscle glycogen, but there is only a limited supply of that, and it will get depleted pretty quickly.  After the muscle glycogen is depleted, glycolysis must rely on the blood glucose for glycolysis.  This can cause a decrease and possible episode of hypoglycemia several hours (up to 24 hours) after exercise.

 In summary, not only do you run the risk of having low blood glucose (hypoglycemia) during exercise, but several hours later as well.

 Some Tips for starting an exercise routine:

If you are going from a relatively sedentary lifestyle to a more active one, you may find the following tips useful:

 How can I find people to work out with?

Ask your roommate.  Make a new friend down the hall, ask him/her.  Ask someone from one of your classes.  Ask me.  Join a team (varsity, club, or intramural) on campus.  Don’t be shy. 

If you are looking for a “work-out buddy” and want to put your name here so others can contact you, please send an email to alively@student.umass.edu with “work-out buddy” in the subject line and any info you wish to give in the text.

 How can I prevent hypoglycemia during and after exercise?

 The first helpful hint is to follow some sort of a routine and exercise at about the same time each time.  This is helpful because you and your body will get used to exercising at the same time.  Many people find it helpful to exercise at least 4 hours after the last insulin injection or bolus of fast-acting insulin.  Depending on how long the effect of insulin remains in your body, you may be able to exercise sooner or later.  The reason is because even though fast-acting insulins such as Humalog or Regular peak at about ½-1 ½ or 2-3 hours, respectively, the durations can be 2-4 hours for Humalog and 3-6 hours for Regular, or even as much as 4-6 hours for Humalog and 6-10 hours for Regular (Colberg 32). 

            Basically, you want a little circulating insulin in your body, but not too much, when you exercise because physical activity has an additive affect on insulin.  That means that exercise enhances the ability of insulin to shuttle glucose from the blood into the muscles and liver.  You may think this is a good thing because your muscles need energy and glucose is one way to supply it.  However, when glucose is being brought out of the blood stream, making the blood glucose concentration lower, which affects the brain’s functioning because as stated earlier, glucose is really the only source of energy for the brain.  At the same time, you do not want too little insulin in the blood because that can result in elevated blood glucose that leads to keto-acidosis.  Some people find that exercising in the (early) morning before insulin injection (or bolus for those on the pump) and breakfast works for them because the circulating insulin levels are low so as not to cause hypoglycemia but still enough so as not to cause hyperglycemia either.  One study found that there were no cases of hypoglycemia after exercising for 45 minutes in the morning in a fasted state without injecting insulin and decreasing the long-acting insulin the night before by 2 units (Biankin et al 2003).  However, this was a rather small scale study and it is possible, especially when starting a new routine, to have unexpected blood glucose readings, so test often to see what your blood sugar is doing until you feel comfortable with and can expect what it will do.  This study was also done with subjects who were injecting insulin and not on insulin pumps.  For those on insulin pumps, it may be necessary to decrease basal rates before exercising (you may need to change it an hour or more before exercising because there is a lag during which insulin is still active) depending on the intensity of exercise.  Yet another study found that the risk of hypoglycemia is always increased after a meal if the insulin dose for the meal has not been reduced (Rabasa-Lhoret et al 2001).

            Other ways of preventing hypoglycemia depend on the type and duration of the activity.  This means you may need to supplement with carbohydrate, change insulin dosage, or do a combination of the two.  Here are the general guidelines.  One study found that carbohydrate supplementation both with and without insulin adjustment was more effective than insulin adjustment alone in preventing hypoglycemia during and after exercise.  However, for long duration activities, you may want to adjust the insulin dosage so that you do not have to supplement with as much carbohydrate, especially if weight gain is an issue.  Here are some guidelines:

Links to helpful exercise sites 

Here is a fact-sheet on exercising with diabetes: http://www.smartplay.net/ouch/medical/diabetes/diabetesfacts.html

Diabetes Exercise and Sports Association: http://www.diabetes-exercise.org/index.asp

Inspirational people: http://www.diabeteshealth.com/read,7,666.html

Beware of Athlete’s Foot! http://www.diabetesuffolk.com/Complications/Athletes%20foot.asp

Carb-counting for the athlete: http://journal.diabetes.org/diabetesspectrum/00v13n3/pg149c.htm

Sports Nutrition for the Athlete with Diabetes http://www.adksportsfitness.com/september2002/columns/nutrition.html

More sports nutrition from the Australian Institute of Sport http://www.ais.org.au/nutrition/documents/FactDiabetes.pdf

References:

Braun, Barry PhD.  Exercise Science 585: Energy Metabolism.  Spring 2006.

Colberg, Sheri PhD.  The Diabetic Athlete: Prescriptions for exercise and sports.  2001.  Champaign, IL.  Human Kinetics. 

Rife, Frank PhD.  Exercise Science 340: Exercise Testing and Programming.  Fall 2005.

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