Back to Sports Corner Blood Glucose Monitoring for Athletic Competition

Monitoring blood glucose is essential for good diabetes control and even more important when participating in sports. Many variables can have an effect on blood sugar response to aerobic or anaerobic activities. One blood glucose result is a snapshot of an athlete's blood sugar but it does not indicate whether it is going up, down, or staying the same. While each person's response to exercise is individualized as it relates to blood sugars, typically aerobic exercise tends to decrease blood sugar. It is important not to rely on signs or symptom of low blood sugar because these are often the same as the response to exercise. An athlete who appears confused, shaky, or fatigued may be that way because of dehydration and not low blood sugar. Typically, anaerobic or more intense activities over a short amount of time will have less impact on dropping blood sugar and may even have an affect of increasing blood sugar. This is due to the hormonal response in the body causing the liver to produce more glucose.

How much insulin is circulating in the blood stream will also influence blood sugars during exercise. For this reason, it is important to know the type of insulin being used and when it typically peaks in the system. The table below shows most of the insulins used today along with its onset, peak, and duration in the body.

Table 1
Insulin TypeOnsetPeakDuration
Regular30-60 minutes2-3 hours4-6 hours
NPH2-4 hours4-12 hours14-18 hours
Lente3-4 hours4-12 hours16-20 hours
Ultralente6-10 hoursminimal20-30 hours
Lantus 1 hournone24 hours
70/30 Mix 10-20 minutes1-4 hours 15-18 hours
Lispro (Humalog) 15 minutes 30-90 minutes3-6.5 hours
Aspart (Novolog)15 minutes 40-50 minutes3-5 hours
Glulisine (Apidra)15 minutes 60-90min1-2.5 hours

Blood glucose level guidelines
The American Diabetes Association (ADA) and the American College of Sports Medicine (ACSM) recommend for people with diabetes who take insulin (and some oral medications that can lower blood sugar) to keep blood sugar above 100 dl/mg and below 250mg/dl to 300 mg/dl with ketones. The associations also recommend delaying exercise when blood sugars are above 300mg/dl whether or not ketones are present. Since most aerobic exercise of moderate to heavy intensity will drop blood sugar, it is important to start with a higher blood sugar (i.e., 150 mg/dl) to provide a buffer so the athlete does not go too low during the activity. This pre-exercise number is individualized and may change due to the intensity and/or duration of the activity. The only way to know what number works best is to check blood sugars frequently during exercise. As long as the blood sugar does not drop below 100 mg/dl or increase above 250 mg/dl with ketones or above 300 mg/dl (regardless of ketones) the pre exercise blood sugar during the activity.

Before competition: Check blood sugar twice.
A good goal is to make sure blood sugar is not too low or too high prior to exercise, during exercise and even after exercise. This can only be achieved through prudent monitoring of blood sugar. To avoid wide swings check blood sugar one hour before and thirty minutes before an activity. This will give two numbers to identify a trend of blood sugars going down (i.e., 150 mg/dl to 90mg/dl) or possibly up (e.g., 150 mg/dl to 260 mg/dl) and it gives enough time to adjust if needed.

For new or atypical exercise, it is better to check blood sugars more frequently. If an athlete who normally participates in half a game is suddenly playing the entire game, it is wise to check more during halftime or a timeout. Before making changes, look for patterns in the glycemic response to a particular activity.

During competition: Check blood sugars periodically
In an ideal situation with good planning and good pre-exercise blood glucose, no changes need to be made during an activity. As most athletes taking insulin know, good plans are often not good enough based on a variety of factors. For instance, blood glucose levels are usually different for practices as compared to games. Practices are often longer and may cause blood sugars to drop more whereas games may increase anxiety causing a hormonal response elevating blood sugars. For this reason, it is important to check periodically (i.e., halftime or a timeout) and more often if a high or low blood sugar occur.

After competition: Check blood sugar upon completion and hourly if problems arise
Athletes who are well controlled usually will not go low during exercise. More often low blood sugars occur several hours after activity and even up to twenty-four hours later. This post exercise delayed-onset hypoglycemia can be prevented with proper planning. Checking blood sugar upon completion of an activity will determine the action needed for replacing glycogen stores lost during exercise. Ideally, an athlete should eat some carbohydrates within thirty minutes of stopping an activity. This will help avoid delayed-onset hypoglycemia as well as aid in performance for activity the next day. It is easy to replace glycogen store immediately after exercise when blood glucose levels are normal or even below normal. The dilemma becomes when an athlete completes an activity and blood sugar is high (i.e., 300+ mg/dl) and glycogen stores need to be replaced. A blood sugar at this level will usually need insulin to bring it down to a normal level as well as water to flush out potential ketones. If insulin is used to bring down a high blood sugar, it is important to check again relatively soon to make sure the levels do not go too far the other way. If using a rapid acting analog (Humalog, Novolog, Apidra) it is best to check within a half hour of giving insulin to see the affect it had on the blood sugar. Is the post exercise blood sugar of 300 mg/dl now 150 mg/dl or possibly 65 mg/dl due to the heightened sensitivity the exercise has on insulin? Once the blood sugar is within a relatively normal range, carbohydrates can be consumed. Using this technique should be used with extreme caution. It is better to error using less insulin causing a high blood sugar than too much insulin causing a low blood sugar.

Environment and blood glucose monitoring - effects of heat/cold/altitude
The effects of extreme environmental factors such as heat, cold, and altitude all make the body expend more energy. The result of expending more energy without adjusting some other factor (i.e., decreasing insulin or adding carbohydrates) will usually cause a low blood sugar. An athlete competing at a higher altitude and a colder environment will utilize more glucose so an increase in checking blood sugars is recommended during these times. During hot or humid conditions, the body expends more energy trying to cool the body thus increasing the chances of hypoglycemia. These conditions can lead to an increase in insulin absorption because blood flow to the skin is increased as well.

In athletes with Type 1 diabetes, blood glucose levels can fluctuate considerably during competition. It is important to remember not to become obsessed with checking blood sugar but to have a plan. It is not the checking that improves control but what is done with the results that matters.

References, August 2004. Retrieved on June 5, 2005 from,10621,40-12,00.html.

Rick Philbin, MBA, M.Ed., ATC

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Last Updated: Friday September 07, 2012 11:16:44
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