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From Phoenix, Arizona, USA:

I am 19 years old and have had type 1 diabetes for five years. I have a few questions regarding blood sugar levels. I was having a problem where I wasn't feeling low until my numbers where down into the low 40s mg/dl [2.2 to 2.7 mmol/L], or sometimes even lower. My endocrinologist and I have somewhat fixed this problem by adjusting my basal rates on my pump (which I have been on for about two and a half years) and having me run higher. We did this for about three weeks, and for a week and a half, I was not wearing my pump at all, but using Lantus and NovoLog injections. Overall, I am now able to feel low when I am in the low 70s mg/dl [3.9 to 4.1 mmol/L] to 60s mg/dl [3.3 to 3.8 mmol/L].

However, when I exercise (like going to the gym or our running), I will have levels in the normal range and then plummet into the very low range again. When I exercise, I take off my pump and don't put it back on until I get home. And, when I get home, I will cut the basal rates to almost nothing (0.3 or sometimes lower, depending on what my number is). I never tend to be low right after exercise, but rather up to seven hours later. A few weeks ago, for example, I went to the gym in the morning and when I was finished working out (my workouts are usually an hour to an hour and a half) I tested and I was 106 mg/dl [5.9 mmol/L], so I drove home. I did not eat foods that contained a whole lot of carbohydrates that day, so I was not bolusing very much. However, that night as I was fixing myself dinner, I tested and I was 23 mg/dl [1.3 mmol/L] and I didn't even feel low. I re-tested with that meter and with another one to make sure there was nothing wrong with the meter and all the readings were under 27 mg/dl [1.5 mmol/L]. This has happened to me a few times. On the days I exercise more, I have tried eating more carbohydrates and I am not even giving the full amounts of insulin I would require if I had been lazier that day. My endocrinologist and I are at a loss as to what is happening. Do you have any ideas as to what is happening and what I can to stop my numbers from being so low?


Hypoglycemia unawareness is a frustrating experience especially around exercise. It is common to have low blood sugars after exercise. In fact, there is a high occurrence for people four to six hours after exercise, but some experience it up to 24 hours later. Once the body burns the food eaten prior to exercise, it uses a stored form of glucose (called glycogen) in the liver and muscles. After that is depleted, if food is not eaten, the body will get glucose from the blood, which causes us to go low. This does not happen quickly but over a number of hours. There is a two-fold reason for eating carbohydrates after exercise. One is to help prevent a low but, probably just as important, is to restore the glycogen lost during exercise. This will not only help with decreasing the chance of a low blood sugar, but will help with performance for the next exercise bout.

Now that you know why it happens, the question becomes what can be done to prevent this? There is no easy answer but, if you follow a few guidelines, the risk of hypoglycemia will decrease. For individuals who experience low blood sugars post exercise, here are some tips:

  • Aerobic exercise (running, cycling etc.) will usually decrease blood sugars so providing a buffer with a starting blood sugar higher than normal is recommended. Many athletes shoot for a starting blood sugar of 150 mg/dl [8.3 mmol/L] prior to exercise, but it may be lower or higher depending on the individual and intensity of exercise. When insulin is involved, the guidelines are to have blood sugars above 100 mg/dl [5.6 mmol/L], but not higher than 240 to 300 mg/dl [13.3 to 16.7 mmol/L] with ketones or 300 mg/dl [16.7 mmol/L] whether ketones are present or not.

  • Anaerobic exercise (weightlifting, sprinting) can cause blood sugars to go up during or immediately after the exercise due to the increased activity of hormones such as adrenaline. Even though there may be an increase, there is a likelihood that the blood sugars will go low hours after the activity. Once blood sugars are near normal, it is important to eat to replace glycogen stores. Drinking a lot of water and discussing with your healthcare team about using insulin to bring down high blood sugars (i.e. over 300 mg/dl [16.7 mmol/L]) is recommended.

  • Decrease basal rate(s) after exercise: a 50% reduction rate is a good starting point over four to six hours, but check frequently to see if more or less is needed and whether to decrease/increase the time of the decreased basal rate.

  • A good rule is to eat 15 grams of carbohydrates for every 30 to 60 minutes of exercise. This is individualized, so checking blood sugars during exercise will help determine how many carbs are needed.

  • Eat carbohydrates with some protein or a good source of fat (i.e. peanut butter) within 20 to 30 minutes after exercise to restore glycogen stores. How much depends on blood sugars and intensity of exercise.

These are guidelines to follow, but are not absolute. If a pattern develops, look to make changes up or down depending on blood sugars and make sure to keep your healthcare team in the loop with any changes made. A good book to read on exercise and diabetes is The Diabetic Athlete by Sheri Colberg, PhD.


Original posting 6 Apr 2005
Posted to Hypoglycemia and Exercise and Sports


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