Exercise: predicting blood sugar (BG) changes up or down

During and after exercise - why does my blood sugar sometimes go up, and sometimes go down?

Different types of exercise will affect you in different ways. For example, if you are doing a very easy activity, you will be using more fat metabolism to fuel the event, and you might not see a rise in blood sugar. If you do a harder more challenging exercise, your body will be using more muscle glycogen, and that might cause more of a blood sugar spike after the exercise. If you are doing very hard exercise that is shorter and very
intense, you may see the spike during the exercise.

Going low or high during or after exercise has reasons that are perfectly normal. Your body responds in much the same way as a non-diabetic’s body. The only difference is that the non-diabetics don’t see their blood sugar rise or fall because their pancreas and liver take care of it automatically.

During intense exercise it is common to need insulin because your body is trying to fuel the work, and also because of the cortisol and stress hormones that are released which will raise your blood sugar. During lower intensity exercise, the rate of glucose uptake from the blood is increased, which can cause a low.

Here are some very general examples of what might happen to a diabetic’s blood sugar after exercise. These things will vary greatly for each person depending on the intensity of the activity and a person’s fitness, so simply use these descriptions as a basis for understanding the different possibilities, and how they may apply.

Low Aerobic - energy is primarily supplied by fat metabolism, with only a small background of carbohydrate metabolism required. The initial perceived effort level (on a scale of 1 to 10) is between 3 and 4. Physical activity can increase the rate of insulin absorption and glucose uptake. Blood sugar can drop during this exercise, and can also drop after.

Moderate Aerobic - slightly faster than Low Aerobic. Energy comes predominantly from stored muscle glycogen, but the body is still metabolizing a fair amount of fat. Initial perceived effort level is 5 or 6. A diabetic’s blood sugar might drop during this exercise, but it can also rise after. When you exercise your muscles need more glucose to supply energy. In response, your liver increases the amount of glucose it releases into your bloodstream, which can lead to a post-workout spike in blood sugar.

Intense High Aerobic to Anaerobic - muscle glycogen provides virtually all of the energy for muscle contractions. Initial perceived effort level is 7 or 8. Intense exercise causes the release of hormones that increase the release of glucose by the liver, and reduce the muscular uptake of it. By releasing epinephrine (adrenaline), norepinephrine, cortisol. Your heart rate and blood pressure increase, the amount of oxygen flowing to the tissues increases, and your liver’s glycogen stores are put into action giving you fuel. It’s the typical response the body has to a “fight or flight” situation. Imagine if you are about to be attacked by a grizzly bear. Your body would prepare for that by providing fuel and doing those things mentioned so that your would have everything you need to either fight the bear or run away. Ultimately what you will see is an immediate rise in blood sugar.

Several other factors can affect blood sugar during and after exercise:
Performing exercise yesterday may also cause lower blood sugar today, because the body has not completely replaced the muscle glycogen and is relying more on the blood glucose uptake to fuel the exercise.

What can make it even more difficult to predict is that the previous day’s exercise and blood sugar can also affect what happens today. Having low blood sugar yesterday can blunt the body’s hormonal response to exercise today, meaning the hormones that would usually raise your blood sugar are not as active. Exercising on the day after severe low blood sugars should be done with extra caution and frequent testing.

The location of the pump infusion site or the injection site used can affect the rate at which insulin is absorbed. Using the muscles that are near the injection/infusion site will increase the rate of insulin absorption and bioavailability of the insulin. Consider the difference an infusion site would have on a leg versus an arm when bicycling!

Temperature can also affect insulin absorption. Higher temperatures will increase the absorption rate and bioavailability of insulin. Heat causes blood vessels to expand, so this should be considered when exercising in hot temperatures, or doing an activity such as hot yoga or using a sauna or hot-tub after exercise.

Even the order that exercise is performed can affect the blood sugar that follows. Doing aerobic exercise first followed by anaerobic exercise can cause blood sugar to be more likely to trend down after the exercise. If done in the opposite order, anaerobic first and then aerobic, blood sugar might be more likely to trend up. (This of course depends on individual factors such as the intensity of exercise and the amount of insulin and carbs, so this concept is mainly useful for comparing the results the exercise order will have when performing different types exercise.)

One of the best tools the diabetic has is keeping good notes! Keeping track of different types of exercise, and how they affect the blood sugar makes it easier to decipher the mystery.

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