Why Does Blood Sugar Spike After Exercising? A Detailed Look at the Stress Response and Its Effects that Exercise Causes

Why Does Blood Sugar Spike After Exercising? A Detailed Look at the Stress Response and Its Effects that Exercise Causes
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Blood Sugar and Exercise

Blood sugar provides the raw materials for the body to produce energy. Fat also is another important source. Both of these substances are broken down by the body during exercise.

Fat is used during moderate activity. A 2009 study by the University of St Thomas in Minnesota explains that maximum fat burn occurs during 60 to 80 percent of your maximum heart rate or 220 minus your age. When you ramp up the intensity of your workouts, the fuel source changes.

Energy Production

During vigorous exercise, the demands for energy are heightened. Your body is put into a situation where it must produce a great deal of energy very quickly. Thus, it will turn toward carbohydrates for energy. As you use up glucose, your pancreas will release a hormone called glucagon. Glucagon will stimulate your liver to release stored sugars to keep fuel flowing.

Using carbohydrates yields energy more quickly than fat because chemically, carbohydrates are not as complex. Your body can keep up with your energy needs better during strenuous activity. That is not to say that your body won’t tap fat reserves, just that using carbohydrates provides a more efficient means for producing energy.

Fight-or-Flight Response

Why does blood sugar spike after exercising? For all of its health value, the body interprets exercise as stress. You are putting forth a greater effort during your workouts, causing a depletion of circulating blood sugar and its reserves found in your muscles and liver. You may be exercising hard enough to elevate your heart rate. These effects are caused by the release of epinephrine or adrenaline. This hormone is associated with the fight-or-flight response.

It is a survival instinct with the purpose of enhancing your ability to respond to some type of stress or threat. A spike in blood sugar is not uncommon during other types of stressful events such as surgery. Another effect of epinephrine release in an increase in sugar metabolism. This response may explain the blood sugar spike after exercising.

While that may appear to be harmful for a diabetic, a 2009 study by the University of Missouri argues that the body needs stress like exercise because stress is part of its evolutionary history. Researchers explain that the absence of stress can actually be the cause of harmful physiological changes. In the long run, exercise will increase your insulin sensitivity so that your body can respond more appropriately to changes in blood sugar.

As a precaution, you should plan on having a small snack prior to exercise to help maintain stable blood sugar levels. The spike in blood sugar following exercise may be accompanied by a subsequent crash for which you need to be prepared. With proper blood glucose monitoring and control, you can exercise safely.


F. Booth and M. Laye. Lack of adequate appreciation of physical exercise’s complexities can pre-empt appropriate design and interpretation in scientific discovery. Journal of Physiology, December 2009; 587: 5527-5539.

D. Carey. Quantifying differences in the “fat burning” zone and the aerobic zone: implications for training. J_ournal of Strength and Conditioning Research_, October 2009; 23(7):2090-2095.

Colorado State University Extension: Glucagon www.vivo.colostate.edu

G. Tortora and B. Derrickson. Principles of Anatomy and Physiology, Wiley, 2008.

Photo by Cheryl Empey, stock.xchng