Glucose Levels During Fasted Training
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Interpreting Glucose Levels During Fasted Training

Performing exercise on an empty stomach requires a lot of endurance and practice. It has been scientifically proven that fasted training sessions are beneficial, but it is important to understand whether your body needs this sort of training and your metabolic health can meet the demands of these specialised training methods. The two primary reasons for undergoing this type of training include- 

  • The capacity of the body to utilise fat as an alternate source of energy while performing an exercise of low to moderate intensity
  • The ability of the body to replenish the reduced glycogen stored in the muscle as a result of exercise 

How to remain fasted and prepare for exercise 

If you intend to plan for an early morning workout session, it is best practice to have an early dinner the previous night. There should be no calorie intake after dinner till you go for your exercise or workout session. However, the instructor must plan any training session regarding hours of training, intermittent gap sessions and training intensity, depending on your health and body capacity.  Read more to know about what level of blood sugar is dangerous?

Interpreting glucose levels in fasted training

1. Rise in blood glucose level with high-intensity training

This happens because due to vigorous exercise, the muscles require energy. Glucose is an energy source readily available in the blood. But due to fasting, the glucose levels in the blood are already depleted, read more about fasting blood sugar. Hence, the body now depends on the stored liver glycogen to provide glucose. The breakdown of liver glycogen into glucose temporarily increases blood glucose levels though you have not consumed any carbohydrate.

Why does it happen?

 This happens because the brain sends signals to metabolise the stored liver glycogen and release glucose. Conducting high-intensity training is possible during this phase. When the stored glycogen level decreases slowly, the ability to perform high-intensity training also diminishes as the potential energy of the body increases.

2. Plateau phase of glucose level in blood

This happens when the training intensity has been reduced to a medium or low-intensity level after some time. Glucose requirement also reaches a stable phase, indicated by the plateau phase in the glucose curve. For a normal healthy adult, the body can maintain a stable glucose level for 2 hours of exercise at low to medium intensity. The glucose curve can also be used as an indicator for metabolic adaptations of the body during exercising at higher intensities.

What does the plateau phase indicate?

This indicates that the body’s metabolism has entered a stable phase. The body can endure long periods of exercise. During this phase, fat depletion also occurs, as fat is used as an alternate energy source to perform an exercise in the absence of stored glycogen. 

3. Fall in glucose levels

Sometimes at the start of high-intensity exercise, the demand for glucose rises as the requirement for energy in the muscles increases suddenly. This might lead to a sudden drop in glucose when enough metabolism of stored glycogen has not occurred to replenish the used-up glucose in the blood. But this is usually a transient phase and gets filled up quickly by the body’s intrinsic mechanism if the body has good metabolic health.

It becomes a matter of concern when liver and muscle glycogen stores have been depleted during long durations of exercise. It must be ensured that the body’s metabolic health is not affected and remains stable throughout the workout session. Drops in glucose levels under these conditions might sustain longer durations.

For trained athletes, the switchover to generate energy via fat oxidation and other pathways occurs quickly, and the body adapts to the changed metabolic pathway naturally. But this can happen only with regular and disciplined exercise under the supervision of a trained professional. In other cases, similar situations might lead to fatigue, dizziness and blackouts because of a lack of glucose. If you witness such drops in glucose levels or symptoms, it is better to take a break and replenish the glucose with some supplement or drink. Also read about fasting blood sugar levels.

Five Zone Model

Before directly jumping into rigorous or high-intensity workout sessions after fasting, it is better to start with practice sessions with intermittent fasting. You can go for morning and evening sessions with minimal calorie intake in between. This can be taken as a practice before you go for a long duration of fasting and then perform high-intensity exercise. Usually, the current approach is that follow the 5 zone model, which states that

  • Perform at zone 2 intensity for 60-90 minutes
  • Then perform at low zone 3 intensity for 60-90 minutes
  • Perform at zone 2 intensity again for 60-90 minutes with intermittent 5-minute efforts at zones 4 or 5

Also read about benefits of dates for diabetes


After completing the workout session, you can compare your pace and power output to the glucose curve. However, as blood glucose is a vital parameter of the body, it is essential to maintain the basal level of blood glucose to ensure that the vital organs get the required amount of glucose to maintain normal functioning. Lack of a proper training schedule and diet chart might disrupt the body’s metabolic health, which can be dangerous. The entire training session must be performed under the strict supervision of a certified instructor. The diet, calorie intake and fasting schedule must also be followed as advised by the dietician. Performing the procedure properly and carefully will produce maximum results associated with fasting and exercise. 

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This website's content is provided only for educational reasons and is not meant to be a replacement for professional medical advice. Due to individual differences, the reader should contact their physician to decide whether the material is applicable to their case.