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Fasting and Exercise - What to Know When You Start

The role of fasting and its effects on the human body is well documented. Recently, we have shifted our research focus to the effects of fasting combined with exercise. Both fasting and exercise alone are often used as tools to improve body composition and fat-free mass (amongst other things). So it would seem like a natural extension to combine the two for an enhanced effect.


Let’s take a quick look at what actually happens when we fast. We’ll call this physiology 101. Every movement or function in the body requires energy. We get this energy, or fuel, in the form of the foods that we eat. Namely, the components or macronutrients of our food. These components are fat, carbohydrate, and protein and each plays a different role in our body.


During exercise, the body will utilize a combination of fat and carbohydrate in order to provide the energy needed to train. We refer to exactly how we use each source of energy as substrate utilization. Substrate utilization (or what form our fuel comes from for energy) can be influenced by activity type, intensity, and our diet.


Carbohydrates are our body’s preferred source of fuel for long-duration of high-intensity exercise. Typically our stores of this fuel can last for 24 hours or more. So only very long duration, high-intensity exercise would put us at risk for depleting these stores. However, when we are in a fasted state, less carbohydrate is available as an energy source to fuel our activity. As a result, we rely more heavily on fat as a fuel source for similar activities.


Based on this, fasting and exercise seems to be best utilized for weight-loss, particularly fat-loss. Some evidence suggests it may not be ideal for building muscle. There is conflicting evidence regarding it’s long term effect on performance although it may help with adapting to fat utilization with long-duration endurance activities.


Many have questions as to its safety with regards to exercise. So, is it safe? Generally, yes. But a few precautions should be taken.


Experience of exercise while fasting may be affected by:

  1. Age

  2. Gender

  3. Exercise type, duration, and intensity

  4. Training age/status (untrained/new to exercise vs. well-trained)

  5. Fasting protocol or schedule

  6. Metabolic conditions - such as diabetes

  7. Uncontrolled blood pressure

  8. Pregnancy


The body of scientific research continues to grow yet the focus on fasting and its effects on exercise is still largely in its infancy. As a result, there are still many questions that are unanswered. Therefore, many of the current recommendations are often based on expert opinion and not yet fully supported by the scientific research or literature.


Recommendations and Guidelines:


  1. Start low and slow - Low-moderate intensity activity is recommended in untrained individuals to prevent hypoglycemia (low blood sugar). Over time, your body becomes more fat-adapted, allowing more exercise with greater intensities. Start low and gradually increase your duration and intensity over time.

  2. Manage your expectations - Just like starting to fast for the first time, this transition takes time. Over a 2-3 week period you may see short-term decreases in performance (although some studies show no change in performance), energy levels, and general capacity. It may also negatively affect your mood and cognitive function.

  3. Communicate - Tell your team that you’re beginning an exercise program while fasted. Talk to your trainer, coach, or medical professional so that they are able to help you navigate some of the new sensations. Individuals with specific metabolic conditions may require more sophisticated monitoring during their fast so it’s best to work with a qualified medical professional.

  4. Be flexible - try different protocols depending on your fast schedule. Afternoon exercise appears to have a more significant effect on post-exercise blood sugar. In one study, blood sugar remains higher post morning exercise than it does post-afternoon exercise. Some may find it easier and more beneficial to train immediately after their last meal or right before their first meal. If you’re sensitive to changes in your blood sugar, focus on timing. I recommend start by training following your first meal.

  5. Stay hydrated - whether fasted or not, drink plenty of water to prevent dehydration and help to stay alert during training.




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