Here’s What Happens if You Water Fast When Working Out

With increasingly more people swearing by the effectiveness of intermittent fasting, scientists have been trying to understand how a fasted workout session differs from a non-fasted one. While it is perfectly safe to fast for a limited period of time (provided one is neither sick nor pregnant), working out in a fasted state is a completely different matter. So what happens if you water fast when working out?  

According to some studies, working out in a fasted state helps you burn more fat as your body is forced to tap into your fat stores for energy. However, several scholars and health professionals reject the claim that fasting promotes fat oxidation, and warn against working out in a fasted state.

In this article, we will see what the latest scientific evidence has to say about the practice of working out during a water fast. Scholars and fitness gurus often refer to this as “fasted cardio” or “fasted aerobic exercise.” 

Why Fast in the First Place?

Ever since Nobel Prize winner Yoshinori Ohsumi discovered that fasting promotes autophagy (a process by which the body discards damaged cells and creates healthier ones), fitness enthusiasts across the world have been encouraging people looking to lose weight and improve their overall health to try intermittent fasting.

The truth is that people have been fasting for thousands of years, mostly for spiritual purposes. Our ancestors regarded fasting as an effective way to cleanse their bodies. This is why this practice is endorsed – if not enforced – by many religions, including Buddhism, Islam, Taoism, and Christianity.  

People fast for a variety of reasons, including:

  • To worship a deity.
  • To cleanse their body and mind in preparation for a religious ritual.
  • To lose weight or improve their overall health.
  • To test themselves.
  • To trigger autophagy (within the context of disease management).

To understand what happens if you water fast when working out, you first need to appreciate what happens to your body during a prolonged fast. 

First of all, you need to keep in mind that a 12-hour fast is not the same as a 72-hour fast and the way your body responds to starvation is going to change dramatically. 

On a normal day, when we eat on a regular basis, our bodies use glucose as their main source of energy. After twelve or more hours of fasting, the amount of glucose in our system declines, forcing our bodies to look for alternative sources of energy

This is when the liver starts breaking down fat cells, which the body then uses for energy as part of a process called ketogenesis. 

Working Out During a Water Fast

Let’s say you have been fasting for sixteen hours: your body is now producing ketone bodies which means that it is using fat for energy. 

If you work out in such a state, logic dictates that since your body will require more energy, it will burn more fat, resulting in greater fat loss. To test this theory, a group of scientists performed an interesting experiment involving twenty female athletes.  

Half of the athletes were made to work out in a fasted state for a period of four weeks, while the other half worked out after consuming a meal. All athletes performed one hour of aerobic exercise three times a week and were required to follow a hypocaloric regimen. 

After a month, the researchers found that both groups had lost a considerable amount of weight and fat mass without reporting any statistically-significant between-group differences. It is possible that since all participants were adhering to a strict hypocaloric diet, working out in a fasted state did not trigger any additional changes in their bodies.

There are also studies that claim the complete opposite. According to a study published in the British Journal of Nutrition, for instance, performing aerobic exercise in a fasted state results in lower glucose and insulin concentrations – two important fat oxidation markers.    

When discussing the benefits of working out in a fasted state, fitness gurus often refer to a study published in the Journal of Physiology in 2005. 

In order to understand what happens in the body when you exercise in a fasted state, a group of researchers conducted an experiment involving nine healthy subjects. Their goal was to see whether working out in a fasted state had any impact on the fat that is stored in our muscle tissue. 

The subjects were divided into two groups: one group was made to work out in a fasted state, while the other one worked out in a fed state. At the end of the observation period, the researchers found that the subjects who had been working out in a fasted state were burning three times more intramyocellular triglycerides (i.e., the fat content of our muscles) than the subjects who had been working out in a fed state. 

Now, your body replaces this particular kind of fat on a continuous basis. This means that if you are burning more of this fat, your body will have to pull fat from other stores, thus enabling you to burn more fat. 

It is for this reason that many fitness experts and health professionals see intermittent fasting and working out in a fasted state as effective ways for overweight people to lose weight. Because working out in a fasted state has been found to have a positive impact on insulin resistance, people with diabetes may also benefit from regular fasted workout sessions. 

Final Thoughts

According to some studies, working out in a fasted state triggers the same physical processes as working out while on a hypocaloric diet. However, the amount of evidence showing the positive effects of fasting on the body is overwhelming. 

If fasting for as little as 12 hours triggers ketogenesis, logic dictates that an individual working out while water fasting would reap even greater benefits as their body requires more energy. 

If you are looking to get in better shape and/or improve your overall health, you may want to discuss working out in a fasted state with your healthcare provider.

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