I have always thought that you couldn’t really exercise while you were fasting. This was because I didn’t really understand the body glucose and ketosis mechanisms. Well, after 20 hours, this is what actually happened.....

Personal Record While Fasting!

I have been intermittent fasting for a few years now, but I just started regularly fasting each week for at least 20 hours for about six months, and I was under the impression that you couldn't really exercise while fasting. I did quite a bit of research, and if you are healthy, regularly eating nutritional foods, and taking vitamin supplements, you should be fine to exercise as much as you would like. You could lose a little muscle mass as you are breaking down muscles without the protein to build them back up, but overall, it probably is not a significant amount. I feel that the health benefits out way the small issue of muscle loss.

I had a pretty standard simple Keto lunch and followed it up with a High-Keto dinner with very minimal carbs. We ended up watching a show and decided to have popcorn and a Moscow Mule drink, so I ended my fast with quite a few carbs including a fair amount of sugars. Not necessarily the best way to jump into Ketosis with my fast, but a healthy life is about balance right? I know my body is pretty good about switching into Ketosis, so I was fine with doing it that way.

After I got home from work, I had an espresso, and I was feeling pretty good physically and mentally. I checked my Keton levels with my glucose blood tester, and I was at 0.8 mmol/L which is in Light-Ketosis, so my body was in the mode of turning fat into glucose and keytones. I got on the bike (in this case a Peloton we got 18 months ago), and decided ”today I would go for it”. I found a 20-minute ride with a fantastic 80s rock set, and put my mind into pushing hard. I started off hard and just kept it going. After about 7 minutes in I was at my all time high pace (Peloton shows you how you are doing based on your all-time best workout), and I decided to just keep pushing hard.

I broke my 20-minute personal record of 147kj of output with a whopping 164kj of output and a Strive Score of 39.2. My typical Strive Score is about 28 with a normal good workout. I just killed it! I am at such a different place than I was months ago when I remember working out while fasting and had such low energy, but before I started extended fasting (20 hours or more) at least once a week and ‘trained’ my body to quickly go into Ketosis.

Here Are My Recent Peloton Workouts

As you can see in the image below, my previous record workout was from May 25, 2024, getting pretty close to topping that a couple of times recently, but nothing close to what I did yeasterday - and believe me, when I went to bed last night my left calf was quite sore from the thorough work it had to do. Now all this isn't from Fasting or Keto, but some of it is.

My Main Point Here - You Can Have Lot's of Energy While Fasting

This wasn't always the case for me, but now that I have been long-fasting each week (more than 20 hours) in addition to my intermittent-fasting for several months now, I see the results. I can get into ketosis very quickly, and my body unfortunately has ample amounts of fat to turn into glucose and ketogens to feed my body and brain. That, with a desire to prove I can do something extraordinary last night, turned into a nice effort that I am proud of.

Ketosis Levels

Here are my Ketone Levels for this fasting period:
after 10 hours - 0.8mmol/L
after 20 hours - 0.6mmol/L
after 39 hours - 1.2mmol/L

If you use a meter to test for ketones in your blood:
- 0.5mmol/L to 1.0mmol/L is Light Ketosis
- 1.0mmol/L to 3.0mmol/L is Optimal Ketosis

Here is a Chart from Keto-Mojo.com

Some Science Behind Fasting and Exercise

1. Fasting and Ketone Production

When you fast, your body enters a state called ketosis. This happens because:

-Reduced Insulin Levels Fasting lowers insulin levels, which reduces glucose availability and encourages fat breakdown.
-Increased Lipolysis With lower insulin levels, fat cells release fatty acids into the bloodstream.
-Liver Ketogenesis The liver converts these fatty acids into ketones (acetoacetate, beta-hydroxybutyrate, and acetone) to be used as an alternative energy source, especially for the brain.

2. Exercise and Ketone Production

When you exercise, several processes enhance ketone production:

Depletion of Glycogen Stores

-Glycogen Use During exercise, muscles use stored glycogen for energy. This is particularly true for high-intensity activities.
-Accelerated Depletion In a fasted state, glycogen stores are already low. Exercise further depletes these stores, signaling the liver to increase ketone production to meet the body's energy demands.

Increased Fat Oxidation

-Fat as Fuel With depleted glycogen, the body shifts more towards burning fat for energy.
-Fatty Acid Mobilization Exercise increases the release of fatty acids from adipose (fat) tissue, providing more substrate for ketogenesis in the liver.

Hormonal Changes

-Catecholamines (Adrenaline and Noradrenaline) Exercise elevates these hormones, which stimulate lipolysis (the breakdown of fats into fatty acids).
-Growth Hormone Exercise, especially high-intensity exercise, can increase growth hormone levels, which also promotes lipolysis and subsequent ketogenesis.

3. Biochemical Pathways

Beta-Oxidation

- Fatty acids are transported to the mitochondria in liver cells, where they undergo beta-oxidation to produce acetyl-CoA.

Ketogenesis

- In the liver, excess acetyl-CoA from beta-oxidation is converted into ketone bodies. This process involves:
-Acetoacetate The primary ketone body formed.
-Beta-Hydroxybutyrate Acetoacetate can be reduced to beta-hydroxybutyrate, a more stable and energy-dense ketone body.
-Acetone A minor ketone body that is often excreted via breath and urine.

4. Energy Utilization

-Muscle Cells During exercise, muscle cells can use ketones for energy, which helps preserve glucose for the brain and other essential functions.
-Brain Ketones can cross the blood-brain barrier, providing a vital energy source during both fasting and prolonged exercise.

Summary

When you fast, your body is already primed for ketone production due to low insulin levels and increased fat breakdown. Adding exercise to the mix accelerates the depletion of glycogen stores and enhances fatty acid mobilization, further boosting ketone production. The hormonal response to exercise also amplifies these effects, leading to higher ketone levels as the body shifts more towards fat metabolism to meet its energy needs.