Exercise While Fasting

How is it possible to continue to exercise without having the food to recover? Won’t you get light-headed and fall over?

Somewhat surprisingly, yes, you can (and should) still exercise while on a fast. It helps achieve ketosis more quickly, it helps preserve muscle, and any additional activity you do while on the fast is “pure profit” (i.e pure fat loss vs just losing water weight). If you walk 3-4 hours per day on a 3-day fast, it is almost the equivalent of fasting 4 days in terms of fat loss.

That said, everyone’s body is different, with varying levels of strength and cardio fitness. For your first fast, I recommend sticking with light cardio: 2-3 hour walks, or a bit less if you’re not used to exercise. On subsequent fasts, you can attempt to keep up your resistance training.

Core Benefits to Exercising While Fasting

• Prevents muscle loss – This is the number one reason to exercise if you are fasting regularly. You are in an extremely catabolic state when fasting. No new calories are coming in, so your body uses what it has stored for its energy. Much of this is body fat, but some of it is  protein, including muscle. The goal is to minimize the latter as much as possible. Exercise and especially resistance training triggers signals in your body to NOT break down the muscles you recently worked out for energy expenditure.
• Achieves ketosis more quickly – This will make your fast much more pleasant, as it helps to avoid potential “hunger pangs” and unpleasantness. Ketosis is the state when your body is no longer primarily reliant on glucose (derived from sugar carbs) for your energy needs, instead using a higher concentration of ketones (derived from fat). Think of ketosis as being in a fat-adapted state, no longer reliant on carbs. As you can imagine, if you haven’t eaten your normal carbs and aren’t yet fat-adapted, your body will feel out of whack and headache, foggy brain, fatigue, irritability, or nausea could set in, otherwise known as the keto flu. The symptoms are a lot milder than the actual flu, but why not avoid them with a little exercise?
• Burns additional calories – While deep into a fast, any extra activity you do will help you accelerate fat loss and accrue all the benefits of fasting more quickly. When you exercise normally (in a regular, non-fasted state), the energy expenditure usually comes from a combination of stored fat and stored carbohydrates in your body. There are many factors at play, including type and intensity of exercise, time of last meal, nutrient composition of last meal etc. If you recently had a meal with carbs, most of the calories burned are from carbs. In a deep fasted state almost 100% of your energy expenditure comes from fat!

Added Benefits to Exercising While Fasting

• Makes you feel better – Moving around and deliberate exercise can be good solutions when feeling a little off. It can help get rid of lethargy or body heaviness. In fact if you are testing your blood glucose, you will notice that your glucose levels will rise 10-30 mg/dL during exercise, so you get a little boost there as well. 
• Helps pass the time and focus the mind – This seems obvious, but when you are fasting you are going to have extra free time that was devoted to procuring, preparing, and consuming food. These activities normally punctuate your day. So it’s nice to have a replacement, and exercise can often be meditative or a nice area of focus when your mind doesn’t have eating to worry about. 

How to Modify your Exercise

• Strength training – This is a crucial tool if you want to minimize muscle loss. You most likely are not going to gain strength or muscle while fasting – the goal is purely maintenance. All of this is much easier if you already have a strength routine that you follow regularly. If you do not, xxx. Aim for 60 minutes. Train all muscle groups versus doing a split that focuses on solely one muscle group. Keep your normal weight/intensity, but drop the volume of sets by two-thirds. High rep sets may feel more fatiguing than normal, but you should be able to lift the same weight you are accustomed to. You may need to increase the rest time between sets. The general idea is to make sure your body knows that it still needs to maintain all the muscle needed to perform your high intensity/heavy sets. Normally when fasted, the “mTOR” nutrient/protein sensors in your body get inhibited which signals to the body that you should recycle existing materials in the body to function instead of expecting new materials. When doing resistance training you can selectively activate mTOR in your muscles despite decreased mTOR in the rest of your body.
  • For example…
    • Normal routine (mostly strength focused routine)
      • 2-day split
        • Leg focused day
          • Squat – 3 sets of 5 reps (i.e 3 x 5)
          • Deadlift – 3 x 5
          • Lunge – 3 x 5
          • Calf raises – 6 x 10
        • Upper body focused day
          • Bench – 3 x 5
          • Barbell row – 3 x 5
          • Pull up – 3 x 8
          • Dips – 3 x 8
          • Shoulder raise – 3 x 8 
          • Bicep curl – 3 x 8
          • Tricep dip – 3 x 8
          • Wrist curl – 3 x 10
    • Modified fasted routine (volume, i.e. # of sets pulled way down, but hit every muscle group)
      • 1-day (full body)
        • Squat – 2 sets of 5 reps (i.e 2 x 5)
        • Deadlift – 1 x 5
        • Lunge – 1 x 5
        • Calf raises – 2 x 10
        • Bench – 2 x 5
        • Barbell row – 2 x 5
        • Shoulder raise – 2 x 8 
        • Bicep curl – 2 x 8
        • Tricep dip – 2 x 8
        • Wrist curl – 1 x 10

• Cardio – Unless you are an experienced runner/cyclist, stick to low intensity cardio. Walking is usually best. It won’t make your muscles as sore, and you’ll be able to go longer. A 2-3 hour walk the first morning of your fast is the most critical (to accelerate ketosis). However, walking and cardio are great every day. While fasting, I find I have almost limitless cardio stamina.
  • Walking outside v. Walking on a treadmill
    • Outside – Doing your long walk outside is fantastic, both on the trail and in the city. On the trail you will likely be going on a much longer hike than usual so it’s an opportunity to hit some of the longer trails that you might not normally do. The city is also great, because you can zig zag to explore new neighborhoods and notice things about the neighborhood or people that you wouldn’t normally notice.
    • Treadmill – The advantage of a treadmill is that you don’t have to map out where you are going, you can set a consistent incline and speed, and just zone out to a podcast or audiobook.
  • If you are an intermediate/advanced runner, cyclist, swimmer, rower, or “rucker” then you could continue to do those activities at a higher intensity than just walking, which is great if you want to save some time. However, if you are not well trained in those areas you may get crushingly sore if you are not used to it (which is no fun if you are fasting).
  • The way I think about it,  for every 10 hours of moderate walking you do, it is the equivalent of fasting an extra day.

Things to watch out for 

• Salt and hydration – Make sure you supplement salt before it becomes an issue on exercise days. I usually chug a relatively concentrated salt solution (LMNT) 1 hour to 30 minutes before walking. I will use a less concentrated shaker bottle full of salty water for lifting workouts. For long walks and outdoor workouts I’ll fill a whole camelbak with LMNT Citrus Salt. It is better to have too much than to run out while out and about, especially if it is hot out.
• Time between sets – You may need to take more time than usual to recover between sets. For example, if you normally take 2 minutes between heavy sets, you may need to take 2.5 minutes.
• General fatigue – You may notice that you wake up in the morning feeling body heaviness or just feeling run down. Try to push through it if you can. Often the act of moving can help you feel better. Make sure you take inventory of your sleep quality, hydrations, and salt status. 
• Strength loss – On Day 2-3+ of your fast, you may notice slight decrease in strength (maybe a 5% decrease or so). Don’t stress about it too much. Keep training through it to prevent further loss. You will gain the strength back relatively quickly when you are done fasting. I notice that when my sleep is good, I have relatively minimal strength loss. However, when my sleep is consistently poor multiple nights in a row, that is where my strength takes a slight hit.
• Muscle soreness – If you train more than usual (strength or cardio) while fasting, your muscles will feel sore. I have noticed that it takes longer for the soreness to go away in a fasted state (probably because there is no ingested protein for muscle recovery). It is best to start a fast with a few weeks of consistent training to minimize this. Push through it if you can! Stretch. If it’s really bad, you can reduce the duration of your subsequent workouts if you want to.