Hi, I am currently on day 7 of what I hope to be a 21 day water fast. I supplement with electrolytes, and honestly feel pretty good. I read up on the science of fasting, and how as your body lacks glucose and carbohydrates, it goes into a state of burning fat for fuel. I have a large amount of excess body fat (about 60 pounds overweight), so as long as I supplement with electrolytes correctly, drink plenty of water, and listen to my body if it gets overly fatigued/dizzy, I can exercise really any amount, right? I mean any extra energy needed will just come out of my fat. Is this science/logic correct?
Let me preface this by saying that I am not a physician or a nutritionist, just someone who has had a lot of success with longer fasts.
Here’s my understanding:
First of all, you really need to watch your water and electrolytes. The “…so long as I supplement with electrolytes correctly” caveat is a lot easier to say than do. As you may know, electrolytes mediate interactions between cells of all types, between blood and cell membranes, basically between everything in your body. Electrolytes are positively or negatively charged particles that are quite literally pumped from one side of a barrier to another in order to create a charge disparity that cascades into some action - they’re really important, and people who are fasting for long periods of time often aren’t getting enough of them. But, ok. Let’s say you’re getting enough electrolytes. Can you exercise any amount? Well, all exercise isn’t created equal. People have around a day of glucose stores on their muscles (very rough estimate). That amount can be burned in a much shorter time period, depending on what you’re doing. Triathletes regularly eat carb-heavy snacks during longer races because they deplete their muscles’ glucose stores over, say, a 25 mile bike. The same thing can happen to fasters on a slower time frame. Within a day or two, you’ll likely have used up that glucose. What does that mean? Well, first, the water that is attached to it in a 1:4 ratio is likely also gone. This results in some weight loss (“water weight”) that will return immediately upon refeed. It makes your muscles look flatter, and can make heavy people look trimmer. The lack of glucose also means that any exercise that would draw on those glucose stores becomes way harder to do. Not impossible, depending on the person and your discipline and what you’re attempting, but pretty hard. Like so much with long fasts, I find my mental and emotional state very effected in ways that are surprising. I am a very disciplined person, but after a few days of fasting, finding motivation to do bodyweight calisthenics is so difficult. I find myself rationalizing not working out, not moving, being sluggish, and I have to consciously remember this is a physiological reaction creeping into my waking thoughts. It’s weird. But, you can work out once your glycogen is gone. You will still be forming ATP (the body’s energy currency) and so stuff will continue to happen. Type of exercise matters, though.
Some other comments have suggested you don’t work out; I don’t think that’s wise. I would suggest slower, deliberate strength workouts paired with daily walking. Studies have shown that doing resistance workouts while fasting helps maintain muscle mass, and prevents your body from catabolizing muscle along with fat. Basically, by stressing your muscles, you can cause your body to even more preferentially burn fat as opposed to muscle. Our bodies have a process called gluconeogenesis that turns protein into glucose for use elsewhere; resistance work can slow or prevent this. You want more muscles because muscles burn more kcals and are a key feature in healthspan and longevity, not to mention aesthetics. I prefer body weight stuff, like squats, push ups, pull ups, etc., when fasting. I suggest walking because it’s a great way to burn kcals without starting certain cascades that will make you feel tired or sore or hungry that can come along with higher intensity cardio.
Ultimately, the limit to how much you can work out is how much energy your body has native to it without eating from fat and protein stores. If you have 60lbs of body fat, you can likely go over two months without eating. Working out heavily will become more difficult, but you can do the math on your remaining body fat vs kcals burned in a day and figure out your theoretical exercise limit. That being said, your organs will shrink, you will likely not keep up with electrolyte and water demands, you probably won’t be supplementing certain vitamins and minerals (as an aside, make sure you’re adding some iodized salt to your water occasionally. This helps with electrolytes and helps prevent goiter), and you’ll find exercising increasingly difficult over time. I do not suggest trying to rid your body of all its fat in one go. You’ll likely just catabolize muscle right before you die from refeeding syndrome.
Finally, let’s talk about re-feeding. Be extremely careful with your refeed. People can and have died from refeeding incorrectly. Basically this, too, comes down to electrolytes; it’s less of a problem if you’re making sure you’re getting enough, which you probably aren’t.
This is way more than I meant to write. I wrote it in one go, off the cuff, so I’m sorry if some things don’t make sense. Fasting is great for you, but does (like most things) carry some implicit risks if you do it wrong. A great thing about it, though, is you can really stop whenever you want. It’s that simple. Good luck!
What is the point of exercising? You need intake of protein to build muscle. Are you thinking you’ll burn more fat? I would just rest and let autophagy do it’s job, then workout when you are intaking food so your body has the nutrients it needs to build/repair muscle.
Exercise basically causes injury/tears to the muscles which the body repairs. I don’t think it uses* fat to do so, and certainly not your own fat.
There’s also a good amount of research showing exercising during prolonged fasts is bad for you, mainly because you end up burning a lot more muscle. That’s not to say you can’t go for walks or stuff like that but when doing a 21 day fast you should try to take it relatively easy.
Can’t remember the source, prob some YouTube video, but fasting and exercising is okay but not so much after 3+ days. Not okay as in, loss of muscle. I definitely would not exercise hard after a week even if there’s the rebound effect of building more muscle after.