How long can I fast without losing muscles?

Question

If I am lifting three times a week, can I have a 24h or 48h fast on the weekend (say, eat last meal on Friday evening and then next have on Saturday or Sunday evening)? Am I going to lose muscle mass because of this or am I safe and I can use this method to reduce my weakly calories intake?

Answer 1

A 24 hour fast is not going to hurt you or magically cause you to lose muscle. In fact, there are several intermittent fasting (IF) approaches that talk about how to balance food, fasting, and lifting. The short list of these are:

• Lean Gains
• Eat Stop Eat
• Warrior Diet

Each of these have slightly different approaches to fasting, eating, and exercise, but the basic principles are the same:

• You have a period of no food (water is OK).
• You have a window of time where you get all your caloric and dietary needs.
• You do your exercise at a time when you can eat afterwards.

The key to successfully pulling this off is to manage your recovery--the rules of which don't change whether you intend to fast or not. For about 48 hours after you lift, your muscles are sensitive to insulin. This is a good thing, because it takes the carbohydrates you eat post workout and feeds that energy to your muscles first. Of course you still need your protein, fat, etc. to rebuild your muscles while you are resting.

The idea behind the approach is that you continue to gain muscle while burning fat more effectively. The idea is not to give yourself a Caloric deficit. It's to give you the Calories you need, in a way that helps you stay leaner in the process. There are some people in the lifting forum I use who swear by IF, but I have yet to attempt it myself. The only thing I can say is that these guys lift very respectable weights, so that tells me you can incorporate fasting and weightlifting.

The biggest difference between the above dietary approaches is how long the fast is, and how long the feeding window is. Eat Stop Eat is closer to what you are discribing in your question where you fast on rest days, but feast on workout days. Lean Gains is where you have an 8 hour feeding window, and the other 16 hours are fasted. Bottom line is that it is possible to lift and fast.

Answer 2

While @berin brought leangains into the topic already, he didn't address @gruszczy's concerns about any actual studies to this end.

Martin Berkhan, of course, addresses this topic to some degree at leangains in point #4 and #6 of the article Top Ten Fasting Myths Debunked:

4. Myth: Fasting tricks the body into "starvation mode".

Truth

Efficient adaptation to famine was important for survival during rough times in our evolution. Lowering metabolic rate during starvation allowed us to live longer, increasing the possibility that we might come across something to eat. Starvation literally means starvation. It doesn't mean skipping a meal not eating for 24 hours. Or not eating for three days even. The belief that meal skipping or short-term fasting causes "starvation mode" is so completely ridiculous and absurd that it makes me want to jump out the window.

Looking at the numerous studies I've read, the earliest evidence for lowered metabolic rate in response to fasting occurred after 60 hours (-8% in resting metabolic rate). Other studies show metabolic rate is not impacted until 72-96 hours have passed (George Cahill has contributed a lot on this topic).

Seemingly paradoxical, metabolic rate is actually increased in short-term fasting. For some concrete numbers, studies have shown an increase of 3.6% - 10% after 36-48 hours (Mansell PI, et al, and Zauner C, et al). This makes sense from an evolutionary perspective. Epinephrine and norepinephrine (adrenaline/noradrenaline) sharpens the mind and makes us want to move around. Desirable traits that encouraged us to seek for food, or for the hunter to kill his prey, increasing survival. At some point, after several days of no eating, this benefit would confer no benefit to survival and probably would have done more harm than good; instead, an adaptation that favored conservation of energy turned out to be advantageous. Thus metabolic rate is increased in short-term fasting (up to 60 hours).

Again, I have choosen extreme examples to show how absurd the myth of "starvation mode" is - especially when you consider that the exact opposite is true in the context of how the term is thrown around.

Origin

I guess some genius read that fasting or starvation causes metabolic rate to drop and took that to mean that meal skipping, or not eating for a day or two, would cause starvation mode.

6. Myth: Fasting causes muscle loss.

Truth

This myth hinges on people's belief it's important to have a steady stream of amino acids available to not lose muscle. As I explained earlier, protein is absorbed at a very slow rate. After a large high-protein meal, amino acids trickle into your blood stream for several hours.

No studies have looked at this in a context that is relevant to most of us. For example, by examining amino acid appearance in the blood and tissue utilization of amino acids after a large steak, veggies and followed up with some cottage cheese with berries for dessert. That's easily 100 grams of protein and a typical meal for those that follow the Leangains approach. We are left to draw our own conclusions based on what we know; that a modest amount of casein, consumed as a liquid on an empty stomach is still releasing amino acids after 7 hours. With this in mind it's no stretch to assume that 100 grams of protein as part of a mixed meal at the end of the day would still be releasing aminos for 16-24 hours.

Few studies has examined the effects of regular fasting on muscle retention and compared it to a control diet. None of them are relevant to how most people fast and some are marred by flaws in study design and methodology. Like this study which showed increased muscle gain and fat loss, with no weight training or change in calorie intake, just by changing meal frequency. While I would love to cite that study as proof for the benefits of intermittent fasting, body composition was measured by BIA, which is notoriously imprecise.

Only in prolonged fasting does protein catabolism become an issue. This happens when stored liver glycogen becomes depleted. In order to maintain blood glucose, conversion of amino acids into glucose must occur (DNG: de novo glucogenesis). This happens gradually and if amino acids are not available from food, protein must be taken from bodily stores such as muscle. Cahill looked at the contribution of amino acids to DNG after a 100 gram glucose load. He found that amino acids from muscle contributed 50% to glucose maintenance after 16 hours and almost 100% after 28 hours (when stored liver glycogen was fully depleted). Obviously, for someone who eats a high protein meal before fasting, this is a moot point as you will have plenty of aminos available from food during the fast.

Origin

An example of severe exaggeration of physiological and scientific fact, not relevant to anyone who's not undergoing prolonged fasting or starvation.

Answer 3

I am a powerlifter I started intermittent fasting 10 weeks ago, I fast from lunch to lunch 2 times a week (from monday lunch to tuesday lunch, and thursday lunch to friday lunch) I don't eat anything, just water black coffee and that's pretty much it, I still lift on fasting days without a problem, I just lost fat and nothing else. While you 're on a fasted state your body will generate growth hormones to keep your muscle mass intact. Don't worry about losing muscle cuz you won't! Hope that helps, talk to you next time.

Answer 4

You are safe.

Up to 3 days and should receive more positive effects than negative. After that, metabolism is likely to slow a lot.

The first 24 hrs, you will burn more muscle than usual. Then from 24 - 72, you should burn predominantly fat.

I generally do 16, 20, and 24 hour fasts. I feel good on these. My longest is 46. I talk about that here: Diet Review: Intermittent Fasting (IF)

Answer 5

You are asking about the Catabolic-Anabolic balance. You are always in a Catabolic state ie: you are always breaking down stuff (food/fat/muscles) so that the Anabolic process can occur (broken down stuff replenishes your muscles). When you aren't eating, you have nothing to break down but your muscles/fat.

When you wake up, you are have an imbalance in processes, meaning that the Catabolic state outweighs the Anabolic state. If you continue to not eat, you will have to get your nutrients from somewhere, and that somewhere will be your muscles/fat stores.

Above all do not fast as a way of reducing caloric intake. You need calories: and more importantly you need the stuff that calories represent: proteins, etc. If you want to reduce your caloric intake, eat higher quality foods. Higher quality foods will give you the necessary nutrients and less of the saturated fats/highly processed sugars that do nothing for your body. In other words, you have to eat more to be satisfied if you eat low quality foods.

In short: fasting will burn muscle.