Recently, I got this comment on the blog:
“Please everyone, the first part of this diet plan is ridiculous! Never fast for 16 or 20 hours, because your body will not burn fat for energy, it will go into a catabolic state in which it burns your muscle because its saving your fat because it believes it is being starved. You will go so far backwards if you ever go almost an entire day without eating.”
Lets examine what this guy is saying, shall we?
Perhaps I am totally wrong, and maybe fasting is somehow dangerous or will cause your body to burn muscle or will cause you to “go so far backwards” on your journey to health?
Let’s examine the SCIENTIFIC evidence …
When You Fast, Does Your Body Burn Fat OR Muscle?
The comment states:
“Never fast for 16 or 20 hours, because your body will not burn fat for energy, it will go into a catabolic state in which it burns your muscle”
Is this true?
Well, it depends on two things:
- How long you’re fasting and
- What you’re DOING while fasting.
In a SHORT fast — such as 16-24 hours — unlike what clueless people (like our commenter) want you to believe — your body will NOT burn muscle.
Instead it will first burn the stored glycogen in your muscles, and then fat, and LASTLY muscle.
Second, if you’re doing some form of exercise — resistance/strength training — during the fast — then your body is “smart” enough to know that it needs to keep your lean muscle and get rid of fat instead.
There was a study where they took a group of men, decreased their calorie intake by 1,000 calories per day and also had them do weight-training 3x per week.
The result? They lost up to 20 pounds of body fat and kept all their muscle mass.
(Rice B, Janssen I, Hudson, R, Ross R. Effects of aerobic or resistance exercise and/or diet on glucose tolerance and plasma insulin levels in obese men. Diabetes Care 1999; 22: 684-691)
As I’ve also mentioned before, a study was done as long as 12 weeks where people were on a VERY low calorie diet. Only 800, liquid, calories per day! But because these people were doing weight training 3x per week (a form of resistance training) – these people kept all their muscle mass.
(Bryner RW, Ullrich IH, Sauers J, Donley D, Hornsby G, Kolar M, et al. Effects of resistance vs. aerobic training combined with an 800 calorie liquid diet on lean body mass and resting metabolic rate. J Am Coll Nutr. 1999;18:115-21.)
Lastly, while there are no well-designed studies at this time that test subjects to see if fasting causes muscle loss, my friend Martin from LeanGains.com posted a great summary about this myth that fasting causes muscle loss (backed up with science, as per usual and emphasis mine also like usual):
“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.”
Basically as long as you’re eating a decent amount of protein every 24 hours — you’re gonna be absolutely fine — no muscle loss.
When You Fast, Does Your Body “Save” Fat?
The comment also states that when you fast, your body is “saving your fat” …. is this true?
Let me answer this question, by asking YOU a question:
WHY does your body store fat?
For ENERGY in case you ever stop feeding yourself.
So when you STOP feeding yourself, your body STARTS using stored fat.
It’s that simple: it’s the way your body was designed to work.
To think that your body will “save” fat if you stop eating for 24 hours or less is ridiculous.
Again, my main man Martin from LeanGains.com (because he has the patience to rebut all-comers with real science and hunting down the appropriate studies, unlike me, I’d rather just yell at you) has the following to say about how your metabolism actually INCREASES during a short-term fast (short-term in this case is up to 60 hours — WAY more than either I or Martin recommend, which proves the point even more):
“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).”
Add to the fact that fasting automatically cuts your calories for the fasting days/hours … and … that your metabolic rate is increased and you have a pretty good recipe for fat loss … NOT … “saving fat”.
What’s more, as you’re about to see you can lose a LOT of fat by not eating.
The ULTIMATE proof of all these points is coming up …
How Long Can You SAFELY Go Without Eating? And How Much Fat Can You Lose?
It was 1968 — the year before the summer of love (I don’t know this because I was there, but that’s what I’m told).
A 27 year old man was VERY obese … extremely overweight … he weighed a whopping 450lbs!
He went on a medically supervised fast.
Here’s what he did:
- Drank water
- Took vitamins
- Did not eat or drink anything else.
He Fasted For 1 year and 2 weeks — 54 weeks — 382 days!
The questions are:
- Did this man burn fat for energy? The energy to survive was coming from SOMEWHERE, he didn’t eat ANYTHING for over a year …
- Did he go into a completely “catabolic” state where he burnt muscle… instead of … fat? I’d say there was some muscle loss after an ENTIRE year of no food … BUT … he burnt MOSTLY fat (otherwise, he would be dead because your body will not function without muscle).
- Why did he burn mostly fat? Because he was 450lbs — severely overweight — and fat is stored energy that gets used when you stop eating (or aren’t eating enough)
This is just common sense!
And what were the total results?
He Lost 275 Pounds!
Now, here’s the point in the argument where people like our commenter will say something like:
“You will go so far backwards if you ever go almost an entire day without eating …”
Which is an attempt to say:
“sure he lost all that weight, but it probably ruined his metabolism, and his body was so catabolic, he lost too much muscle mass, and he’ll regain all that weight back, blah blah blah”
Well, here’s what you should know:
The fast took place in 1968 and the paper wasn’t published until 1973. At that time, he had gained back only 6 lbs.
You can read the full abstract here, where it states:
“A 27-year-old male patient fasted under supervision for 382 days and has subsequently maintained his normal weight.”
Also, and just as importantly, the abstract of the study says this:
“Prolonged fasting in this patient had no ill-effects.”
So What’s The Bottom Line?
Don’t believe “bro science”.
What is Bro Science?
It’s when people repeat “facts” about health, diet and fitness that they keep hearing … without … ever examining the evidence for themselves.
Don’t believe the hype!
- Short fasts WILL primarily burn fat for energy
- Short fasts will NOT make your body eat all its muscle
- Fasting does NOT make your body “save fat”
- Short, smart, fasting will NOT make you “go backwards” in your weight loss goals
- 24 hr or less fasting is a piece of cake compared to what your body is capable of
All that being said, here’s my disclaimer:
DISCLAIMER: PLEASE do NOT go without food or water for that long without medical supervision. I would never recommend a fast longer than 24 hours. You don’t NEED to do that to lose weight, or be healthy, or for any other reason.