If CICO doesn‘t work (as per Fung) why does IF work?

(The crazy German guy) #1


So, in various interviews and even his book, Jason Fung states that calories are not an adequate measurement for food intake. I fully agree. They are merely a measurement of how well stuff burns in a laboratory.

However, he also says it doesn‘t work because the body adjusts its calorie expenditure. You cut your calories, your body cuts its expenditure. He further states that the body reacts to less caloric intake quickly, but only reacts to higher caloric intake slowly. —> JoJo.

I somehow get all of this. Coming from our ancestors a thousand years ago when they weren‘t sure when they‘d find food again it made sense for our bodies to learn to react quickly and focus on what‘s necessary.

The question i have is: if the body really responds to less caloric intake in a quick way with less caloric expenditure, why does fasting work? And remember: as per Fung it doesn’t need to be KETO + IF, it can also be carbs + IF, so take the fat-adaptation out of the equation.

From my perspective IF is nothing but cutting calories. Yes, theoretically i can drink a bottle of oil and eat 4.000 calories in one sitting, but come on. The whole deal is that we have less chances to eat too much. So, in the end it does come back to cutting calories.

(Rachel) #2

Ok so with my non existing scientific knowledge I might answer that question this way:
If you confined your calorie intake within an 8 hour period vs say a 16hr period,
Your body has more time to reset, use the energy and renew it’s processes where when the calories are spread out over 16 hours, the metabolism becomes less effective? And perhaps starts storing energy instead of burning it?
I read somewhere that optimum fat burning begins at 13th fasted hour?

(Jack Bennett) #3

tl;dr version - IF enables you to burn stored body fat for a greater fraction of the day, leading to body fat loss.

Part of the theory of IF is that it allows insulin levels to drop off during the fasting period. Lower insulin and higher glucagon allows fat to come out of storage and allows the body to use its stored fat for energy needs.

I’m not sure if this research has been done, but it would be useful: what if people eat the exact same food (type, quantity) in one big meal versus three or five separate meals?

The “spread out” timing would elevate insulin multiple times through the day and make it harder to burn stored body fat.


One day, I am going to create an ultimate list or glossary, if you will, of topics that have been discussed to death on this forum, create a topic called “For the love of God look here first if you can’t or won’t use the search feature” and BEG the mods to pin it.

Anyone who doesn’t look there first must then post a video of them eating sardines as an apology (wait, I love sardines…maybe brussel sprouts…no…let’s decide on this later!)

One day.

For the love of Richard Morris look here first if you can’t or won’t use the search feature
(Robert C) #5

The goal of IF is to provide long periods of low insulin.
2000 calories spread out into 3 meals and a couple of snacks will keep insulin up all day (making fat burning impossible and fat storage possible).
2000 calories between 4 PM and 6 PM will spike insulin one (less so if keto) but, for the majority of the day - fat usage (from diet and body) is possible due to very low insulin.
Same calories but very different outcome with IF vs. non-IF.

Not sure where you heard that, if you calorie restrict to 75% of your total caloric needs, you will lose weight for a while - several weeks to several months. Until you plateau - that is when the body has responded to your lowered intake by lowering expenditure. Of course, now your screwed because eating at 100% of your previous caloric needs puts fat on quickly.

(The crazy German guy) #6


here’s where he expresses this in a nutshell.

But, it’s also included in his lengthier videos.
The way he expresses it, it sounds like energy expenditure will reduce immediately if you eat fewer calories one day.

That said, i totally get the 2000kcal scenario you’re creating.

(Robert C) #7

Also, simply eating to satiety all the time (which is part of the reason keto works) or throwing in a couple of over-fed days a week (if you are more CICO based) will keep things lumpy (food intake wise). The body has less of a reason to lower expenditure in that case - putting off plateaus.

Still should be in the context of exercise though. If, on your overfed days, you try to compensate with 10+ mile runs (to “keep your weight loss going”) - you could manufacture the same deficit anyway and fall victim to metabolic slowdown.


It’s pretty clear to me, Fung is wrong.
I never could wrap my mind around this thinking… Zillions of people loses fat by eating less, it’s even logical if someone’s body works in a certain way. It always very nicely works for my family members, including me. I just can’t eat little so under a certain energy need, I stuck.
I understand not everyone is this “simple” but it’s always CICO, just the complicated one where CO is a complete, CI is a partial mystery. But some people can guess it pretty well because they are “stable” enough or not sensitive to various things. Or their body doesn’t believe it’s famine if the food is a tad less. So they can use the simple but in general clearly wrong CICO too.
Other people may find their own way. I need some tricks myself.

A huge extra energy intake for a long time may result in no or tiny gain due to genetics (I have lots of personal experience). Sometimes the metabolism quickens quickly and very much. I’ve read it’s not even very rare but somewhat unusual, yes. So that part is individual too.
I have no knowledge, personal experience with metabolism slowing but some people loses fat at a quick, almost stable pace for long when starving too. It’s probably very complex but I would think their metabolism doesn’t “like” to slow down too much or too soon. Probably the energy need matters too, it’s problematic to have a huge energy deficit if one needs very little energy to begin with. Starving at 200 kcal and starving at 1300 kcal is probably quite different and the reserves of the body are there too… Way too individual.

Fasting. If “works” mean it causes fat-loss, well, it clearly doesn’t work for me (except under special circumstances. A 5 minutes long eating window would have a very good chance, for example. 30 minutes is dubious, it depends on my diet, probably). It worked only when I ate little enough calories but that worked anyway, I never noticed a difference (but I always was very close to 16/8 and it’s just me, with my body surprisingly insensitive to many factors).
It automatically works for many, of course and some people can’t not starving on IF so it works “too well” for them.

I believe there is some extra help due to hormones and other things. It makes fat-loss easier for many. But calories are heavily involved, at least almost always. And some of us never have a noticeable bonus anyway. We just eat less and get satiated better and for longer easier - or not. IF isn’t for everyone.
I heard many stories and IF clearly reduced calories in most cases. Many intermittent fasters (especially beginners who aren’t used to “big” meals yet) often eat quite little, not even enough for a healthy fat-loss. Others eat at an okay amount but still less than their needs.
And the OMADers who eat 4000 kcal in one sitting because they are that hungry, they usually don’t lose fat at all even if they have fat to lose. It’s not magic, working for everyone, no matter what.
I only lost fat on OMAD when I went below my usual energy need (it’s quite stable in the last several years) though my experience is little, I usually do 19/5 with low-carb/keto. I inevitably gain fat on a carby IF if I eat at least twice every day, that’s serious enough overeating even with my genetics.

We are very, very different. Let’s keep that mind. I accept some people function in interesting ways compared to me but they shouldn’t say something works for everyone, no matter what when many of us experienced that it’s not true, even when we did it way stricter and put effort into it, for a very long time.

I focused on the cases similar to mine, of course there are explanations for the ones who have some bonus on IF. Even my body probably does those things just not to a noticeable extent. I just wanted to say it’s clearly not huge for everyone and it’s definitely not the reason why fasting “works” for so many.


It always made very much sense to me. I am still sure metabolic slowdown is very individual and doesn’t happen easily for everyone but for most people having a higher-calorie day may be a very good thing to do. I can’t test it on myself as I always have higher-calorie days, I just need them sometimes and they inevitably come pretty frequently if I eat under my energy need. And my metabolism stays higher than average for my stats, even after months of fat-loss. Maybe it’s due to those days, it’s logical.
I usually hear those days should have maintenance calories (not like we surely know that number, whatever). I don’t enforce that either and usually eat more than that. It works when my usual days are lower. My hypothesis is that such a higher-calorie day might help more than just avoiding metabolism slowdown. Maybe it makes the metabolism even quicker it normally is for a while. If the one in question has those genetics. The people who eat like crazy for a week and gain a noticeable amount of fat aren’t like that. The ones who eat crazy for a month without gain might be.

(Prancing Pony) #10

Simple answer is hormones. IF lowers some and up regulates others. To a certain point, with certain people. There are 8 or more that we know of that directly effect metabolism.

We are basically 4 year olds twiddling what switches we can reach trying to change the speed of a car we barely understand (and each car is different) hence conflicting advice that never works for everyone.

IF and keto have more science and success in general than CICO but it still has to be an individual experiment and answer.

(The crazy German guy) #11

For me it works quite simple. I can eat around 800kcal in a healthy meal. No doubt I can eat 1.600 or even more when eating crap. But vegetables, moderate protein and high fat = 800 for me. IF 16:8 or 18:6 result in 2 meals. 20:4 results in 1 meal and a snack. Omad results in 1 meal.
This way for me it is either to consume around 1.600kcal, 1000kcal or around 800kcal = calorie deficit.

I think Fung explains the mechanics of all this nicely in this video

After watching it for the 5th time or so in 1 year, I sort of get that CICO with carbs and low low low calories doesn’t work.
Keto + IF + low calories does work because the body has an alternative energy source.

Don’t get my initial question wrong. I lost the most weight on keto and omad. But my head keeps revolving around this low calorie thing.
I also don’t think that Fung is entirely correct. CICO and low calories do work for a lot of people. I’ve lost 50% of my 100lbs weight loss with it. And there’s literature saying the metabolic rate Adaption is only very little.

(John) #12

without getting into the cico argument my 2 cents is that fung is dealing with a group of people that is pretty much on the obese side with t2d. any way you look at it when you IF coming from a diet that got them there they will most likely be eating less calories. Group that with extended periods of lower insulin and you get good results. IMO its both the insulin and the cico that is working


CICO DOES work. If you are one of those lucky persons who doesn’t like to eat much, OR if you have huge willpower. There are many many examples out there.

Unfortunately most people do not have that willpower long term, so they will gain weight again – because they will get terribly hungry and begin to eat more. That bad hormones, that Work when you eat all the time and high carb will not make you gain bodyfat out of no calories, they will just make you want (and do) eat more.

IF makes it easier to eat less, because of the psychological reason that “all or nothing” is easier for your willpower, and in the second step, because of the decreasing hunger hormones.
That’s the whole secret. IF makes you want to eat less, so that you eat less calories and gain less weight (or lose some).

(Thomas) #14

No. It has nothing to do with willpower. There are many studies that overweight people tend to consume less calories on average than normal weight people. And on the other hand there have been studies where people were supposed to gain weight by eating more. Some of them failed, in others most people would gain weight but lose it even quicker than they gained it once the “fattening” ended. Gary Taubes “Good calories, bad calories” has tons of the stuff. There are mice that will have significant fat depots even if you starve them to death, they die fat. Fat is only superficially related to calorie intake, but in reality you can be really fat and eat almost nothing.

Having said that, the laws of thermodynamics DO apply and CICO does work if done correctly. The principle itself is correct, it’s the conclusions that are way off.

  1. “calories out” does not equate to calories burned in mitochondria. We excrete calories, we exhalte ketones that have caloric value, we use calories to break down food (especially protein).
  2. The basic error of the CICO people is that the variables are independent, that is, we can lower “calories in” or increase “calories out” without affecting the other. That’s were it fails miserably. Both resting energy expenditure (REE) and calories exhaled/excreted will go up, and it seems the body burns off excess calories after eating. That is, eating both increases the long-term metabolic rate and the immediate energy consumption (people burn more energy until the food is processed and we’re once more in fasting state).

We can lose weight on keto because the diet affects the adjustment to calories. If we eat carbs and lower calories, the REE will be decreased to compensate. If we keto and lower calories, the REE will not change as much.


On a much higher level, that’s what I’ll tell someone who doesn’t want to go into detail: Evolutionary we have two basic modes, a “fattening up” mode when there is food surplus where we build up fat for the winter, and a “normal” mode where we burn body fat if we can’t get enough food. The fattening mode is triggered by insulin, or carb content in our diet. Our western diet keeps us in “fattening” mode all the time, and the body will do everything it can to preserve the fat depots in that state. It will lower the base metabolic rate as much as required to maintain the fat reserves. If you want to lose weight, you need to get the body into “winter” mode.

(Jack Bennett) #15

Exactly - if you truly do measure CI, and closely control it, you can potentially make a CICO approach work for you.

The issue is when people stop closely tracking, or make estimates that aren’t accurate (forgetting to write things down, underestimating the spoonful of almond butter, etc). In that case, CI often drifts. Usually it goes upward but it can also go downward in the case of loss of appetite due to anxiety or illness (for example).

This also assumes that CO can be estimated accurately and that it doesn’t change much over time. This is the source of those tables that show how long you need to work out to “burn off” that donut. But we see experiments that show (e.g.) changes in CO in response to increases in CI. It’s not a simple system where input and output are independent. The counterexample of calorie-restriction dieters plateauing in weight loss despite “compliance” with the restriction is also well known.

Fundamentally, CICO is a statement of mass and energy conservation. It’s a correct scientific principle but isn’t always useful to help people to lose body fat.

In a sense, it’s always “true” - if you eat a gram of fat/protein/carb, then your body is going to do “something” with that. And conversely, if you metabolize or use a gram of fat/protein/carb it obviously came from “somewhere”.

But so what? Does knowing this make people more likely to feel full after two cookies instead of the whole box?

I’ve always found the CICO-guided approach hard because of appetite: if I am not reasonably full when I stop eating, it’s very hard for me to stop eating. I also find certain foods very hard to stop eating - that 47/47/6 ratio for maximizing addictiveness is no joke.

I think the keto approach works for me because I focus on highly satiating foods (protein-rich), eat until I’m pleasantly full, and then stop eating and/or thinking about food until next time.

(Thomas) #16

That’s not true. There are documented cases of up to 40% in “starvation” experiments (mind you, 60 years ago a 1600-calorie-diet was considered “starvation” so we’re not talking about fasting here). But it may take months to kick in, and most weight loss studies will consider only 3-6 months.

(Thomas) #17

Once more, this is wrong. It’s just as saying that if you’re 1m above sea level in low tide that you’ll drown in high tide, ignoring that you’re in a boat.

There are two levels of “wrong” here. Even if you ignore the fact that you’ll constantly have to fight hunger if you reduce your calories on high carb.

(1) short term: If you eat more then you’ll burn off some of the excess calories right away. If you eat 1000 excess calories, you will never accumulate even close to 1000 calories worth of fat, instead you’ll produce excess heat. Obviously the type of food and the rate of eating will matter. If you eat a candy bar every hour you’ll gain more weight than if you eat the same amount of calories in 2 meals with fatty meat (or whole grain bread), but even with candy bars you will accumulate less than 100g fat a day. (If anybody has numbers here, I’d be very interested.)
If you eat 1000 calories less you won’t burn 1000 calories worth of fat, because you’ll be cold and miserable and spend less energy.
Glycogen stores may smooth this over of course. That is, you may see different effects in fasted state than in a state will full glycogen stores.

(2) long term: The energy expenditure will change, which takes a few months. Say you need 2400 calories a day given your REE and activity level. You start eating only 2000 calories. In the first month you may lose 2 pounds and think the approach is working. However, after half a year your expenditure may be down to 2000 calories a day, and your weight is stable. After a year you’ll burn 1800 calories and actually gain a pound every month. And yes, your body will overcompensate.

In the womens health study I think they had a group eat 300 calories less for 8 years. In the end, the net difference in weight to the control group was 2 pounds on average. According to energy differential the difference should have been 250 pounds.

If you do consider hunger and eat when hungry (and until full), you’ll see additional effects.

(3) Only part of the carbs are converted to glycogen and become available for the muscles. Another part will be converted to fat and go into fat cells. As insulin blocks ketone production (so you can’t use the food for energy) this is a one way street. This one depends on insulin levels, the higher the insulin the higher the fat conversion. Bottom line, if you spend 2400 calories a day you may need to eat 3000 calories to keep your glycogen stores filled if 20% of the food goes directly into fat cells.
(4) The satiety signal does not work, we overeat.
(5) We become addicted especially to sugar and may eat even if we aren’t hungry.


I looked at it more simplistically. If longer fasting slows down your metabolism in order to preserve energy, then IF won’t let that happen. So during intermittent fasting, your metabolism is still at a normal rate, instead of suppressed. This means that in those few hours of not eating anything, your body will burn energy at a normal rate instead of a suppressed one.

(Thomas) #19

This works if (and only if) you’re burning ketones in that 16h time span. That’s the thing with metabolically healthy people – my daughter (8yo) can eat pasta in the evening but she’ll have ketones in her breath the next morning. So fat is burned once the blood glucose is gone.

If you’re metabolically sick (like me), a meal of pasta will block ketone production for at least 24h. I may be cold and miserable but I won’t burn fat. MetS is a sucker :slight_smile:

(Jack Bennett) #20

I should specify, in the short term. Clearly, the downsides of a naive CICO/eat-anything-IIFYM approach mean that it’s hard to make it a permanent lifestyle.

People can achieve short-term weight loss with it, but that’s a low bar to clear, because you can do that with a lot of questionable approaches.