People are indeed different and the scientist you quote is saying precisely that.
Taken literally calories in vs out is of course true.
No, they actually dont assume that only physical exercise has an effect on calories burned. In fact, CICO is agnostic to the means by which calories are burned.
> This is why just restricting calories in and exercising more won't necessarily mean you loose weight.
Actually, so long as you burn more energy than you consume, you will lose weight. Again this is agnostic to how the energy is burned. What the body does to adjust is merely effecting the amount of energy burned. It does not effect the formula concerning CICO.
You are trying to find a loophole here that doesn't exist. If two people burn the same amount of energy E_O ("out") and take in the same amount of energy E_I ("in") with E_I < E_O, then both of them must take those calories from somewhere else in their body and must therefore undergo weight loss. There is no way around that.
Now, notably you didn't specify E_O, you only said
> the same level of physical fitness and physical activity
which can mean a lot of things.
All the supposed loopholes work in exactly the same way: It might not be entirely clear what the values of E_I and E_O are, so "calories in, calories out must be wrong". (Which, again, is impossible because it's just conservation of energy, i.e. a law of physics.)
In any case, just like you can bound E_I from above by counting calories, there are ways to constrain E_O from below: If both people have the same mass (say 100kg) and walk up the staircase in one of the towers of Cologne cathedral (~150m), then they will both have spent at least E_O >= 100kg * 150m * 10m/s² = 150,000 J.
This[1] is a good summary of a talk she gave about a year ago and in it she talks a bit about the non-CICO factors in weight gain or loss. Here's one example:
“The gut microbiota of those that are lean versus those that have obesity are quite different, so much so that we can often take the gut microbiota out of individuals that are lean and place it in those that have obesity and see weight shifts with no other modifications."
Why did I know this before I even looked?
>“The gut microbiota of those that are lean versus those that have obesity are quite different, so much so that we can often take the gut microbiota out of individuals that are lean and place it in those that have obesity and see weight shifts with no other modifications."
Yeah, so all that effects is CO (calories burned). This does not disprove CICO. I see Fatima has a limited grasp of logic.
Not necessarily. In this case I think she is referring to calories that are not absorbed but are excreted.
Meaning if you are naively doing calories in vs calories out, or doing desired estimated calories out based on a table, you have an error factor significant enough to skew your results.
What we call it doesn't matter, but what GP said is actually incorrect as written.
Please do point out which part of my comment is incorrect.
> Meaning if you are naively doing calories in vs calories out, or doing desired estimated calories out based on a table, you have an error factor significant enough to skew your results.
What does "naively doing calories in vs calories out" mean exactly? I agree that some people do it naively but where exactly was my comment naive?
> Calories In vs Calories Out is mediated by a factor, usually labeled S, which accounts for the individuals own metabolic factor.
Yes, the metabolic factor affects the numeric value of E_I and E_O (I never disputed that), but once you have determined their value (or bounded them as in my comment – which, as demonstrated, is orders of magnitude easier), the statement "weight loss occurs if E_O > E_I" remains correct.