In a lab with controlled portions people lose weight.

Outside the lab where they are free to revert to gluttony they also typically revert to their original weight (exactly as harris/benedict predicts).

Cals in/cals out works fine. But only if you actually work it. I've successfully used it myself to modulate my weight in both bulking and cutting cycles, as well as reducing my weight from obese to healthy back in the day.

"and pretty much every study for the last 100 years has shown reducing calories does not keep weight off."

That is possible if it's a small reduction of calories and you're not very far off your ideal weight.

Otherwise is a thermodynamical impossibility.

And yes, losing weight is one problem, keeping it off is a different matter.

It is not thermodynamicly impossible because your body is not a fixed system. It can (and does) regulate energy usage based on how much energy it is receiving, what kind of energy it is receiving, what the environment is (primarily reflected through cortisol and stress), etc. all of these intertwine, making it nearly impossible to look at calories alone to measure, well, much of anything.

Reducing calories causes the body to reduce its calorie usage and starts pumping hormones that cause fat to be retained and your desire to eat to increase, until the hormonal system forces an "override" where you have to eat more. Sure, there are some people who manage to overcome the hormonal override, but they are by far the exception and not the rule, and any weight loss program that tries to override the body's built-in protections against starvation (which go back who knows how long and exist in pretty much any animal) is a recipe for failure.

The keys to the whole hormone cycle are insulin and leptin. Any successful weight loss regimine will have to address those hormones, which calorie reduction "inflames" rather than "pacifies".

Yes, you are right (and explained it quite nicely)

But there's an absolute minimum of calories the body need to operate, even at 'low gear' (I'm not sure, but this is around 1000kcal per day), so if you're really limiting the calories below a certain point you will lose weight.

Apart from that, if you're trying to lose weight, but not actually consuming less calories, of course you may have a weight gain.

Fair enough, but I would say that's only useful if you are wanting to lose a little weight before a cruise or want to get into a lower weight class for a wrestling match. Weight loss that doesn't "stick" is probably more dangerous than not losing, because it tends to focus as visceral fat instead of subcutaneous fat; and, unfortunately, study after study[1] show that dieting does not result in long-term weight loss[2].

1. This is a review of 31 studies that comes to the same conclusion: http://mann.bol.ucla.edu/files/Diets_don%27t_work.pdf

2. My anecdotal story, I tried restricting calories for 15 years with the result of hitting 350+ pounds. Seven years after a gastric bypass, I've found low-carb to be the only way I can maintain my weight. Even with the gastric bypass, I can't limit myself to the 1200 calories or less a day I would need to maintain (and I feel like crap). Low carb, I never have to even think about the number of calories (I trade one number for another :) and my energy goes up.

I was experimenting with this and, when the results were positive, it really drove me to understand why the results are positive. I'm (slowly) starting to get an understanding of the biochemistry involved (that is a lay understanding, for sure!).

If people choose to overeat and do not lose weight (as you assert occurs), that is not evidence against cals in/cals out.