Yes, some of it is willpower, and probably for lots of people it's entirely willpower, but there is definitely a metabolic component to it.
If you eat less than you expend you must lose weight, but it's naive to discount a decrease in the expenditure part as well. Keep in mind that just living requires the vast majority of your calories, and any exercise is a blip covered by a couple cookies.
Often on prolonged diets, your NEAT can unconsciously decrease from a series of small decisions like choosing to take the elevator when you usually take the stairs, parking nearer to your destination, etc. and even things like fidgeting less.
You can kinda get around this with things like step counters so you can track how many steps per day you normally walk, and then if you notice you're suddenly walking fewer steps, you can consciously choose to walk more to keep your activity level at your baseline.
For example, you might average 8,000 steps per day but after a few months on a diet, you find yourself walking 4,000 steps per day naturally so you can just choose to go out for a walk after dinner to maintain or increase your baseline level of activity.
When people talk about changes in metabolism, this is normally the culprit.
You’re not losing a meaningful amount of fat in the first week. You’re just getting dehydrated.
I’ve seen this process up close multiple times. It usually starts with a dramatic weight loss goal like “I’m going to lose 2kg/week for a month.” The first few days of crash dieting seem to be working, then the rapid progress stops and it becomes clear that the original target is impossible. This makes it easy to justify quitting the diet altogether to oneself, which was always subconsciously the goal of this approach.
But as others have noted, it often doesn't seem to be sustainable as your body will react to that kind of calorie deficit.
My max. sustainable weight loss rate seems to be about half a kilo a week. Any more and I start yoyo-ing.
I know I am losing weight when I am hungry between meals, most of the time I eat because it’s time to eat.
Down 10 pounds in two weeks. It’s nothing special just eating what I should be eating in terms of quantity. Eating sweets? Skip a meal, etc.
by switching antidepressants. the one I was on before caused weight gain and sugar cravings. I gained 40lbs when I started taking it and lost it when I stopped. I just suddenly found sugar and carbs unappetizing, whereas before I'd get intense cravings.
the experience further convinced me that weight regulation is metabolic or something. I used zero self-control, diet or exercise, and a chemical change brought me back to a healthy weight. my eating habits changed but it was subconscious, not willpower.
2 months is hard.
2 years is really hard
Yeah, loosing 2 kg/week for two years is physically impossible for almost all obese people.
At least for me, we're talking massive amounts of water, by the way. ~500ml every 2-3 hours, all day for the whole time.
Ayran/yoghurt with more salt and water works better, likely because of the salt, but I've gotten too many small kidney stones that way - 100% sure, because I had to painfully piss them out and cut my urethra several times. And, well, yeah, bring a portaloo everywhere.
The hunger gets to me every week, though, in which case I found it's OK to eat a lot of protein/fat (eggs, cheese, meat) for a day. Well, by a lot I mean maybe 3 times the normal food intake.
I drink more water than most and I usually go through 4-5 litres in a regular work day.
Absolutely. For instance only counting the calories of their scheduled meals, but "snacks don't count because its just a snack". Or counting a heaping plateful of spaghetti as a single serving because they put it on a single plate. Underestimating calorie counts is much more common than estimating it accurately.
Eat a few meals with a huge person who "just can't lose weight" and it will usually become obvious that they eat a huge amount of food.