The root cause of weight gain is either overeating or lack of sufficient exertion in almost every case. Sleep apnea may absolutely cause someone to overeat in part because of those dozens of biological feedback loops you mention. But that doesn't mean that sleep apnea makes people gain weight. It's still too much food at the end of the day.

It's all four fundamental physical forces actually, if you want to go that route. Which isn't helpful.

That meant measuring calories until I could generally estimate what I was taking in each day. 1000-1500 calories a day sucks at first, but seeing the scale drop is motivating. I dropped 30lbs in about 6 months.

You can’t gain weight if you are taking in less calories than you expend. That’s pretty simple. Not eating that cheesecake, on the other hand, not so easy.

I gather that you are unaware of how sleep apnea causes poor sleep, which in turn causes insulin resistance, which in turn causes an elevated production of insulin, which in turn forces glucose into your adipose tissue, which in turn makes the rest of your tissues starved for energy, which in turn causes you to be hungrier, overeat and gain weight.

This is an oversimplification, of course, as there are hormones other than insulin and cortisol involved in the complex process of insulin resistance, but it is at least a starting point that goes beyond the useless truism that "people gain weight because they overeat". Of course they do, but the mechanisms why they are hungrier than they need to be are crucial if you want to address the problem, because in the long run every human being with free access to food will eat precisely until they are no longer hungry, whether or not they are metabolically healthy or not.

There definitely is the issue of how to reliably maintain a caloric deficit when your body is screaming at you to eat more, but the underlying mechanism is much simpler.

At the most basic levels, food choice doesn’t even really matter (specifically for weight loss, not general health), but choosing satiating foods with low calories is going to help with implementation. Eating 2,000 calories of oranges (45) is going to be a lot harder than 2,000 calories of fast food hamburgers (2).

> Being hungry does not cause you to eat

Yes, it does. That's why people eat: to alleviate hunger. You may will power your way to a caloric deficit for some months, maybe a couple of years, but eventually hunger will win and you will eat until satiety.

Metabolically healthy people aren't slim by being hungry all the time. They eat to satiety, and they are lucky enough that their satiety signalling is still working well enough. If you make them chronically hyperinsulinemic, they will be hungrier and they will gain weight -- it happens in humans and in animal models when allowed to eat ad libitum.

> At the most basic levels, food choice doesn’t even really matter (specifically for weight loss, not general health), but choosing satiating foods with low calories is going to help with implementation

As long as you are hyperinsulinemic, you will store a portion of the energy you consume in your adipose tissue, because that is one of the major effects of insulin in the body. The remaining energy left in you will not be sufficient to supply the energy needs of your body, and as a result you will be cold, tired and hungry.*

If this is the premise of your argument, it is flawed.

You have no idea if “eventually” happens to everyone. You’re making a wildly strange assumption.

Have you tried? Are you willing to try? How many people do you know that intentionally remain hungry every day for more than a couple of years, while otherwise having free access to food?

Any weight loss advice that does not adequately address hunger and satiety is naive to the point of ridicule. And any person who advises others to address their weight by telling them to be hungry in perpetuity hasn't given much thought to what they are asking for.

> Yes, it does.

Nearly 1:10 people in the world are chronically hungry. Being hungry has no impact on their ability to eat. Likewise, those with access to food eat when not hungry and (believe it or not) may not eat when hungry.

I really think you’re missing the point, though. We have a host of signals in our body that can misfire due to various conditions and people cope with them. Hunger is no different and many of us have spent months ignoring it to our own benefit (and some to their own peril).

Caloric intake in excess of expense causes weight gain. What factors lead to that excess do not.

There are certainly those with a variety of issues that make caloric deficit difficult. Fortunately, that’s not most people.

The problem with that is you can reach a point were trying to reduce calories further will only further sabotage your metabolism and makes you feel like shit. Which is why treating the symptom repeatedly fails to work long term.

I’m a 5’11” male who gained weight up to ~220 pounds on ~1800 calories eating a very consistent meal rotation. I now maintain ~190 pounds at ~2100 calories. My problem was likely a combination of my food acting as a chemical signal to gain weight (high in branch chain amino acids, polyunsaturated fats from fatty pork and chicken), work stress, bad sleep, and possibly even chronically elevated blood sugar or thyroid effects from essentially what was an almost carnivore one meal a day sort of diet. My body temperature was hitting lows of 96.3 degrees F. It’s now roughly two degrees higher, which seems small, but feels so much better.

If someone is eating healthy foods and making a genuine effort to get good exercise, then we can talk about other factors. But too many people use “this is so complex” to avoid the very basics of healthy living.