You’re begging the question. You assert it’s not particularly rare for people to lose weight and keep it off long term, and therefore it’s trivially obvious to prove that it’s not particularly rare. Except that all studies to date show that it -is- rare, especially for the group of people that are candidates for an intensive intervention like medically managed weight reduction or bari surgery.

The data show that over ten years average sustained weight loss in diet-and-lifestyle interventions is zero (actually, there’s usually weight gain). That doesn’t rule out particular rare exceptions, but they’re just that, and not useful for evaluating the intervention as a whole.

That's not what 'begging the question' means. If you say "iron doesn't rust" and I show you a rusty piece of iron, that's not 'begging the question' regardless of how many studies you can cite that show iron not rusting under various conditions.

And 'average sustained weight loss over a cohort' is the wrong measure, especially if your cohort are candidates for bariatric surgery (ie. pre-selected for being medically recognized as incapable of losing weight voluntarily) rather than just overweight people in general (a fair few of whom do choose do get back in shape.)

But long-term, 10, 20 30 years out?

I can't name a single friend who has successfully dieted short term that has kept it off long term.

Is 30 years a reasonable expectation? Does someone who loses weight in their 50s have to literally die of old age before you'd grant that they'd 'kept it off'?

Actually... Yes.

Obesity related complication occurr with excess weight. So, if they were at their ideal weight at 50 yrs old and gained it back at 80 when they also incurred high blood pressure and diabetes, then they weren't successful.

That's why it's difficult. Most can lose weight at some point in their lives. But the weight eventually comes back, along with the other complications.

This seems like an odd way to split the data. Is it not the case that healthy-weight people sometimes gain weight in their old age? Your claim seems unfalsifiable: Whatever mechanism is at play that makes healthy-weight people gain weight later in life could also be at play for people who lost weight, and 30 years later regained it in their old age.

To put it another way, it's a silly bar to use to say that weight-loss programs must move their adherents level of health wrt weight past those of normal slim people, and that any weight gain from a former dieter is proof of a diet's failure of efficacy (while any weight gain from a formerly-healthy-weight, non-dieter is just happenstance).

It's tough to look at just the existing data to tease this out, but if you need to resort to a bar as absurdly stringent as that to make your point, I'm afraid it's not a very convincing one.

So, I dove into this a bit (as a professor in a related discipline but who doesn't study weight) and had some heated conversations with researchers in this and related areas. My impression is that those studies are controversial because they "define away" diet effects.

They often engage in a bit of semantic game-playing about the meaning of the term "diet."

The problem is a lot of the "diets don't work" conclusions are based on very strict assumptions about "diet" meaning "telling people to just lower their calorie intake" or "telling people not to eat certain foods." They don't necessarily mean giving strategies for lifestyle changes, or ways to avoid increased calorie intake, or replacements.

As a concrete example, one researcher I spoke to is a very publicly prominent advocate that "diets don't work," and gave a talk I attended. She was discussing studies of weight loss and was explaining how studies that instructed participants to do things like use smaller plates (to change perspective on how much food is being consumed) or strategies for coping with parties, etc. were not "diet" studies because they did more than just tell participants to diet out of sheer willpower.

So, in effect, diets, if they are defined as just telling people to lower intake, and nothing else, don't work. However, if you support them, and give them alternative behaviors or foods, or strategies for lowering intake, they do work.

What also became clear to me, and which people on this forum are maybe pointing out, is that there's also a lot of gamesmanship in these papers about individual trends and what counts as a control group. There are in fact people who do reliably lose weight on diets (you can see it in their individual-level-data graphs), but their trends are often explained away by switching controls ad hoc.

I was left very frustrated by these studies, because it wasn't clear to me why these researchers wouldn't be promoting the strategies, etc. more, or advocating more for understanding why people who lose weight on diets do lose that weight, to gain strategies. I got the impression they were trying to be sensationalistic or something, to gain attention, by trying to sound counter-intuitive.

As a side note, I've worked in bariatric clinics, and I can say from my experiences there they have their own brand of burying side effects and defining outcomes in such a way as to achieve success (for example, by only selecting "appropriate" individuals for surgery).