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.)