https://www.nature.com/articles/ijo201094

>More than one out of every six US adults who has ever been overweight or obese has accomplished LTWLM [1y] of at least 10%.

review: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4777230/

As far as the doctor's advice is concerned, focusing on weight loss as the sole outcome is misguided. The typical trajectory of an obese individual is to keep gaining weight; if doctors could even make their patients stop gaining weight, that would already be a major development in standards of care.

It's also important to consider the limitations of the metric. Studies don't evaluate sustained cultural shifts like "do doctors regularly talk to the patients about nutrition", because it's hard enough to track a patient for five years, let alone get a doctor to do blinded controlled interventions for five years. So what the study tests is a controlled diet for a short period followed by a period of unsupervised maintenance (with a recipe book), not the consistent availability of nutrition guidance over a period of years.

It's perhaps more important to note that while the results in adults are quite bad, it has been shown possible for adolescents to drop by more than a standard deviation in BMI after an intervention and maintain this for two years:

https://www.sciencedirect.com/science/article/pii/S002234760...

https://www.sciencedirect.com/science/article/pii/S1054139X1...

Professional interventions for weight loss and/or maintenance aren't infeasible, they're just hard, and it pays to start early. It's not too surprising that older people are more reluctant to change their habits.

[fuller2014]: Finds that Weight Watchers underperforms standard national guidelines: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4282338/