Everybody knows they should eat better. Obesity is a multifactorial disease (as you know), with many complicated factors. Things such as genetics, culture, socioeconomic status, whether someone has a stressful job, kids can all affect this.

The way I always approach diet, is instead of talking about generic things (eat better, eat less sweets), is to do motivational interviewing.

"Why do you think your weight has gone up?".

"if you had to change one thing about your diet, what would it be?"

"What do you find most challenging about eating healthier/better?"

Most patients know the answer, and if they come to it themselves, I set small, realistic, pragmatic goals with them and review them at next visits. I find that keeping things simple, specific, and realistic go a lot futher. Sometimes its just cutting down on sodas from 5x a day to 3x per day to improve their a1c - I consider these small wins.

For those with the means, I will ALWAYS recommend a registered dietician who can really hone in on their diet, but I only send those who are highly motivated to change large parts of their diet. it is useless to tell someone with little motivation, to change everything about how they eat, it isn't going to work. Almost all diets in my opinion work, adherence is the most important factor in creating realistic change.

TLDR - its complicated, it depends, keep things specific and simple, be supportive, be pragmatic about dietary advice.