There is no best diet. Eat real food, mostly plants, less of it is good advice from Michael Pollan. Avoiding highly processed food helps. It is best to eat for good health, exercise and the weight will follow.
Whether or not labeling makes a difference to make people aware of dangers to health there is a long running battle on the subject between big business and public health. This goes from tobacco to food.
That is a question difficult to answer in the context of our medical and pharmaceutical oriented culture. The answer seems to be about half the time for mild to moderate depression and then for a short initial period of treatment. The more serious issue is that physicians and patients are willing to take some medications for long periods of time (years) without consideration for consequential side effects.
The best diet for health and weight is to eat whole foods and avoid the fads.
Exercise keeps us healthier and younger.
The evidence indicate some foods might do that.
How to eat less sugar.
Eat good food, mostly plants, avoid highly processed junk and get moving.
Add mindfulness to your program for weight control.
Places in the world with longer life.