As for diet and mental health; I’m a psychologist and I can tell you that your argument (in this context) is complete hogwash. While I agree that diet likely plays a role in mental health (to some degree), the research is limited and inconsistent. Most psychiatric disorders are not caused by diet alone or even likely mostly by diet. That’s not to say a diet high in sugar, food additives, and other toxic things won’t exacerbate certain conditions. Rather, at best, diet is only one of a multitude of factors in the vast majority of disorders.
On the other hand, weight loss, or more specifically bodyfat loss, can prove to be more difficult. Occasionally people will present with metabolisms that are in the dumps, eating disorders, or symptoms of overtraining that will preclude them from reaching their body composition goals until they take the time to tease out any issue that is preventing them from creating a sustainable caloric deficit. Metabolic Damage is a term being used these days and, in general, the definition of this admittedly vague term is squishy. Many people argue about this term on the internet, which can be interesting and engaging at times, but to be fair it is not well characterized. Currently, metabolic damage suggests that there is a smaller-than-predicted alteration in the amount of calories someone burns in a day given a dietary change without a change in other variables like emotional stress or training. Take, for instance, a female who has dieted for 16 weeks or so to get on stage and compete in a figure competition. She has been in a caloric deficit for some time and as an adaptive process, the body slows down many calorie-burning processes to preserve homeostasis. Now, we would expect that after the show her metabolism would increase back to previous levels if she returned back to her baseline levels of caloric intake and training (pre competition prep). Unfortunately, there are many eating disorders, psychological issues, and other emotional problems that are covered up by “figure prep” and/or “constant dieting”. Many times there is a subsequent period of overeating or binging (significantly above pre competition diet intake) that results in an abnormal adaptive response. Occasionally, people will store fat rapidly, gain more than predicted weight, and have lower than predicted metabolic rates, ultimately leading to a person being unhappy with their physique and an immediate reentry back into a calorie restricted state. Sometimes the lower than predicted metabolic rate produced from these episodes can stymie weight loss at caloric levels that previously produced weight loss. In other words, a person now has to calorie restrict more than previous because they have lowered their metabolism from where it was in before the contest prep began because they did not allow themselves the time required to get back to their baseline or above it.