One of my cousins was born with type 1 diabetes. That's the kind which reflects an innate deficiency in ones body's ability to regulate insulin rather than the type which is brought on by lifestyle. In terms of current thinking about health, it is the kind which one for which a person cannot be "blamed".
I didn't know this particular cousin especially well, but I did see him occasionally. He was about 5 years older than me and was always thin. He also seemed to have some emotional or mental difficulties. Though he wasn't in any sort of special education classes, he was not particularly bright and had some temper issues. He was capable of doing certain types of work and eventually married and had children.
Despite having grown up with his illness, my cousin was never capable of regulating his diet. In particular, he consumed candy bars and other sweets such that his diabetes was not under control. Though he never gained weight or got fat, his body was constantly being damaged by his diabetes as a result of a poor diet. In the end, he died young as a result of his inability to control his eating. He wasn't fat, but he literally ate himself to death.
There is no doubt in my mind that my cousin experienced a certain biological pressure to consume sweets. However, my uncle (not the father of this particular cousin) also was born with type 1 diabetes which he has kept under control for his entire life through medication and a controlled diet. The main difference between the two was emotional. One could not control his relationship with food and the other could.
The question that I'm throwing out there for myself after pondering their respective situations is whether or not the inability to regulate your diet when you have a life-threatening health issue is a question of mental health. Very often people approach diet as if there were a single "best" way to eat which would result in all people experiencing optimal health. Bodies don't work like that. There are people who can eat a candy bar (or two) everyday and experience no ill effects. There are those who can eat bread all day and those who can't tolerate a single slice. All processing of food by a particular body is individual with varying consequences.
This fact is one I realized long before I thought about my cousin's situation. Some time ago, I finally internalized the fact that there are people out there who can gorge on junk food and not get fat while I am likely going to have to be very careful about what I eat to simply avoid being morbidly obese. Some people will remain or get fat on 2000 calories. Some will stay thin on over 3000. That's the way it is and railing against it and the unfairness of it doesn't change the biological reality. Just as my cousin's reasonable desire to indulge in sweets that he likely saw his sister and others consuming resulted in his death, my diet, "healthy" or "reasonable" though it might be in some eyes, can result in a physical state which will make me miserable.
Getting back to the main point, however, if my cousin could not adhere to a diet which was stricter than most, was it a matter of mental health? Is the inability to control your behavior above and beyond what "average" folks are required to do because of your particular special circumstances an issue of psychology? My answer would be, "yes." Psychology isn't a matter of averages anymore than biology is. Maximum quality of life cannot be achieved by applying a template of actions or thinking patterns according to what everyone believes is best, but rather what is best for the individual. My cousin could not do what was best for him, even though similar actions would not have been destructive to someone lacking his health issues. If there had been behavioral intervention to help him master his destructive urges, he may have lived a lot longer.
There is a lot of debate about whether or not obesity is the result of a mental health problem and there are vociferous folks who rail mightily against the idea. However, I think context is very important. If someone washes their hands 30 times a day, do they have a disorder? They do, unless that person is a doctor who sees a lot of different people and washes his hands between patients and for regular routine hygiene. If someone becomes so fat that their mobility, joint health, or blood sugar are affected and their quality of life compromised by their weight and they cannot alter their habits to affect a positive change, is it a mental health issue? I say it is. If you can't make behavioral changes in your life to be happier, then it is a matter of psychology and it doesn't matter that only you need to make those changes. It's about mastering your own life, not living as an "average" person. That goes for everything, not just weight and food. If you can't control your temper, concentrate long enough to study, etc. and these conditions are particular to you, they are issues of mental health.
Note that I am in no way saying that these are psychological issues which must be dealt with. No one is obliged to deal with their particular problems at all. Just as I do not believe physical health is an obligation, I do not believe mental health is one either. People have the right to live as they please, even to their own detriment. However, I increasingly believe that thinking about our lives and choices in a relative fashion (i.e., "everyone does it and I should be able to as well") is a big part of what erodes quality of life for people. We often say fatalistically that "life isn't fair", but we don't internalize that reality and consider it when making choices in our lives. We still compare ourselves to others and feel that similar choices should yield similar results, and more so when it comes to health than many other areas. I think it's time to develop more sophisticated thinking in this regard and work out that "life isn't fair" isn't a pessimistic conclusion, but the starting point for understanding how to make the best choices for each of us as individuals.