I gave this some thought over the last 2 days. Two ideas came to mind:
(1) Perhaps your uncle, like so many others, diligently followed the PC weightloss methods (low-cal, low-fat, aerobics, etc) and never succeeded. Then (again, like so many others), he tried the "eeeeeeevil" Atkins diet, the one that all the "right" people abominate, and voila, it worked! Yet, when he's finally found something that works, everyone turns against him. That, coupled with the feeling that the government, the medical profession, the health food/fitness advocates, and just about everyone else has been lying to him for 30 years, can make a man just a bit "moody", "fanatical", and "irritable". The fact that he has someone in the family with a "Master in Dietetics" who is just as biochemically wrong as everyone else, only added fuel to the fire.
Have you tried just laying off of the Atkins subject and congratulating him on his weight loss? He just might be a little less " "moody", "fanatical", and "irritable" if everyone would quit persecuting him.
But perhaps I'm projecting, as that is sort of how I felt. Having a Ph.D. in chemistry, however, I was better able to fend off people's quasi-scientific attacks but I still faced a lot of social scorn and a lot of people who just kept on preaching my doom and gloom to me no matter how many times I gently, patiently explained to them that they (and the government, and a million doctors, etc) were simply biochemically wrong. All that nagging gets really old, sometimes. At one point I got so sick of explaining myself (futilely) that I did lash out with something to the effect of, "I DON'T CARE IF IT SHORTENS MY LIFE, I'D RATHER HAVE A LEAN, ACTIVE, HAPPY AND SHORT LIFE THAN A LONG MISERABLE FAT ONE!!!"
(2) The second thought I had, was "serotonin". For some people the adjustment to high-fat/low carb may be difficult NOT because of any fuel shortage in the brain, but because low carb means lower serotonin levels, at least temporarily. If that's the case, and it doesn't resolve, then an alternative to Atkins might be Dr. Mauro DiPasquale's "Anabolic Diet" (no, there are no drugs involved, it's a diet originally designed for bodybuilders.)
The basic difference is, you're on essentially an Atkins induction phase for 5-6 days, then on free carb intake for 1-2. For me, this allowed me to eat freely on the weekends, for dating, etc, and for your uncle it might just give him the serotonin boost he needs to tolerate low-carbing the rest of the week. (I lost a lot of weight on that, with no increase in running mileage. And running got easier,even before the weight started to drop.)
Te wierd thing is that he wasn't fat at all. He was in great shape. He just got on Atkins to help his fat wife (reeeeealy fat).
I know it's an unhealthy and innatural thing to do to yourself, but I have not given him a hard time about it. Not at all. Really, nobody does. The guy was, as far back as I cqn remember, a stable and decent guy. Now, he is such an unhappy, wierd and unstable guy. Bitchy and moody. Fanatical about his Atkins god, which he didn't even need.
I cannot explain it, but I know that low carb regimens affect the brain like that. Now I avoid him, which is easy, because he avoids all us foolish, carless, carb-eating-devils.