There's a Mark Twain tale, by way of Hal Holbrook, that his doctor told him he had to cut down on smoking and eating and drinking and swearing to improve the state of his health. Mark Twain said he couldn't cut down on them.

The doctor said "why not"
"Because I lack the will power. I can cut them out entirely, but I can't cut down on them!"

the doctor told him to cut down on them entirely. Once he felt better, Mark Twain started them up again.

I had a similar experience on the low calorie, very low carb, low fat diet. As long as I rigidly adhered, I lost weight like I was on an elevator. There were protein and vitamin supplements and I had to drink a great deal of water. It was tough going through the supermarket. All I was there for was some meat and cruciform vegetables. Oh, and as much lettuce and cabbage as I wanted. I was in ketosis, but once in, my mind functioned well and I don't recall any special breath problems. I do recall having no spare energy, none. I would make myself get to my floor via stairs, but I didn't do so with brisk enjoyment. Either of those. On the other hand, I found myself fitting into sizes of clothes I hadn't been into in many years.