There’s a limit here, because as we keep reducing calories, the body keeps reducing the metabolic rate, requiring a further reduction in caloric intake, which results in a further lowering of the metabolic rate, and so forth. Various studies have shown that this is the reason weight loss is never as great as the supposed calorie deficit would have us expect. Rather than intentionally restrict calories, it’s better to eat to satiety and let the appetite hormones regulate our intake. Kevin Hall’s follow-up study of Biggest Loser contestants showed that, even several years on, their extreme calorie reduction had resulted in an apparently permant 500-calorie decrease in their metabolic rate.
The government’s minimum daily recommendation is 0.8 g of protein per kg of lean body mass. This is because the definitive study measured an average daily nitrogen loss equivalent to 0.6 g of protein/kg of LBM. Though if you look at the data plot, participants’ nitrogen loss was all over the place, so some people can get away with less protein, while others need more.
Our recommendation for a ketogenic diet is 20 g/day max of carbohydrate, 1.0-1.5 g protein/kg of lean body mass/day, and enough fat to reach satiety. Some experts recommend going as high as 2.0 g/kg, especially if one is trying to add muscle mass.
I don’t know why you are counting protein as metabolic fuel. There is a high cost of metabolising amino acids, because they have to be deaminated, then converted to either glucose or a fatty acid (depending on whether they belong to the glucogenic or the lipogenic type), and then metabolised. Glucose and fatty acids both yield far more net ATP per gram than protein does, because of the extra energy cost of proteolysis. Excess amino acids beyond the capacity of the labile pool cannot be stored, and must therefore be disposed of.
This is true if (a) protein intake is sufficient to replenish proteins lost to autophagy and amino acids lost to deamination (the body needs the nitrogen elsewhere, and the liver needs substrate for gluconeogenesis), and (b) so long as there is abundant caloric intake, to encourage the body to part with its excess fat. In famine/starvation mode, the body actually hangs on to its fat store until quite late in the process, and will metabolise protein for a while before starting in on the fat. If you look at photographs of the survivors of the Nazi concentration camps, you can tell that they were in the late stages of starving to death, because they had lost most of their body fat. By contrast, many forum members have reported that their fat loss did not commence until they increased their caloric intake sufficiently. By comparison with starvation, this is a much more comfortable way to lose fat.
Actually, at some point, ammonia toxicity becomes a problem. I believe Richard Morris calculated that protein intake of 3.1 g/kg is the point where the uric acid cycle starts to get overwhelmed. If people tell you that you smell of ammonia, then you have exceeded the capacity of your uric acid cycle and should reduce protein intake.
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I almost never lost any weight on keto… Only a very little and it came back anyway. But I know the reason, I eat way too much. Our numbers are very similar except I am younger and eat WAY more than you do. I can’t always just stop at 2000 kcal, it’s so little food, a decent but not big meal to me… But good timing and food choices help. If I just can stick to them but I trained and changed enough so I think 2022 will be my year!