Answer by Steven Fowkes:
The simple answer is that you would lose weight and get healthier (better energy regulation, better redox regulation, lowered insulin resistance, etc.). But this assumes that your protein intake is not excessive, which might be a big assumption based on the language of your question. And the “negligible” carbohydrate could be drastic enough to impair your gluconeogenesis and divert amino acids into the manufacture of glucose.
During beta-oxidation and ketogeneis, fat is burned for energy and glucose is synthesized; the opposite of the state in which carbs are plentiful (glucose if burned and fat is synthesized). Fat burning can become limited by your body’s fat-composition set point (you become too lean from your body’s homeostasis perspectives). And gluconeogenesis can be limited by a lack of substrate; the glycerine from triglyceride catabolism is way to little and amino acids are diverted into making glucose. You might be able to monitor this in terms of lean body mass (muscle and bone), but to most people, this is difficult to assess by any symptomatic clues.
When a high protein diet is catabolized for energy, or for elimination of excess amino acids, organic acids are produced. This poses a detox burden to your kidney regarding the nitrogen load that needs to be excreted, but the organic acids chelate minerals as they are excreted. You can lose calcium, magnesium and other trace- and micro-minerals, and you can lose bone if the mineral reserves fall low and your body demineralizes bone to correct falling electrolyte levels.
So if you had asked, “What would happen if I eat a high healthy-fat diet with moderate protein and low carbs?” the answer is more clearly good things and not bad things. This is the Ezrin diet (after Calvin Ezrin, the endocrinologist). It is a slight modification of the Atkins diet, but a bit higher in fat and a bit lower in protein. The degree of carb restriction, if tied to the generation of ketones in the blood or urine, tends to be self-correcting. If you keep carbs from being too low, and periodically eat carbs to “break” ketosis, the body thrives on the metabolic exercise of switching from burning carbs to fat and back again.
I suspect that this is a birthright of our heritage. We are historically adapted to a variable diet involving episodic feasting and fasting, and seasonal feasting (in fall) and fasting (during winter). But long-term starvation is over the line regarding survival. In such situations, you should be incentivized to relocate to a more conducive environment.