This is a book-marking post for thoughts I had in https://www.facebook.com/TheFatEmperor/posts/1442430506020812.
"The human body does not need carbohydrates from an external food source, because it is capable of very precisely and correctly assembling its own amounts of glucose that is needed in very small amounts for auxiliary and specialized functions." - Igor Butorski.
1) It's not precise. See How eating sugar & starch can lower your insulin needs.
2) It's not enough to fuel sustained medium-intensity exercise for everybody EDIT: if there's insufficient protein intake. See "Funny turns": What they aren't and what they might be and Everyone is Different.
From Blood Sugar is Stable:-
After liver glycogen has been depleted in starvation or on Nutritional Ketosis (Ketogenic Diets with less than 14% of total energy from Protein), total glucose production from liver & kidneys is ~100g/day.
From It's all in a day's work (as measured in Joules), the body oxidises carbohydrate at a rate exceeding ~4g/hour at exercise intensities exceeding ~25%, on a LCHF diet. This is unsustainable.
EDIT: If protein is consumed, total glucose production increases, up to a maximum of ~400g/day.
3) It's wasteful. Glucose production from protein converts ~50% of the most expensive macronutrient (protein) into the cheapest macronutrient (carbohydrate). It creates expensive urine, as the nitrogen part of amino acids is detoxified by being converted into urea by the liver and then wee'ed out by the kidneys.
4) Using the above argument, the human body does not need saturated fats & monounsaturated fats from an external food source, because it is capable of very precisely and correctly assembling its own amounts of saturated fats & monounsaturated fats (out of carbohydrate) that are needed in very small amounts for auxiliary and specialized functions.
If we consume only Essential Fatty Acids, Essential Amino Acids, Vitamins, Minerals, Fibre/Fiber, Water & Anutrients, there won't be much to eat. Also, there won't be a source of chemical energy to generate heat energy & mechanical energy. That's what dietary carbohydrates & fats are for!
Respiratory Exchange Ratio/Respiratory Quotient (RER/RQ) varies with carbohydrate & fat intake, as the body preferentially oxidises the fuel that's most readily available, when it's working properly. If it's not working properly, due to Insulin Resistance (IR), fix the IR rather than kludge the diet (by eating LCHF) to compensate for it. See Insulin Resistance: Solutions to problems for how to do this.
RER/RQ varies with Exercise Intensity.
Low-intensity exercise results in mostly fats being oxidised.
High-intensity exercise results in mostly carbohydrates being oxidised.
Medium-intensity exercise results in a mixture of fats & carbohydrates being oxidised.