Warning: post contains maths!
Breast milk is 1% protein, 6.9% carbohydrate and 4.4% fat by weight, or 5.6% protein, 38.8% carbohydrate and 55.6% fat by calories. It is low in protein and high in both fat and carbohydrates... Or is it?
Due to babies huge energy requirements*, they actually get ~0.89 grams of protein per pound body weight**, but this is more than an adult needs as the infant is growing rapidly so this amount can be seen as the maximum we would need (especially considering formulas made with higher protein levels are toxic to infants). This translates as 150lb adult needing a maximum of ~134 grams of protein a day, and at 2000 calories a day, this is 27% of calories.
* An infant weighing 5kg/11lbs needs 700 calories; this is ~64 calories per pound. A 150lb adult may need 2000 calories, or ~13 calories per pound. The infant’s energy requirements are almost five times that of an adult’s on a per weight basis.
**700 calories @ 5.6% protein = 39.2 calories from protein = 9.8g protein / 11lbs = 0.89 protein/lbs.
How little protein do we need though if that's the maximum? Well many keto dieters eat as little as 60g of protein a day fine (~12% @ 2000 calories), but this is usually done in fear of slightly higher fasting blood glucose levels on a ketogenic/low-carb diet even though such a thing is perfectly natural and safe. They're eating the absolute minimum they can get away with, which I think is a poor strategy. The body has an efficient system for getting rid of excess protein, via urea and uric acid. As previously discussed uric acid is a potent antioxidant and why I think we don't get scurvy on all meat diets, so protein in excess of our pure tissue requirement can be seen as a good thing!
Phinney/Volek in The Art And Science of Low Carb Living/Performance generally recommend 1.5g/kg protein (~0.68g/lb), this is ~102g at150lbs (~20% @ 2000 calories). Other recommendations include 0.7+g/lb of lean mass (84g/150lbs@20%bf, 17% @ 2000 calories), this seems much more reasonable minimum. Basing your protein needs off lean mass rather than whole mass is much smarter, as fat tissue doesn't increase protein requirements but muscle mass does. 102g of protein is almost exactly half way between 84g and 134g, so is a good middle ground target of protein intake.
The brain is the biggest user of carbohydrates in the body; in infants, it uses 50% of the body’s total energy requirements, but by adulthood only uses up 20%. This means the adult brain uses 40% of the energy required by an infant brain: this works out to ~15% of calories from carbohydrates (38.8% x 40% = 15.5%).
BUT... the carbohydrate source is lactose, made of glucose and galactose. Now galactose is very special, it's not used as an energy fuel like glucose, it's used for myelin synthesis (that is making nerve insulation), this is why human breast milk is so high in lactose, for the galactose! So that ~15% becomes ~7% of calories coming from carbs for an adult (~38g @ 2000 calories).
BUT... we can make glucose from protein (via gluconeogenesis) and glycerol from fats. Gluconeogenesis is constant regardless of diet, as we've discussed before, but as far as I'm aware there is no cap on glycerol->glucose synthesis. Estimates are at about 10% of whole fat can be converted to glucose from the glycerol.
The actual dietary requirement for carbohydrates is zero, and even infants do very well on ketogenic diets; we can synthesis all the glucose we need (which is less while in ketosis) from GNG and glycerol. The main reason for muscle wastage in starvation is it's being used for GNG (which is constant even in starvation), this supplies the glucose requirement easily and blood glucose levels remain steady right up until you die.
The rest of the body’s calories come from fat, which in an adult eating ~27% protein and ~7% carbohydrates is ~66%, or ~147 grams of fat on a 2000-calorie diet. This would be the minimum fat, as this is the maximum protein and carbohydrates we need. Taking in our minimum 84g protein and no carbs would mean we need to eat 83% of our calories as fat or ~184 grams of fat on a 2000-calorie diet.
So understanding the macronutrient ratios of breast milk in context of the body it is designed for gives up a good idea of what ratios are healthy for an adult human. This works out to ~17-27% protein, ~0-7% carbohydrates, ~66-83% fat, or 84-137g protein, 0-38g carbs, and 147-184g fat on a 2000-calorie diet. Such a diet would almost always be ketogenic (very metabolically damaged people may not handle even 38g carbs).
As calorie expenditure increases, say due to increased exercise, the extra calorie should come from fat, little extra protein or carbs are needed. During very heavy exercise some non-insulin-stimulating carb source may be used to aid recovery such as super starch, palatinose, or possibly pure fructose in order to refill glycogen stores (as discussed previously our ability to refill glycogen is limited by GNG and is less than that of other carnivores).