Name one credible case of a low-protein bodybuilding competitor. If they say they are, I would just chalk it up as another story to shock people or get people talking. That excerpt from Justin's book is complete nonsense. First, applying the laws of thermodynamics to dieting as if the body were a closed system is just beyond silly. Does that mean the laws of thermodynamics don't apply to the body? Of course not. But good luck figuring out what those numbers are.
Your body handles all macros and micros very, very differently. Fiber isn't digested at all and I assure you it has calories. Anything that isn't digested fully in the digestive tract goes out the the back and if did absorb properly into the blood but can't be used fast enough it passes in the urine (e.g. sugars, proteins, albumin, ammonia, chlorophyll, you name it).
Protein may or may not be digested depending on extremely variable absorption rates from individual to individual and under wildly varying conditions. Saying things like, "He eats a pound of protein per day but he doesn't gain a pound of muscle per day," is completely lacking a full overview of the complexities of body maintenance, cell repair, food burned as energy, even the heat that we radiate, etc. Your body can make pretty much every thing it needs except a few EAAs, fatty acids, and trace vitamins and minerals.
We can convert protein to carbs, fats to carbs, carbs to fats, build amino acids all based on chemical signaling and genetics by the highly complex systems of the body depending on what your body needs at that moment. You lose calories in so many ways in the form of work and heat, cell repair, energy storage (e.g. fat, glycogen, etc). Plus your body can add calories to your body by itself by cannibalizing fat and muscle.
A high protein diet is just simply necessary to what we do. Yes, it's overkill. But so is being 300lbs of muscle. What do the studies show? Pretty much anything they want them to. Because truth be told, they're not even sure how all of this works and what keeps us in homeostasis in terms of general bodyweight. There's no way to quantify that precisely.
Below is a complete list of what we MUST take in. Less than half of the twenty amino acids. Our bodies can make alanine, asparagine, arginine, aspartic acid, glutamic acid, cysteine, glutamine, proline, glycine, serine, and tyrosine as needed.
Essential Fluids:
Water
Essential Amino Acids:
Isoleucine
Histadine
Leucine
Lysine
Methionine
Phenylalanine
Tryptophan
Theonine
Valine
Essential Fatty Acids:
Alpha-linolenic acid
Linoleic acid
Essential Vitamins:
Vitamin A
Vitamin B1, B2, B3, B5, B6, B7, B9, B12
Vitamin C
Vitamin E
Vitamin K
Choline
Essential Minerals:
Calcium
Chloride
Chromium
Copper
Iodine
Iron
Magnesium
Manganese
Molybdenum
Phosphorus
Potassium
Selenium
Sodium
Zinc