[dualogy]

> Fat and sugar (a carbohydrate) account for two of the three most common macronutrients, the third being protein. Eating a diet that's low in both fat and carbs would imply a mostly-protein diet which can lead to illness

Spot-on observation. Furthermore, while proteins can be metabolized for energy, that's not advisable as a major energy source, proteins should be spared for day-to-day repairs (most-all multicellular structures ---incl tissues/bones/organs/tendons, the brain, not just simply "the muscles"--- are constantly tearing-down and rebuilding and we also synthesize much other crucial stuff partially from amino acids, red/white blood cells, enzymes, the list goes on, even glucose if&as necessary) and burning them instead not just removes them as building materials but also creates various additional stresses on the body, produces extra detrimental waste products to be run through the kidneys and get excreted, etc.

All mammals excel at beta-oxidation of both carbohydrates and fatty acids for energy, I know of no animal thriving off the meagre amount of energy their bodies may safely obtain from proteins. Many humans don't seem do particularly well (in the long run) combining fats & carbs in ~equally high amounts and find better results emphasizing one and sharply reducing the other. (Personally I'm a fat advocate but no need to get into that discussion ;)

Consider 2000kcals from protein-only (not that your body will allow this) would require 500g of protein, so about ~2kg of super-lean meat or over 100 egg-whites.. and that's not yet counting any protein usage as building-blocks!