Humans have large brains made up almost entirely of fat (cholesterol and fatty acids). This big brain requires lots of energy to run properly.

Humans (unlike cows, or gorillas) do not have the digestive system to synthesize saturated fat from plant material. Our lives literally depend on eating fish/meat to fuel our brains.

That is why vegans start out great, having a wonderful time, while their body feeds off it's own reserves (starvation). I've done numerous fasts and know the great feeling. Eventually though the interal reserves are depleted, and the vegan begins to fall apart. Some resort to drinking their own urine. Their teeth rot and often fall out. Mental capacity fades, and they come very angry and irritable, often violent (because they are literally starving).