There are several essential amino acids we need to survive but don't produce in our bodies. No plant has all of those amino acids, but every animal product does. You can get complete plant protein by consuming multiple different plants, so if a vegan has a varied diet they're usually fine.

[–] mojorizer@feddit.de 14 points 1 year ago

There are a few plants with a complete amino acid profile: quinoa, soy, amaranth or buckwheat. But as you said, a varied diet is better.

[–] AbeilleVegane@beehaw.org 3 points 1 year ago

Also, animal meat maybe, but not every animal product contains a complete protein profile.