Wikipedia: Amaranth and the Aztecs

Amaranth has a similar nutritional profile to grain (the Aztecs got up to 80% of the calories from it prior to the Spanish conquest), and it is even today termed a "pseudograin" because it can be ground into flour like wheat or other seed grains, which biologically are grasses. Even today, Amaranth is used to replace wheat flour in gluten-intolerant patients (e.g., celiac disease) or to increase the nutritional content of standard whole-wheat flour.