First got onto this topic via “amaranth wafers”, which are popular in Mexico and, increasingly, the US. You could think of them as similar to those puffed rice crackers, I guess, but they are instead made of amaranth. So what is amaranth? well…
Amaranth is a “pseudocereal”, like quinoa or buckwheat. It has been cultivated in Mesoamerica for its edible, starchy seeds for at least 8,000 years. These seeds can be processed and eaten in similar ways as other types of grains.
Amaranth grain was a staple food of the Aztecs, but banned by the conquistadores when they invaded and colonised Mexico. In modern-day Mexico, it is still used to make a type of sweet called alegría. The seeds are also presented as offerings on Day of the Dead. The candy skulls associated with that festival also used to be made from amaranth, but are now made out of sugar. Amaranth grain can also be popped and eaten like popcorn, or as a breakfast cereal with milk and stuff.
As for amaranth wafers, they’re eaten as a kind of “health snack food” because in the US they’re marketed as one calorie per wafer. Someone on Reddit sent them off for testing and found they are actually 3.76 calories per gram (very close to the figure for straight carbohydrates).
There is also a food dye called “amaranth” that has been banned in the US since 1976 due to toxicity. It has the red colour of amaranth but is not actually related to or derived from amaranth grain in any way.