Collagen is a protein made up of lysine and proline. Lysine is an essential amino acid, but proline is not. Essential amino acids are amino acids that the body cannot adequately synthesize, whether because it cannot synthesize it at all, or if it cannot synthesize enough for proper body function. (There is a subcategory of amino acids referred to as “conditionally essential”, meaning that the body might normally be able to synthesize enough, but there are situations where the body cannot keep up.)
The essential amino acids (EAAs) are: histidine, lysine, leucine, isoleucine, threonine, valine, methionine, tryptophan, and phenylalanine
Because collagen only contains one essential amino acid, it is an incomplete protein. Complete proteins contain at least a little bit of all essential aminos, whereas incomplete proteins are missing one or more.
The primary issue that I, and many other dietitians, have with collagen supplementation is that there’s a large amount of people who - through no fault of their own - don’t realize that although supplemental collagen in the diet provides protein, because it is not a complete protein (far from it, considering it’s missing 8 of the 9 EAAs), it should not be considered or become a major source of dietary protein. A person attempting to reach recommended protein intakes solely (or predominantly) through collagen supplementation will still develop protein-calorie malnutrition.
EAAs are essential and you need them in your diet. However as long as you eat more than one thing and eat enough food you typically will always hit 100%.
Collagen is non-essential and there is (as far as I know) a lack of evidence showing that dietary collagen is helpful. There are studies based on supplementation but they are also typically funded by companies with a vested interest in them (i.e., supplement companies).
“Essential” means that your body cannot produce it. That does not mean the “essential” amino acids are better, or that you don’t need to eat the non-essential ones. You can produce your own glycine, but not necessarily to the ideal amount.
Adult animals tend to live longer when fed diets with more glycine, less tryptophan, and less methionine, relative to standard diets. Compared to “complete” protein, collagen has more glycine, less tryptophan, and less methionine.