[rypskar]

ok, so a protein source, or a source of protein, would cover it. Might be me getting old, but the term "a protein" rubs me the wrong way

[amanaplanacanal]

It's just another example of an English word having multiple meanings. The listener (or reader) has to divine which one was intended by context. Much like the word "organic".

Protein is used this way quite a bit when talking about diet.