But an analogy to cars is extremely useful. The need for fuel and water dwarfs the need for new oil and new parts. For humans, the fuel is carbohydrates. The oil is fat. The parts are amino acids.
Amino acids are the building-blocks of protein.
Different combinations of amino acids form the basis of blood cells, brain cells, hair cells, skin cells, and so on. Many amino acids we can make ourselves, some we need to get from food. The 9 or 10 amino acids we need from our diet are found abundantly in mother's milk, starches, vegetables, and leaves. They want us to think amino acids have to be combined to imitate the proportions of amino acids found in meat and eggs. They want us to think this is a terribly complicated business.