Proteins are chains of amino acids joined end to end. There are 20 standard amino acids used to build proteins. Nine of these are essential — the body cannot make them, so they must come from food. The remaining 11 are non-essential because the body can synthesise them, though conditionally essential ones become dietary requirements during illness or rapid growth.
Animal foods (meat, fish, eggs, dairy) provide all nine essential amino acids in proportions the body uses well. Most plant foods are lower in one or more, but combining varied plant proteins across the day — legumes, wholegrains, nuts, seeds — meets requirements for most healthy adults.
Adequate protein intake supports muscle maintenance, immune function, hormone production, and tissue repair.