Lysine
Why you need it: Lysine, also known as l-lysine, is an amino acid that helps the body absorb calcium and form collagen for bones and connective tissue. It also plays a role in the production of carnitine, a nutrient that helps regulate cholesterol levels.
Where to get it: Protein-rich animal foods, especially red meat, are good sources of lysine, as are nuts, legumes, and soybeans.