Collagen is a protein that is responsible for skin elasticity, stretchiness, and healthy joints. It makes up three quarters of your skin and a third of the protein in your body, and it's found in your bones, muscles, and blood. However, your body begins to lose collagen in your 20s. By the age of 40, you'll have lost roughly 1% of your body's collagen per year, and menopause accelerates that loss, resulting in wrinkles, stiff joints, worn-down cartilage, and diminished muscle mass.