This protein makes up 80 percent of the skin, and its fibers give skin its firmness and strength. Collagen naturally breaks down over time, but certain ingredients, such as retinol and peptides (including Matrixyl), can stimulate new collagen production. The most abundant protein in the human body, it makes skin thick, strong, and smooth. Laser treatments and retinoids build it up; UV rays and free radicals tear it down.