In young men undergoing 12 weeks of muscle-building resistance training, taking collagen peptide supplements leads to a fivefold increase in the production of specific muscle proteins involved in contractile function, such as myosin and Z-disk components, compared to those taking a placebo.

When young guys who already work out do a lot of high-volume weight training for 6 weeks, their muscles grow mostly because of fluid and non-contracting parts inside the muscle cells, not because they’re building more of the actual muscle-building proteins — and this change sticks around for over a week after they stop training.

In young men who exercise recreationally, taking 15 grams of collagen peptides daily along with a 12-week strength training program results in greater gains in muscle mass and more changes in muscle proteins than strength training alone with a placebo.

Taking collagen peptides while doing strength training leads to specific changes in muscle cells that support muscle function and repair, whereas taking a placebo during the same training only triggers general metabolic changes.

Even if you do a lot of heavy weight training for 6 weeks, your muscle fibers don’t actually get more of the main building proteins—so the idea that your muscles grow by diluting those proteins with extra fluid probably isn’t right.

Taking collagen peptides while doing strength training does not lead to greater increases in maximum strength in young men compared to strength training alone, even though it may increase muscle mass and protein markers.