GHK Cu vs AHK Cu: Which Copper Peptide Is More Effective? By age 30, your skin produces about 25% less collagen. By 40, that number can reach 40%. This decline affects more than just how your skin looks. It changes how it heals, GHK Cu is a naturally occurring tripeptide in human plasma. Levels average 200 ng ml at age 20 and drop to 80 ng ml by age 60 (Pickart et al., 2020). That decline tracks with The Human Tripeptide GHKCu in Prevention of Oxidative Stress and Degenerative Conditions of Aging: Implications for Cognitive Health Pickart 2012 Oxidative Medicine and Cellular Longevity Wiley Online Library