The GH IGF 1 axis in ageing and longevity Nature Reviews Endocrinology 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, Frontiers Growth hormone insulin like growth factor I axis in health and disease states: an update on the role of intra portal insulin NAD+ vs GHK Cu: Why Energy and Repair Are Two Completely Different Systems GHK Cu Peptide Progress: Week 3 Results