This study found that compared to healthy young adults, older humans have severely elevated oxidative stress, glutathione deficiency, impaired mitochondrial function, increased inflammation, insulin resistance and endothelial dysfunction, and lower muscle strength and mental cognition. We tested and found that supplementing GlyNAC (combination of glycine and N-acetylcysteine) improved all these defects, and that stopping GlyNAC resulted in a loss of benefits. The results of this trial suggests that GlyNAC supplementation could be a simple, safe and effective nutritional strategy to boost cellular defenses to protect against oxidative stress, correct mitochondrial defects to improve energy availability, increase muscle strength and cognition, and thereby promote healthy aging in humans.