When oxygen is missing, or when pyruvate is temporarily generated more rapidly than it may be consumed because of the citric acid cycle (as in extreme muscular exertion), pyruvate is converted to lactate because of the enzyme lactate dehydrogenase, a method that also oxidizes NADH again to NAD+ for re-use https://stressreduction90111.blogkoo.com/getting-my-renew-health-supplement-to-work-48480768