Although this effect may remain in infertile men, in men with otherwise normal T, levels return to baseline within 28 days. The sun provides a major source of vitamin D, but we just don’t spend enough time outdoors anymore. As a result, a global trend of vitamin D deficiency has emerged. For starters, it helps to maintain muscle mass, bone density, and sex drive. Testosterone production is at its highest in a man’s early adulthood and drops a little bit each year thereafter.