One of the many problems with the Western diet is that it’s lacking key micronutrients that we need to create hormones, specifically vitamin D, which is essential for testosterone production. As you read earlier, almost everyone is now deficient in vitamin D because of our overavoidance of UV light. This is likely a major reason behind the decrease in testosterone levels. A study published in 2010 looked at the vitamin D and testosterone levels of more than two thousand men over the course of a full year. The results showed that men with healthy vitamin D levels had more testosterone and lower levels of sex hormone binding globulin (SHBG) than the men who were vitamin D deficient. SHBG binds to hormones so your cells can’t use them. If you have too much of it, your testosterone levels will drop.

• E. Wehr et al., “Association of Vitamin D Status with Serum Androgen Levels in Men,” Clinical Endocrinology 73, no. 2 (August 2010): 243–48, https://doi.org/10.1111/j.1365-2265.2009.03777.x.