Tuna, just like sardines and other similar fish-products is rich in Vitamin D. Vitamin D is one of the essential precursors to testosterone production - but due to the way most people in developed countries spend their days (usually at home or in office spaces), most people are Vitamin D-deficient. This is because apart from consuming it from food, Vitamin D is usually naturally secreted by the body once it is exposed to direct sunlight. As another negative aspect - sunscreens seem to decrease the amount of Vitamin D.
