The researchers studied how not having enough zinc in the diet affects the testicles and testosterone levels in rats. They divided the rats into different groups and fed them diets with varying levels of zinc for 8 weeks. They found that low zinc levels led to decreased zinc in the blood and testicles, slow testicle growth, and lower testosterone levels. However, when zinc was added back to the diet, the testosterone levels returned to normal. This shows that zinc deficiency can impact testicle development and testosterone production in rats.