Of course, it would be best if we all ate diets that provided an optimal vitamin and mineral balance every day from the foods we eat, but that's not always easy or possible these days. A supplement is particularly important for vegetarians or those on other limited diets because, unless one is very careful, vitamin and mineral deficiencies can result.
Science has not found any particular diet that reliably improves testosterone or fertility. Everything points to the general idea that if a man eats for whole-body health, he'll be eating for his sexual health as well. That means the following:
That may sound overly simple, but you don't need complicated regimens, fancy diets, or other faddish-ideas such as eating a low-carb diet, or a low-protein diet, or a low-fat diet. Most people instinctively know how to eat well--the problem is avoiding the temptations produced by our in-born cravings for fat and sweets, cravings that served our species very well ten-thousand years ago, but which now cause us grief in our current environment.