Vegan diets with a low fat and high fiber intake increase chance of lower testosterone levels according to this study:
''The concentrations of serum total and free testosterone were studied in 30 healthy, middle-aged men during a dietary intervention program. When men were transferred from their customary diet to an experimental diet, which contained less fat with a higher polyunsaturated/saturated ratio (P/S ratio) and more fibre, there was a significant decrease in serum total testosterone concentrations''
STUDY: Decrease of serum total and free testosterone during low-fat high-fibre diet