Regularly consuming a high protein, low carb diet might greatly impact the testosterone levels of men. In a recent study published in the journal Nutrition and Health, researchers collected data from over 300 healthy males in their late twenties. They studied levels of cortisol, a stress hormone, and testosterone, a sex hormone involved in sperm production and muscle strength. They found that a high protein diet had little effect on cortisol levels, which gradually returned to normal. But they found a 37 percent average decrease in testosterone levels for those on high-protein, low-carbohydrate diets. Most nutrition experts recommend keeping protein levels in your diet to less than 25 percent of calories.