While a high-protein diet can cause a very mild decrease in testosterone in sedentary people, it causes an increase in IGF-1, which also stimulates muscle growth. However, for active people, a high-protein diet, combined with strength training, can increase testosterone levels. In a study published last week in the medical journal Nutrition and Health, the results of 27 different trials were analyzed to see how low-carb and high-carb diets affected men's long-term health. Researchers from the University of Worcester examined a total of 309 patients, all healthy men around 27 years old, who consumed varying degrees of protein in their diets.
The study examined the two short-term (three weeks) and long-term diets, or more than three weeks, and their impact on the subjects' resting cortisol and testosterone levels. The study itself is subject to a number of limitations, including the limited cohort included in the research. While the homogeneous trial is representative of the average man in his twenties, the research may not be applicable to other age groups or demographic groups. The production of testosterone and the increase in muscle mass depend largely on the total protein intake and the type of protein used.
In some studies, low-carb groups consumed large amounts of protein (defined here as 35 percent or more of total calories); in others, low-carb groups consumed moderate amounts of protein. There is only one study to suggest that animal protein is much more effective at increasing basal androgen production, compared to vegetable or soy protein.