From the publication:

Seven studies (including eight trials) were included in the analysis for a total of 230 patients, five using normocaloric KD [ketogenic diet] and three very low calories KD (VLCKD). Only three studies enrolled overweight/obese men. A significant total testosterone increase was recorded after any kind of KD considering 111 patients. This increase was more evident considering VLCKD [very low calories KD] compared to normocaloric KD. Meta-regression analyses highlighted significant correlations between the post-KD testosterone raise with patients’ age and weight loss.