Fat gain and the associated increase in chronic disease risk, such as cardiovascular disease and type 2 diabetes, are strongly linked to decreases in testosterone, particularly in middle-aged and older men.[1][2][3] If you gain weight (as fat), your testosterone production drops. Fortunately, if you lose weight, your testosterone production can climb back up.
Effect of weight loss on testosterone levels
Reference: Grossmann and Matsumoto. J Clin Endocrinol Metab. 2017.[4]
A meta-analysis of 24 RCTs looked at weight loss caused by diet or bariatric surgery.[5] In the diet studies, the average 9.8% weight loss was linked to a testosterone increase of 2.9 nmol/L (84 ng/dL). In the bariatric-surgery studies, the average 32% weight loss was linked to a testosterone increase of 8.7 nmol/L (251 ng/dL). You need not lose huge amounts of weight to see a bump in testosterone levels, either: a 5% loss in weight may increase total testosterone by 2 nmol/L (58 ng/dL).[6]