Leptin is released by fat cells in amounts mirroring overall body fat stores. Leptin is a protein hormone that plays a key role in regulating energy intake and energy expenditure, including the regulation of appetite and metabolism. In addition to regulating both appetite and metabolism, it also regulates testosterone levels as well. In the Archives of Andrology, researchers examined a sample of 77 men (ages: 20 to 60 years old) to determine what factors correlated to testosterone suppression. After careful analysis, testosterone suppression was related to BMI, waist circumference, insulin resistance, and high leptin levels10. There have been several studies in healthy young men and older men that have reported that high leptin levels are associated with decreased testosterone levels12, 13. Recently, it was reported that testosterone and its active metabolite dihydrotestosterone (DHT) were able to suppress leptin secretion and leptin mRNA in human adipocytes, suggesting a direct effect of testosterone and its metabolites at the level of adipocytes13.

Obesity is clearly associated with lower levels of testosterone and free testosterone, the decrease in serum hormones are thought to be mediated by: 1.) reduced sex hormone binding globulin (low levels of sex hormone-binding globulin are considered to be due to hyperinsulinemia, predicting the later onset of insulin resistance) 2), reduced LH amplitude, and excess estrogen; however excess estrogen may not be the only fat derived hormones suppressing testosterone, leptin may be an even greater suppressor of testosterone than estrogen.

In one study, males who were of normal weight and moderately obese had blood samples drawn to determine hormone levels in addition to body fat distribution. The obese men had ~30-40% lower testosterone levels than the normal weight men; more important, the degree of testosterone suppression was related to leptin, but not to estrogen levels or other hormonal variables (10). Another interesting finding was that male obesity reduced LH-stimulated androgen secretion and was inversely correlated with leptin levels (the higher the leptin levels, the lower the LH production). The impaired androgen response to LH was due to a defect in the enzymatic conversion of 17-OH-progesterone to testosterone. This is in conjunction with another study, which reported that excess leptin produces a significant decrease in the amount of LH produced from the hypothalmus (11). The results of the study strongly suggests that excess bodyfat which increases leptin levels can lead to reduced androgen levels by inhibiting LH production in the brain.

The abrupt drop in LH levels in the most viscerally obese subjects may reflect a sudden incapacity of the pituitary to keep up its LH secretion. The identification of a possible link between visceral obesity and decreased LH levels might be due to excess free fatty acids. Excess free fatty acids have been linked with inhibition of another pituitary hormone- growth hormone. In addition to the suppression of LH pulse amplitudes and aromatization of androgens to estrogens, some have suggested that putting on bodyfat with an increase in leptin levels directly suppresses testosterone at the testicular level. Isidori et al proposed that elevated leptin levels may directly inhibit Leydig cell (testosterone producing cells) function (16).

All bodybuilders need to increase calories during the off-season to put on quality muscle but this does not give you the green light to eat everything in sight. Some bodybuilders put on as much as 30-50 pounds in the off-season. The following research may persuade bodybuilders to not put on excessive amounts of fat in the off-season.