As people age, their metabolism slows down along with testosterone production. This age-related hormonal decline leads to life-threatening abdominal obesity and increases one’s risk for a wide array of serious medical conditions. Interestingly, published studies have shown that low testosterone and obesity are strongly linked, trapping testosterone-deficient individuals in a spiral of weight gain and hormonal imbalance. [536-539]
The fact that obese men have lower blood levels of testosterone compared to lean men has been recognized for more than three decades.  Since then, numerous studies have consistently found a strong link between obesity and low testosterone levels in men.  In a group of 3219 men from the European Male Aging Study (EMAS), Tajar et al. found that obesity was associated with a 3.3-fold increased relative risk of secondary hypogonadism.  In a cross-sectional study of 314 Chinese men, Cao et al. similarly found that older obese men had lower blood testosterone levels compared to age-matched lean men.  In a cross-sectional study of 1849 community-dwelling obese U.S. American men, Dhindsa et al. found that 40% of the subjects had low testosterone levels.  Of note, Allan et al. reported that reductions in testosterone levels correlate with the severity of obesity. 
Testosterone replacement therapy may be a therapeutic option for obesity since it generates physiological levels of dihydrotestosterone (DHT), which inhibits fat cell formation.  In addition to this, De Maddalena et al. reported that testosterone helps regulate the balance of leptin (satiety hormone) and ghrelin (hunger hormone) in the body.  For these reasons, clinical and pre-clinical studies have implicated a role for testosterone in the treatment of obesity.
In a study by Saad et al., it was found that testosterone treatment reversed fat accumulation with significant improvement in lean body mass, insulin sensitivity and biochemical profiles of heart disease risk in men with testosterone deficiency.  Researchers observed that aside from significant improvement in body composition, testosterone treatment did not cause any adverse side effects.
In a study of 261 overweight men with testosterone deficiency, Yassin et al. reported that long-term treatment with testosterone undecanoate 1000 mg every 12 weeks for 5 years produced marked and significant decrease in body weight, waist circumference and body mass index.  Researchers observed that all of the study participants who received testosterone reported no adverse side effects of the treatment.
A similar study by Traish et al. found that long-term testosterone therapy in men with testosterone deficiency was associated with significant and sustained weight loss, marked reduction in waist circumference and BMI and improvement in body composition.  In addition to this, researchers observed that testosterone therapy improved components of the metabolic syndrome as well as quality of life of the study participants, as evidenced by increased energy utilization, increased motivation and vigor, and enhanced physical activity.
In one clinical trial, Ng Tang et al. evaluated the effects of 10-weekly intramuscular testosterone undecanoate injections in obese men with testosterone deficiency.  A total of 100 obese men received testosterone injections in addition to a very low energy diet (VLED) followed by 46 weeks of weight maintenance. At study end, researchers observed that the subjects had greater reductions in fat mass and in visceral fat.
Similarly, a study by Rebuffé-Scrive et al. also found that testosterone either given as a single injection (500 mg) or in moderate doses (40 mg x 4) for 6 weeks in an oral preparation, can dramatically decrease abdominal fat.  Researchers observed that middle-aged men who received the treatment had significant reductions in waist/hip circumference without any adverse side effects.
Other clinical trial data are also consistent in showing significant reductions in body fat mass during testosterone replacement therapy. [553-556] These results suggest that testosterone does have a beneficial effect in improving body composition by significantly reducing body fat percentage and increasing lean muscle mass in obese patients.
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