The researchers found that after treatment, testosterone levels rose significantly for those in the treated group (256 ng/dL to 562 ng/dL, P=0.001) while there were no significant changes in the placebo group.
They also found that free testosterone rose dramatically (4.1 nmol/L to 12.4 nmol/L, P<0.001), while placebo patients had no significant changes in this parameter.
Dandona said there was a "dramatic" 25% increase in insulin sensitivity as measured by euglycemic clamp after these men took testosterone for 6 months (P=0.01), but there was no significant change in insulin sensitivity for the placebo group.
There were no changes in weight or in waist-to-hip ratio for either group, but total lean body mass increased significantly for testosterone patients (P=0.004). At the same time, fat mass significantly diminished in this group (P=0.02).
"The short of this message is, 2 kg (4.5 lbs.) of fat were replaced by 2 kg (4.5 lbs.) of lean body mass," Dandona said.
The researchers also found, as expected, that mean insulin concentrations fell with the sensitization to insulin in the drug-treated group (11.6 to 7.1, P<0.05), and HOMA-IR also fell significantly for these patients (3.5 to 2.8, P<0.05).
Lipid concentrations didn't change in either group, but there was significant improvement in sexual desire among those on testosterone (P=0.05), Dandona said.
He noted that concentrations of free fatty acids also fell dramatically for men on testosterone therapy.
"It's important to realize that free fatty acids are ... associated with oxidative stress, inflammation, and abnormal vascular reactivity," Dandona told MedPage Today. "When levels are high, as they are in insulin-resistant patients, reducing them on their own is extremely worthwhile."
Dandona noted that his group's study is ongoing and that they will next assess fat and muscle biopsies to determine further metabolic effects of testosterone.
The study was supported by the National Institutes of Health.