This unexpected finding suggests that estrogen may have independent effects on insulin sensitivity. If estrogen indeed has an independent regulatory role in this regard, this would suggest that aromatase transformation of testosterone to estrogen in men could be important in the effects of testosterone on insulin sensitivity. In addition, this would be consistent with the important role already demonstrated for estrogen on bone metabolism and with the metabolic effects observed in aromatase deficient males.

Effect of Testosterone and Estradiol on Insulin Sensitivity in Healthy Older Men -- Lee et al. 34 (3): SUN-799 -- Endocrine Reviews