A number of studies have shown that the reduced form of Coenzyme Q10 (ubiquinol) may be more effective than the oxidized form (ubiquinone).
One such study showed that ubiquinol had significant decelerating effects on aging problems in middle-aged mice. The mice were given either ubiquinol for ubiquinone 6 or 14 months, then the conditions of their livers, hearts, brains and kidneys were examined.
According to the study, as reported by Green Med Info:
"Stringent evaluation of the resulting data revealed that [ubiquinol] has a stronger impact on gene expression than [ubiquinone], primarily due to differences in the bioavailability. Indeed, [ubiquinol] supplementation was more effective ... to increase levels of CoQ(10) in the liver".