The ubiquinol form of Co Q10 dominates most human tissues. Co Q10 is mostly concentrated in the mitochondria and other organelles, whose primary purpose is energy production. Ubiquinol can be directly used in human metabolism as a lipid-soluble antioxidant. As demonstrated in clinical studies ubiquinol is the most effective suppressant of free radicals and also increases mitochondrial energy output (1, 2, 3-7).

While Co Q10 exists in both ubiquinol and ubiquinone forms, the ubiquinol form has been shown to be vastly more bio-available than conventional ubiquinone. In a study measuring absorption in humans supplementing with 150 mg and 300 mg of Coenzyme Q10, far lower doses of ubiquinol CoQ10 produced blood plasma levels of CoQ10 that were equivalent to those produced by much higher doses of ubiquinone (3). A review of published research in humans indicates that ubiquinol increases blood CoQ10 levels eight times more efficiently than ubiquinone (8, 9, 10)

References

1. Ernster L, Forsmark-Andree P. Ubiquinol: an endogenous antioxidant in aerobic organisms. Clin Investig. 1993;71(8 Suppl):S60-5.
2. Siemieniuk E and Skrzydlewska E. Coenzyme Q10: its biosynthesis and biological significance in animal organisms and in humans. Postepy Hig Med Dosw (Online). 2005;59:150-9.
3. Frei B, Kim MC, Ames BN. Ubiquinol-10 is an effective lipid-soluble antioxidant at physiological concentrations. Proc Natl Acad Sci USA. 1990 Jun;87(12):4879-83.
4. Bowry VW, Mohr D, Cleary J, Stocker R. Prevention of tocopherol-mediated peroxidation in ubiquinol-10-free human low density lipoprotein. J Biol Chem. 1995 Mar 17;270(11):5756-63.
5. Ingold KU, Bowry VW, Stocker R, Walling C. Autoxidation of lipids and antioxidation by alpha-tocopherol and ubiquinol in homogeneous solution and in aqueous dispersions of lipids: unrecognized consequences of lipid particle size as exemplified by oxidation of human low density lipoprotein. Proc Natl Acad Sci U S A. 1993 Jan 1;90(1):45-9.
6. Okamoto T, Matsuya T, Fukunaga Y, Kishi T, Yamagami T. Human serum ubiquinol-10 levels and relationship to serum lipids. Int J Vitam Nutr Res. 1989;59(3):288-92.
7. Mohr D, Bowry VW, Stocker R. Dietary supplementation with coenzyme Q10 results in increased levels of ubiquinol-10 within circulating lipoproteins and increased resistance of human low-density lipoprotein to the initiation of lipid peroxidation. Biochim  Biophys Acta. 1992 Jun 26;1126(3):247-54.
8. Hosoe K, Kitano M, Kishida H, et al. Study on safety and bioavailability of ubiquinol (Kaneka QH(trade mark)) after single and 4-week multiple oral administration to healthy volunteers. Regul Toxicol Pharmacol. 2006 Aug 17.
9. Shults CW, Flint BM, Song D, Fontaine D. Pilot trial of high dosages of coenzyme Q10 in patients with Parkinson’s disease. Exp Neurol. 2004 Aug;188(2):491-4.
10. Regul Toxicol Pharmacol. 2006 Aug 17; Absorption Comparison Using Different Form of CoQ10