Two studies originally published in April 2010 reported conflicting results with respect to the association of circulating coenzyme Q10 (CoQ10) with the risk of breast cancer. Both studies are summarized below. The first study found that U.S. postmenopausal women in the highest third of circulating CoQ10 levels had 1.6 times the risk of breast cancer compared to those in the lowest third. The strongest association was found for postmenopausal women under age 60 (3.9 times the risk for the highest versus the lowest thirds).

The second study, based on a Chinese population, reported no association between postmenopausal breast cancer risk and circulating CoQ10. However, premenopausal women in the highest fourth of circulating CoQ10 were found to have a 50% lower risk of breast cancer compared to those in the lowest quartile. Both sets of authors comment that future studies are needed to define the origin of circulating CoQ10, its physiological functioning, and potential role in cancer development and progression.

Implications of the studies regarding CoQ10 and breast cancer

CoQ10 is known to be beneficial for congestive heart failure patients and has been suggested as a treatment during anthracycline chemotherapy to prevent heart damage. The first study results suggest that such treatment would be unwise since it might contribute to breast cancer recurrence. Other antioxidants (for example, beta-carotene) have been found to be associated with lower cancer risk when consumed as part of food while promoting cancer when taken as a concentrated supplement. It is possible that the U.S. population with the highest circulating CoQ10 were more likely to achieve this level through supplementation than the Chinese population. While CoQ10 is found throughout the body, it's normal functioning is not well understood. How CoQ10 functions in breast cancer cells has not been determined. Therefore, we would advise against taking CoQ10 supplements based on the available evidence.