Chrysin (5,7-Chrysin, 5,7-Dihydroxyflavone) is a naturally occurring flavone (a type of flavonoid) found in the passion flowers (e.g. Passiflora caerulea). It is also found in chamomile. Chrysin is available as a bodybuilding supplement and is marketed as a nutrient which can further elevate testosterone levels by reducing aromatization.
Anti-Estrogen Action of Chrysin
It is well-known that conversion of testosterone to unwanted estrogen may can cause breast tissue development in men (gynecomastia). This concern is especially present if anabolic steroids are used. Therefore, anti-estrogenic agents should be consumed. Inhibitors of the enzyme aromatase are also often used in high doses to boost testosterone concentrations.
In vitro studies are reporting that chrysin, at high concentrations, may be an aromatase inhibitor [1,2,3]. One in vitro study  even noted that chysin is similar in potency and effectiveness to aminoglutethimide, a pharmaceutical aromatase inhibitor used clinically in cases of estrogen-dependent carcinoma. However, in vivo studies failed to show aromatase inhibiton when chrysin was administered orally . The inability of chrysin to inhibit aromatase may be due to their relatively poor absorption and/or bioavailability [5,6]. When orally administered, bioavailability of chrysin (estimated around 0.003–0.02%), and possibly other flavonoids, in humans is very low . Cell membrane penetration doesn’t seem to be a limiting factor for chrysin [5,7]. Studies in humans also fail to show any changes in serum testosterone or reduced estrogenic effect .
There is one in vivo study  that showed significant improvement in overall sexual functions in the rats after chrysin and benzoflavone moiety were administered for a period of 30 days. However, benzoflavone moiety was more potent than chrysin as an anti-aromatase agent and exhibited better effects on the sexual system .
There is no clinical evidence for chrysin’s effect on aromatase action.
Chrysin (5,7-Dihydroxyflavone) Toxicity and Side Effects
Chrysin seems to be well tolerated when used appropriately for up to 8 weeks and is possibly safe . So far, no side effects have been reported. In trout liver cell line, chrysin effectively shut down the de novo DNA synthesis and demonstrated cell toxicity .
Van Meeuwen, J. A., et al. “(Anti) estrogenic effects of phytochemicals on human primary mammary fibroblasts, MCF-7 cells and their co-culture.” Toxicology and applied pharmacology 221.3 (2007): 372-383.
Kao, Yeh-Chih, et al. “Molecular basis of the inhibition of human aromatase (estrogen synthetase) by flavone and isoflavone phytoestrogens: A site-directed mutagenesis study.” Environmental health perspectives 106.2 (1998): 85.
Kellis, James T., and Larry E. Vickery. “Inhibition of human estrogen synthetase (aromatase) by flavones.” Science 225.4666 (1984): 1032-1034.
Campbell, Deborah R., and Mindy S. Kurzer. “Flavonoid inhibition of aromatase enzyme activity in human preadipocytes.” The Journal of steroid biochemistry and molecular biology 46.3 (1993): 381-388.
Saarinen, Niina, et al. “No evidence for the in vivo activity of aromatase-inhibiting flavonoids.” The Journal of steroid biochemistry and molecular biology 78.3 (2001): 231-239.
- Walle, T., et al. “Disposition and metabolism of the flavonoid chrysin in normal volunteers.” British journal of clinical pharmacology 51.2 (2001): 143-146.
Walle, U. Kristina, Alema Galijatovic, and Thomas Walle. “Transport of the flavonoid chrysin and its conjugated metabolites by the human intestinal cell line Caco-2.” Biochemical pharmacology 58.3 (1999): 431-438.
Brown, G. A., et al. “Effects of anabolic precursors on serum testosterone concentrations and adaptations to resistance training in young men.” International journal of sport nutrition and exercise metabolism 10.3 (2000): 340-359.
- Dhawan, Kamaldeep, Suresh Kumar, and Anupam Sharma. “Beneficial effects of chrysin and benzoflavone on virility in 2-year-old male rats.” Journal of medicinal food 5.1 (2002): 43-48.
Tsuji, P. A., and T. Walle. “Cytotoxic effects of the dietary flavones chrysin and apigenin in a normal trout liver cell line.” Chemico-biological interactions 171.1 (2008): 37-44.