When women took hop supplements every day for 5 days, the natural compounds in the hops were found in their breast tissue at measurable amounts, showing that these compounds can be absorbed by the body and stored in breast tissue.

This study gave women hop supplements with the same compounds (xanthohumol, isoxanthohumol, 8-prenylnaringenin) at the same doses for 5 days, then measured what reached their breast tissue during breast reduction surgery. They found measurable amounts of these compounds in the breast tissue - exactly what the claim says would happen.