That search lead me to a website describing something called "copper-deficiency anemia." Beer is actually a good source of copper. Read more.
Something new to investigate... and after a quick search, there seems to be something to it. This study on rodents concludes: "lactation markedly enhances the avidity of the mammary gland for copper, diverting most of it from liver and kidney to that tissue" Here is the entire study. Any thoughts? I am reminded of all the copper-rich seafood traditionally recommended to increase lactation: oysters, octopus, eel, fish.