The Simpsons Wiki describes this mysterious substance:
Bart thought that the school served milk, but when he cracked his knuckles, they snapped painfully. Bart was shocked about the brittleness of his bones because he had always drunk plenty of milk. However, when he looked closer at the carton, he realized it was written “MALK” instead of “MILK.”
You have to figure that Superintendent Chalmers wasn’t about to spend big on mere beverages. But that was 1995. Today:
I wonder if Matt Groening gets a cut … pic.twitter.com/g2bHfDc3QY
— Preston Jones (@prestonjones) June 23, 2016
Modern-day MALK was born in Houston in 2014. I have no idea whether it contains any Vitamin R.