When you add milk to coffee, the proteins in the milk might help keep the healthy compounds in coffee from breaking down too quickly — but scientists weren’t sure exactly how this works.

When coffee compounds meet milk proteins, the glow they produce gets stronger if the milk proteins have more of a specific building block called proline — so proline seems to help them stick together better.

When you add milk to your coffee, the protein in the milk (casein) latches onto the healthy compounds in coffee, making it harder for your body to absorb them—so you get 30% to 60% less of their benefit.