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.

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Evidence from Studies

Supporting (1)

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An Overview of Interactions between Goat Milk Casein and Other Food Components: Polysaccharides, Polyphenols, and Metal Ions

Narrative Review

2024 Sep 13

This study shows that the main protein in milk (casein) sticks to plant compounds called polyphenols, which are also in coffee. That means those healthy compounds might not be absorbed as well by your body—exactly what the claim says.

Contradicting (1)

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Molecular mechanism of the interactions between coffee polyphenols and milk proteins

Computational/Algorithm Study

2025 Feb

The study found that coffee chemicals stick to milk proteins, but it didn’t check if that makes the coffee chemicals less useful to your body—so it doesn’t prove the claim that milk cuts coffee’s benefits by 30–60%.

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According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.

Similar Assertions

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.

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Milk has a protein called casein that grabs onto calcium like a magnet, helping it form tiny clumps that stay suspended in milk instead of sinking or separating.

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72%

When casein (a protein in milk) meets polyphenols (natural compounds in tea, wine, or fruit), they stick together in different ways—like magnets and glue—and this changes the protein’s shape and how it works.

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