Casomorphins, peptides derived from dairy casein, bind to opioid receptors and reduce satiety and promote increased consumption.

Study: Hydrolysis of Casein Accelerates Gastrointestinal Transit via Reduction of Opioid Receptor Agonists Released from Casein in Rats

Dairy proteins can release tiny opioid-like pieces that slow down digestion, making you feel full slower and possibly eat more. This study showed that when these pieces are removed (by breaking down the protein), digestion speeds up — and when opioid blockers are given, the slowdown disappears, proving the opioid connection.

Study: Post-weaning A1/A2 β-casein milk intake modulates depressive-like behavior, brain μ-opioid receptors, and the metabolome of rats

This study found that milk with a certain type of protein (A1 casein) changed brain chemicals linked to opioid receptors in rats, making them act more depressed. Since opioid receptors are involved in how we feel full or crave food, this suggests that dairy proteins might affect our appetite through these same brain pathways.

Study: Beyond A1 milk concerns: dietary-relevant concentrations of β-casomorphin-7 show limited absorption but retain opioid-like activity in an intestinal cell model.

The study found that a protein fragment from milk can attach to opioid receptors in the gut, which is exactly what the claim says. While it didn’t test if people eat more, this is the first step that could explain why.