Orally administered milk-derived tripeptide improved cognitive decline in mice fed a high-fat diet

The functions of the brain, which is thought of as an organ highly independent from the periphery, are often affected by the peripheral environment. Indeed, epidemiologic studies demonstrated that diabetes was a risk factor for dementia. It was also reported that the intake of dairy products, such as milk, reduces the risk of developing dementia. We found that mice on a short-term high-fat diet (HFD) for 1 wk had reduced cognitive function. Thus, using this acute model, we investigated the effects of milk-derived peptide on cognitive decline induced by HFD. Tyr-Leu-Gly (YLG), a tripeptide derived from alpha(S1)-casein, a major bovine milk protein, is released by gastrointestinal proteases. We found that orally administered YLG improved cognitive decline induced by 1-wk HFD intake in the object recognition test. YLG also improved cognitive decline in the object location test. Thus, we found that YLG improved cognitive decline induced by HFD. Next, we examined the effects of YLG on the hippocampus, a brain area essential for cognitive function. HFD intake decreased the number of 5-bromo-2'-deoxyuridine (BrdU)-positive cells, and this decrease was improved by YLG administration. HFD intake decreased nerve growth factor (NGF) and glial cell line derived neurotrophic factor, whereas YLG increased NGF and ciliary neurotrophic factor, suggesting that these neurotropic factors play a role in hippocampal neurogenesis after YLG administration. In conclusion, we demonstrated that 1-wk HFD reduced cognitive function. Furthermore, we found that YLG, a milk-derived tripeptide, improved cognitive decline in mice on HFD. The HFD reduced neural stem cell proliferation, and YLG improved this reduction. YLG is the first reported milk peptide to improve cognitive decline induced by HFD intake.