“Does eating influence brain function?” begins a 1974 Scientific American article titled “Nutrition and the Brain” by 2 MIT professors. It is mainly about how changes in carbohydrate affect the brain — especially what happens after a carbohydrate-rich meal. A few studies of protein variation are discussed. Nothing about the effects of varying fat intake, although the brain is mostly fat.
Out of interest, what does happen?
A carbohydrate-rich meal increases serotonin synthesis, the authors (John Fernstrom and Richard Wurtman) found. The rats ended up with more serotonin an hour or two after a high-carbohydrate meal. No behavioral effects were measured.