Back to "brightening up" an older dog with dementia:
Long story short: a friend of ours who drag races had been on the Atkins (low carb) diet, and his reaction times had become laughably slow. I told him that carbs fuel the brain - he ate a peanut butter sandwich, and his times drastically improved.
As I saw my Aussie Ike slowing down physically and mentally, it dawned on me that maybe extra carbs could help him, too? I actually made him oatmeal with a scrambled egg for breakfast (his preference was brown sugar/maple) - and his mornings were great! He was lucid, playful - his usual silly self :-) Of course I have to qualify this diet with his declining ability to utilize the fats in his dog food, and he was slowly losing weight. I figured going back to easily-digestible "baby food" would help him; and the sugar in the oatmeal sure didn't hurt. He still got dog food at lunch and for dinner, to get the nutrients he needed. I think the higher-carb diet kept him happy for an additional year, until a hip issue finally sealed his date with the Final Needle.