Why do protein shakes increase cognitive ability?

I've been have cognitive issues commonly referred to as brain fog. I have also had fatigue and other nonspecific symptoms. I notice that when I drink protein shakes my brain seems to function better for a short period of time. The protein shakes I drink do not contain sugar. My diet consists of lots of protein in the form of meat. I'm trying to figure out of a nutritional deficiency is the root of my health issues. The whey protein shakes also include amino acids. Is there a reason why my body would not digest protein from solid foods? Possibly low stomach acid or another digestive disorder.

Thanks in advance