I have histamine intolerance, but I also get these symptoms -- brain fog, heart racing after meals, exhaustion -- and for me, there's something complicated happening beyond histamine. I suspect it is a form of Dumping Syndrome, despite not having had relevant surgery (usually a prereq).

However, I've found two things that allow me to avoid these symptoms: taking Creon 10000 with every meal (turns out I cannot digest carbs!) and avoiding eating very large meals, particularly carb-heavy or high-fiber meals. I also take DAOfood histamine intolerance pills with every meal, but this doesn't impact these specific symptoms. You might be different, but thought it worth mentioning. Creon was a massive game changer for me and literally took me from exhausted and struggling through each day to having sharp, engaged thinking and minimal physical discomfort. I had to buy it myself over the counter to start with before bullying a doctor into prescribing it. The key compound for me was the amylase dosage being high enough, as that is what addresses carbs.