Thiamine, or vitamin B1, is also found naturally in whole grains, legumes, liver and salmon. The "sweet spot" for the amount of thiamine to consume, according to the study, is 0.68 mg per day.
A 30-gram bowl of cereal has about 1.2 mg. Every 1.0 mg/day above the safe limit was associated with a 4.24-point decrease in global cognitive score. The associations were stronger among people who were obese, had high blood pressure or did not smoke.