Glutamate
An amino acid that is the brain's main excitatory messenger: nerve cells release it to make the next cell more likely to fire. It carries most fast signals in the brain and is central to learning.
Glutamate is the most common free amino acid in the brain and its main excitatory messenger, the kind that pushes the next cell towards firing: one nerve cell releases it and the next becomes more likely to fire. Evidence: solid Most fast signalling between brain cells runs on it, and the strengthening of connections that underlies learning depends on it. Evidence: solid Too much of it outside the cells damages them, so the brain clears it away fast and keeps the level there very low. Evidence: solid The glutamate in food, including the flavour enhancer MSG, isn't a way in. The barrier between blood and brain keeps almost all of it out. Evidence: decent
Where it gets oversold: tiredness after hard thinking. One French study found more glutamate in the side of the front of the brain by the end of a day of hard mental tasks than of easy ones, measured with a scanner that reads brain chemicals. The researchers proposed that this build-up is why hard thinking gets harder late in the day. Evidence: worth trying All the scans were taken once the tasks were under way, so there was no true starting level, and a critic argued the gap, about 10%, could be mostly glutamate falling in the easy group rather than rising in the hard one. Evidence: worth trying "Mental fatigue is glutamate" is an idea from one study, not a fact. Evidence: decent
Why it matters
Swap the task, don't push through rests on that one study. Evidence: worth trying Read it as a reason to switch to easy work at the end of a long, hard stretch, not as proof your brain is clogging up. The idea sits with effort cost and the prefrontal cortex. It isn't the old willpower tank, which failed its big retests. Evidence: solid