Whaddaya mean I have brain fog?

For years, researchers thought when we effected difficult tasks, we used more energy, so our brains would feel overloaded. (That’s why those same folks suggested we should imbibe sweets- a milk shake, pop, etc.- with REAL sugar- to revive our brain functionality.  They hypothesized fatigue was related to a depletion of glucose in the brain. However,  testing could not confirm this as the cause.)

Well- that’s wrong.

It seems that glutamate accumulation may be what creates the tired sensations of our brains. That would make sense since glutamate is a neural stimulator (a key factor in learning and memory).  This hypothesis, that it is the accumulation of glutamate that accompanied brain fatigue, was confirmed by using  in vivo proton magnetic resonance spectroscopy (1H-MRS), focusing on the lateral prefrontal cortex of the brain.  (That is the portion of the brain that we employ when we make plans and are focusing on accomplishing tasks.  Not surprisingly, when we are exhausted, activity in this region attenuates dramatically. The researchers had found no such changes in the primary visual cortex [the vision control portion of the brain].   it could have been possible that eye fatigue led to brain fatigue.)

All of this was revealed in a publication (A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions) in Current Biology.   Drs. A Wiehler and M Pessiglione (both of the Paris Brain Institute) were the senior authors, assisted by  Drs.   F Branzoli (Paris Brain Institute) , I Adanyequh (Sorbonne) , and F Mochel (Sorbonne).  (They were all associated with the Pitié-Salpêtrière University Hospital in Paris.)  The protocol involved a total of 39 subjects (paid!), with some of them assigned to difficult mental tasks to cause them to become exhausted.   These folks worked 6.5 hours in the lab (about a full workday) effecting repetitive, but mentally challenging, tasks. Half of the subjects were assigned more difficult versions of the tasks, the other half were given simple tasks.

To determine their exhaustion, the researchers provided them questions whose answers would require impulse control or not; if the subjects were exhausted, they would make more impulsive choices. (It turns out their brain glutamate levels rose about 8% as well.)

Why impulse control as the parameter?  Because the two senior authors had determined some 6 years ago that after long periods of effortful tasks, folks opted for more immediate gratification, instead of waiting for a bigger, more lucrative one.  Yes- we succumb to simple pleasures when we are exhausted.

However, the authors are unwilling to declare that toxic buildup of glutamate is the total reason for brain fatigue. (We also know that some drugs that treat depression and Alzheimer’s disease target the same neuronal receptors to which glutamate attaches.) Part of the reason for this reticence is that the magnetic resonance spectroscope is not powerful enough to discriminate between glutamate and glutamine.  Another reason to withhold final judgment is that we manifest both physical and mental routes to brain fatigue- and there is no proof that the mechanisms would be similar.

But, that fact that glutamate is the trigger for brain fatigue would explain why sleep has such restorative powers.  After all, one of the primary functions of sleep is to flush out metabolic wastes- and the glutamate is, indeed, flushed away.

We will have to wait for further scientific data to solve this problem.  But, the hypothesis is very intriguing!