There was a really good thread on microglial activation and excitotoxicity on this forum a couple of years ago, but I can't find it. In any case it's integral to understanding the mechanics of brain fog and most other mental issues people w/ CFS experience. "Glutamate is a prime example of an excitotoxin in the brain, and it is also the major excitatory neurotransmitter in the mammalian CNS....When the glutamate concentration around the synaptic cleft cannot be decreased or reaches higher levels, the neuron kills itself by a process called apoptosis. This pathologic phenomenon can also occur after brain injury and spinal cord injury. Within minutes after spinal cord injury, damaged neural cells within the lesion site spill glutamate into the extracellular space where glutamate can stimulate presynaptic glutamate receptors to enhance the release of additional glutamate." http://en.wikipedia.org/wiki/Excitotoxicity#Pathophysiology There is a negative feedback cycle that can be established here where excess glutamate, causes cell death, which causes more glutamate to be released which results in more cell death. If neurons really do use lactic acid as a fuel source, and they are dying in large numbers due to excess glutamate activation then it's feasible lactic acid levels may rise as a secondary effect as the neural contents spill out in the Cerebral spinal fluid (CSF). This mechanism would explain the link @Sidereal posted and the one I posted, about lactic acid build up in people w/ CFS and mechanical brain injury. Microglial activation relates to this as well as microglia are the CNS's immune system. IIRC they can be triggered by ammonia (produced by yeast in an alkaline gut) as well as LPS. They also get triggered when neurons die from the previous mentioned glutamate issue as a clean up protocol. Part of the way they work is that when activated in an inflammation response they release glutamate which can exacerbate the previous noted issue. This is how endtoxin die off could lead to brain fog. In addition they also release NO, nitric oxide; NOO-, peroxynitrite; O2, superoxide; PGE2, prostaglandin E2; TNFalpha and tumour necrosis factor-alpha. All wonderful things that cause all kinds of inflammatory and mental symptoms! In the following diagram this process is called "Self-perpetuating neurotoxicity" http://www.nature.com/nrn/journal/v8/n1/fig_tab/nrn2038_F1.html Again to reiterate, there are well established process's here that explain brain fog perfectly well. There is no need to jump to conclusions and wrongly blame lactic acid itself and that it is somehow magically making peoples brain tissue more acidic thereby explaining their issues. Lactic acid is pretty important stuff. According to the Lactate shuttle hypothesis: "In addition to its role as a fuel source predominantly in the muscles, heart, brain, and liver, the lactate shuttle hypothesis also relates the role of lactate in redox signalling, gene expression, and lipolytic control." It's therefore feasible a lactic acid deficiency due to gut dysbiosis could compromise those organs and aspects of human health. Just like a butyrate deficiency compromises colon health.