Even if glutamate does not cross the healthy BBB, there are many factors which may contribute to a leaky BBB, potentially allowing too much glutamate to enter the brain. In his podcast on the “gut-brain axis,” Chris explained that having a leaky gut (which itself can be due to a number of underlying causes, including food intolerances, dysbiosis, or small intestinal bacterial overgrowth) can contribute to a state of chronic low grade inflammation. This low grade inflammation then also makes the BBB leaky, which essentially loosens the control over what enters the brain. More recent research has identified a specific molecule that damages the cells to create microscopic gaps allowing material through, bypassing the normal regulatory pathways, and explaining how general inflammation within the body can cause a leaky BBB.
Thus, it may be that in the setting of inflammation, we have a leaky BBB, which allows more glutamate to enter the brain than normal. Moreover, since some people have a genetic predisposition to glutamate sensitivity, it may be that a combination of excess glutamate in the diet, combined with chronic low grade inflammation, and an associated leaky BBB, contribute to symptoms. It seems less clear if people without an underlying genetic predisposition to glutamate sensitivity experience any adverse effects from excess dietary glutamate. Further research is clearly needed to elucidate the contribution of dietary glutamate to symptoms.