A study showing that a certain molecule in involved with some serious diseases does not necessarily meant that dietary intake of that is harmful. Measuring elevated levels of glutamate in brain cells might mean that the mechanisms for removing the glutamate created in those cells isn't functioning properly.
What removes glutamate from brain?
Glutamate is removed from the synaptic cleft by several high-affinity glutamate transporters present in both glial cells and presynaptic terminals. Glial cells contain the enzyme glutamine synthetase, which converts glutamate into glutamine; glutamine is then transported out of the glial cells and into nerve terminals."
Does glutamate pass the BBB?
This explains studies that show that the BBB is impermeable to glutamate, even at high concentrations, except in a few small areas that have fenestrated capillaries (circumventricular organs).Jul 1, 2009"
I haven't checked to see why some people are sensitive to dietary glutamate, but it's certainly not true that glutamate is bad for everyone.