Glutamate only causes toxicity when it is found floating free outside a neuron, and the brain possesses several safety measures to protect itself. One is based on quickly removing glutamate once it is secreted from a synaptic terminal, whisking it away to be stored safely in a nearby cell called an astrocyte. The whisking away process is carried out by special carrier proteins, which can be thought of as escorts.213
A second safety measure is to protect the brain from glutamate floating free in the blood stream. Glutamate is normally found in many foods, including vegetables and meats. To prevent this glutamate from entering the brain and setting off destructive excitotoxicity, God created a special boundary called the blood-brain barrier that prevents most harmful substances in the blood from entering the interior of the brain. It can be thought of as a gatekeeper.
Under normal circumstances this gatekeeper is very efficient. Unfortunately, persistent manipulation of the foods we consume has endangered the balance necessary for this protective system to operate correctly. By artificially adding very high concentrations of free glutamate to food, blood glutamate levels in most people are far higher than were ever intended, taxing the ability of the protective gatekeeper. If blood levels remain high, the glutamate can gradually seep into the brain past the gatekeeper, creating havoc.214
Also, some areas of the brain—the circumventricular organs—are not protected by the blood-brain barrier. One area is the hypothalamus, which contains control systems for the endocrine glands, sleep-wake cycles, higher level control of the autonomic nervous system, and connections to the limbic system of emotional elaboration. Numerous studies have revealed that in every species of animal tested, high blood levels of glutamate occur after ingestion of glutamate, causing destruction of a vital group of cells in the hypothalamus called the arcuate nucleus.
It is now recognized that many conditions cause the blood-brain barrier to become defective. Hypertension, strokes (both major and silent), head injury, infections, Lyme disease, heat stroke, brain tumors, certain medications, autism, multiple sclerosis, Alzheimer's disease, lupus, and even aging itself have been shown to result in impairment of the blood-brain barrier.215 From birth until two or three years of age, the barrier system is also deficient.
FIGURE 7.1 Demonstration of the effect of excitotoxins on nerve endings. "A" shows a normal dendrite with numerous synaptic connections. The nerve endings in the dendrite shown in "B" have been exposed to an excessive concentration of glutamate leading to widespread synapse loss.
Yet anyone suffering from any of these disorders almost certainly bears a substantial brain-glutamate burden.
It also should be appreciated that differing degrees of damage result from excessive buildup of excitotoxins. With very high doses, neurons swell and die within a few minutes. Slightly smaller doses produce a delayed death that takes approximately an hour to occur. All of the effects described so far can be seen with an ordinary light microscope: cells appear to shrivel and eventually disappear.
At still lower concentrations, neurons themselves do not die but their connections, called synapses, will shrivel and retract. This phenomenon may not be seen with a light microscope and may require electronmicroscopy. Defenders of glutamate safety frequently point to studies—albeit older ones—in which only light-microscope observations were used, and no examinations were done of the synaptic connections. Interestingly, the greatest damage in Alzheimer's disease is to the synaptic connections and not the cell bodies themselves.
Because of the effects of glutamate and aspartate on brain-cell excitation at even lower concentrations, it is even possible that no physical damage will occur on microscopic examination, yet brain functions may still be adversely affected. One of the more obvious effects of such exposure is the occurrence of seizures. We know that excess glutamate in the brain can precipitate seizures without causing physical changes in the brain, even though continued seizures can produce destruction of brain cells and its connections.216
This means that at lower doses, such as those easily attained by eating processed foods, one may feel confused or disoriented and have difficulty thinking clearly without having a seizure or experiencing obvious physical damage to your brain. However, should excito-toxic assault to your brain continue because you continue eating processed foods, physical damage will eventually result.
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