As you may have read in my link on Testosterone and Excitoxins, the standard argument for allowing excitotoxins, such as MSG and aspartame, in our food is because they cannot easily penetrate the blood-brain barrier. (Of course, I have pointed out that this still leaves the hypothalamus unprotected!) However, this argument is proving to be weaker with every month that passes by.
The problem is that the blood-brain barrier is much more porous than previously imagined. Yes, under normal circumstances the blood-brain barrier is a veritable fortress and actually thwarts the efforts of researchers to get cancer treating drugs into the brain for example. However, circumstances are not always normal, in our modern lifestyles especially, and can leave us potentially vulnerable to excitoxin damage.
Remember: only minute amounts of excitotoxins can render huge neurological damage, causing neurons to "overexcite", miswire and even self-destruct. Many guys do not realize that excitotoxins are actually neurotransmitters, so common sense would tell you that you don't want to flood the brain with these chemicals.
We know from animal studies the nasty things that excitotoxins do and, again, every month or two a new study comes out showing more about short term issues, such as Excitotoxin Syndrome, and longer term issues such as kidney decline. Of course, what I'm doing is trying to warn guys to avoid excitotoxins at all costs because the blood-brain barrier ain't what it used to be.
There are other reasons as well: the blood-brain barrier (BBB) also blocks out almost all cancer cells, viruses and bacteria. This is why there are few cancers and diseases of the brain. In addition, MS and Alzheimer's may have a BBB component. The bottom line is that you need this barrier fully intact and working.
Here are several key items that can increase the permeability or porousness of your blood-brain barrier:
1) PDE5 Inhibitors. Mick Jaggar would have undoubtedly called these Father's Little Helpers and they include Viagra, Levitra and Cialis. One interesting discovery about these Rock Stars of the pharmaceutical world is that increase the permeability of the blood-brain barrier. Of course, researchers are quite excited about the fact that they can potentially use PDE5 inhibitors to push brain cancer-killing drugs through the blood-brain barrier. One recent study did just that in mice using Levitra in order to push the anti-cancer drug Herceptin through the blood-brain barrier. Caution is definitely in order here, because if guys on Levitra, Viagra and Cialis are allowing even very minute amounts of excitotoxins into their cranial neurons, it could wreak havoc.
2) Electrosmog. This is a subject that I covered in my link How Dirty Electricity Can Alter the Blood-Brain Barrier and, like PDE5 inhibitors, can increase permeability. Cell phones, computers and many other devices emit a certain kind of electrical "signal" or "noise" that could be allowing minute amounts of toxins into their brain.
3) Inflammation. Researchers have long known that inflammation can open, at least partially, the blood brain barrier. In fact, scientists have suspected that this could partially explain how MS  and Alzheimer's progress,  as both require damaging proteins to cross the BBB (Blood-Brain Barrier).
4) Aging. Several studies have shown how aging thins the delicate endothelial capillaries of the blood-brain barrier, potentially leaving some of us vulnerable.  Mini-strokes are another potential problem. Mini-strokes are imperceptible strokes that can, among other things, compromise the blood-brain barrier and are much more common as we age.
1) PLoS ONE 5(4): e10108, Received: February 23, 2010; Accepted: March 18, 2010; Published: April 19, 2010, "Phosphodiesterase Type 5 Inhibitors Increase Herceptin Transport and Treatment Efficacy in Mouse Metastatic Brain Tumor Models"
2) Brain, 1990, 113(5):1477-1489, "BREAKDOWN OF THE BLOOD-BRAIN BARRIER PRECEDES SYMPTOMS AND OTHER MRI SIGNS OF NEW LESIONS IN MULTIPLE SCLEROSIS"
3) Neurobiol Aging, 2007 Jul, 28(7):977-86. Epub 2006 Jun 16, "Microvascular injury and blood-brain barrier leakage in Alzheimer's disease"
4) J Gerontol, 1979, 34(5):642-650, "Thinning of capillary walls and declining of numbers of endothelial mitochondria in the cerebral cortex of the aging primate, macaca nemestrina".
5) Advances in Neurology, Vol. 30, 1981, "Blood-Brain Barrier, Againg, Brain Blood Flow and Sleep".