There's a lot of you out there that feel like crap and you have no idea why or how or when it happened. You're tired all the time; you ache all over; your joints and muscles hurt; you feel like you are walking around in a fog. To your dismay you find that the doctors are little to no help and sometimes suggest that you're "depressed" or "stressed" instead of offering any real solutions. Well, I want you to know that these symptoms are a plague in Western societies and you are not alone. Doctors, generally smart and well-intentioned, just can't know everything. And so I want to hopefully help a good percentage of you out there with these general Feel Like Crap symptoms.
First of all, all of the above symptoms can be caused by Low Testosterone or hypothyroidism as my well-rounded and well-read readers already know. So if you hurt all over, can't concentrate and are incredibly fatigued most of the day, then do your due diligence and get tested. But some of you may have been tested for thyroid and testosterone and had fairly reasonable readings and so you are left without answers.
Well, there is another avenue that you may need to travel down as well: Excitoxin Syndrome. I have told many people about Excitotoxins and the dangers of diet drinks such as Diet Coke and Diet Pepsi. Most of the time their comment is something similar to this:
"But why would they do this to us knowingly?"
"The evidence can't be that strong or they would allow it."
Sorry, but this is one time where the world has gone crazy mad and dangerous. Excitotoxins wreak their havoc through a variety of pathways. First of all, we have discussed how they go straight into the hypothalamus and alter negatively the hormonal factors that the hypothalamus secretes to the pituitary and thyroid. And hypogonadalism (low testosterone) and hypothyroidism (low thyroid) will make you feel like Crap.
But there are several other key reasons that Excitoxins probably create the CFS/Fibromyalgia-like symptoms: the hypothalamus is the dopamine manufacturer for your body. Dopamine is the neurotransmitter most responsible for the highs and rushes that we as males so enjoy in life. Both sex, cocaine and other opiates lead to burst of dopamine flooding our brains with pleasure.
Interestingly enough, mild to moderately low dopamine levels have been found in CFS (Chronic Fatigue Syndrome) patients.  This has not been supported yet by any epidemiological studies that I know of, but it is very likely Excitotoxin-induced damage to the hypothalamus can lead to low dopamine levels which in turn can lead to Fibromyalgia/CFS-like symptoms. Excess glutamate depletes glutathione, which can lead to neuronal damage. And, sure enough, one study that clearly indicates this is a study where women who had been consuming excitoxins in the form of diet drinks were taken off of the diet drinks (and other excitotoxins) and had a complete recovery from their fibromyalgia! 
It is important to point out that the authors did not say that all fibromyalgia is caused by excitotoxins. But they clearly stated in my opinion that a certain percentage of fibromyalgia was caused almost strictly by this excitotoxin syndrome and could be dramatically cured through removal of the dietary excitotoxins: "the elimination of MSG and other excitotoxins from the diets of patients with fibromyalgia offers a benign treatment option that has the potential for dramatic results in a subset of patients".
The may also be a non-excitotoxin related reason for some of the standard symptoms as well: one animal study shows that aspartame consumption by animals resulted in a dangerous build-up of formaldehyde. Ingested aspartame, the artificial sweetener in diet Coke and Pepsi and Nutrasweet, breaks down into various nasty compounds such as methanol ("wood alcohol") and the excitotoxin aspartic acid. Just as bad, the methanol then breakes down into formaldehyde, which is an extreme tissue irritant and is associated with retinal damage and birth defects. Another big concern with formaldehyde is that is not easily eliminated from the body and, sure enough, one animal study showed just such a dangerous build-up of formaldehyde in tissues. 
Regardless of whether it is hypothalamus damage, neurotoxicity or formaldehyde build-up, consumption of Excitotoxins can lead to the typical Excitotoxin Syndrome symptoms that are so closely related to Chronic Fatigue Syndrome and Fibromyalgia.
1) J of Chronic Fatigue Syndrome, 1998, 4(2):3-22
2) The Annals of Pharmacotherapy, 2001, 35(6):702-706, Relief of fibromyalgia symptoms following discontinuation of dietary excitotoxins, JD Smith, CM Terpening, SO Schmidt, and JG Gums 1998
3) Life Sciences, 63(5):337f, "Formaldehyde Derived From Dietary Aspartame Binds to Tissue Components in vivo"