P/S/A: There's a treatable genetic mutation that half the population has which has more or less recently begun to be treated called MTHFR that causes several vitamin deficiencies (due to you not processing them into the usable forms - and it's treatable because you can take the usable form as a supplement) and homocysteine issues, and it's symptoms can range between none to raging horrible problems with depression, anxiety, IBS, fatigue, and a list of other things.

Specifics:

It reduces the body's ability to convert folic acid into the usable form, methyl folate and reduces the body's ability to convert vitamin B12 into the usable form (called methylcobalamin). This same mutation also tends to cause homocysteine levels to be too high or too low.

Caution:

Knowledge about this is kind of new, because we only mapped the genome so long ago (and figured out what this gene does, and figured out how to treat it, and began producing the supplements to treat it, etc). It can be tricky to treat. If you pursue this, you should seek a medical professional who has significant experience treating people with MTHFR.

What are the symptoms:

"Research is still pending on which medical conditions are caused by, or at least partially attributed to, the MTHFR gene mutations. From the partial list I recently went through on Medline, these are the current symptoms, syndromes and medical conditions relating to the MTHFR gene mutations" - www.mthfr.net This site lists 64 different conditions and symptoms ranging from miscarriages to schizophrenia. See Also: Disclaimer.

Disclaimer:

There's a reason I chose the symptom link above, but you should know that it is not a perfect list of symptoms. For an alternative list and an explanation about why I chose this symptom list, please see my response to Yvain about that under "the guy you're linking to".