Often CFS patients have their problem ascribed to depression. MDs then attempts to treat the patients with conventional depression medication with results contrary to what they expected [Distinguishing between depression and chronic fatigue syndrome 2013]. As Dr.Bell mentions, “The constant fatigue and other symptoms would cause anybody to be depressed. Claiming depression causes the fatigue is naive and dismissive”. As a result of a recent conversation, I thought a little review on the state of research on depression may be nice. There have been a lot of very interesting recent publications.
Depression can have several sources:
- Epigentic (environmental stress that alter methylation) causing DNA to be express differently
- Childhood stressors in the development of fatigue syndromes: a review of the past 20 years of research. 
- Seeing the future: epigenetic biomarkers of postpartum depression. 
- DNA methylation in interleukin-11 predicts clinical response to antidepressants in GENDEP 
- Epigenetic Risk Factors in PTSD and Depression.
- Early life stress alters behavior, immunity, and microbiota in rats: implications for irritable bowel syndrome and psychiatric illnesses.
- Chemical-Biological agent – at one extreme infections and at the other gut bacteria.
It is my belief the CFS is primarily the last one – specifically biological agents(gut bacteria alteration), although the others may be a factor in some. This is not my theory — it was proposed 70 years ago and there has been little scientific followup – see http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3038963/ for a good write up.
One of the key predictor of the severity of depression is vitamin D level (Relationship between vitamin D and depression in multiple sclerosis. , Vitamin D Status during Pregnancy and the Risk of Subsequent Postpartum Depression: A Case-Control Study(2013), Low vitamin D levels are associated with symptoms of depression in young adult males (2013). It also impacts gut bacteria: Vitamin D regulates the gut microbiome and protects mice from dextran sodium sulfate-induced colitis. (2013), Vitamin D and prebiotics may benefit the intestinal microbacteria and improve glucose homeostasis in prediabetes and type 2 diabetes (2013)
An interesting article that I spotted while reviewing the literature was Could yeast infections impair recovery from mental illness? A case study using micronutrients and olive leaf extract for the treatment of ADHD and depression (2013) — which agrees with some reports of Olive Leaf Extracts lessening some CFS symptoms over the years.