Does reductive stress drive an adaptive inflammatory response, in depression linked to diet and lifestyle?

Everyone knows (I hope, because if they do it will save me a lot of time explaining) that inflammation can cause depression by activating the enzymes that degrade tryptophan, thus depleting the brain of serotonin.
Basically, macrophages hoover up tryptophan and pass it through Indoleamine 2,3,-Dioxygenase (IDO), where it is broken down. This is the pathway for the synthesis of niacin and nicotinamide, which is used to make NAD+.

(Note the other product, picolinic acid, thought to assist in absorption of minerals chromium and zinc)

There is the "sickness behaviour" explanation, whereby this response to infection, by making us less active, assists recovery, and the "sequestering" explanation whereby the macrophages act to deny tryptophan to pathogens, so that they don't have the advantage of extra NAD+. However, inflammation has many forms, and neither of these explain the response to chronic inflammation, when more activity and nicotinamide supplements are usually beneficial (nicotinamide seems to help in fighting infections too).



Supposing you are in the state known as reductive stress (not enough NAD+, too much NADH). This is associated with metabolic inflexibility, metabolic syndrome, and so on.
Reductive stress is mentioned by Peter D in this post. It is associated with fatty liver (steatosis) of chronic Hep C infection here, in these words:

the impairment of NADH oxidation to NAD, with consequent NADH accumulation, is a characteristic figure of mitochondrial dysfunction occurring in fatty liver due to high fat diet (HFD) in rats*.
So, how can NADH be converted back to NAD+ in these states? If lactate is available, metabolising this will restore NAD+, if ketone bodies are available, ditto for their interconversion (so exercise or carbohydrate restriction/fasting are protective against reductive stress). Electrophilic methyl groups in the diet - choline, carnitine, SAMe - may accept the H+ from NADH to form methane, which is dissipated.
But electrophilic methyl groups are hard to come by in the type of diets that cause reductive stress. No choline in flour, sugar, or vegetable oil. (Brilliant analysis of why this matters by Paul Jaminet here, also explaining that "high fat diet (HFD) in rats" line above*).

Also, fresh NAD+ can be supplied from outside the cell. From B3 if you're supplementing or eating good food (in which case your cells shouldn't have easily got into a reductive stress state, but requirements for B3 are unusually high for a co-enzyme vitamin). Or, if you're in the fasting state or your dietary B3 is inadequate, from tryptophan via IDO (Tryptophan 2,3-Dioxygenase is the hepatic equivalent). Which inflammation will upregulate.

So the hypothesis is, that reductive stress is an emergency (perhaps mimicking pellagra) that warrants an inflammatory response if this is what it takes to supply extra NAD+. But this process can not only deplete tryptophan and serotonin, but also produce a number of intermediate compounds that can are potentially neurotoxic.

The concentration of potentially neurotoxic compounds, such as 3OH-kynurenine, 3-OH-anthranilic acid, and quinolinic acid (QUIN) that are formed along the metabolic pathway leading from tryptophan to NAD (the kynurenine pathway) significantly increases in blood and cerebrospinal fluid of patients affected by a number of inflammatory neurological disorders and in animal models of immune activation.
So what is the messenger that reductive stress state cells produce, which triggers IDO in macrophages?
Why not the extra superoxide that is produced? This potentiates NF-kappaB,

Under normoxic conditions, NFκB is bound to one of several inhibitory proteins (e.g., IκB) that prevent its nuclear translocation. Hyperoxia or elevations of ROS cause the ubiquination and destruction of the inhibitory proteins, freeing NFκB and allowing it to bind to target gene promoters.

These target genes include INF-gamma and TNF-alpha. INF-gamma drives IDO in macrophages.
Nicotinamide is an inhibitor of poly (ADP-ribose) polymerase-1 (PARP-1) that, through enhancement of nuclear kappa B-mediated transcription, plays a pivotal role in the expression of inflammatory cytokines, chemokines, adhesion molecules, and inflammatory mediators. Through interaction with CD38 and inhibition of IL-1, IL-12, and TNF-α production, nicotinamide produces a mild TH2 bias. 

(in other words, the product of the inflammatory cascade that begins with reductive stress will inhibit it.)
So this may be part of a homeostatic survival mechanism in B3 deficiency (pellagra), which reductive stress imitates. In which case inflammation and depression is secondary to metabolic disregulation in diet-and-lifestyle related diseases. It is, so to speak, a perfectly natural, adaptive consequence of eating the wrong things and forgetting to exercise.

Edit: Aubrey de Grey's mitochondrial theory of ageing describes a mechanism by which surplus electrons from cytosolic NADH can be exported from the cell, recycling NAD+, via the plasma membrane redox system (pdf). Jettisoned into intercellular space, the electrons combine with O2 to form the superoxide reducing radical; reduction of transition metals (ferretin, ceruloplasmin) by superoxide also creates conditions for generation of oxidising radicals such as hydroxyl and peroxide. (note that RBCs lack the ability that other cells have to transfer electrons to O2 and form superoxide; RBC PMRS is an antioxidant system, other cell-types' PMRS are more likely to produce pro-oxidant effects)
In both that paper and this one, by two different sets of authors, there is a role in the PMRS for ascorbic acid (AA) and its oxidised
form dehydroascorbic acid (DHA is the product of 2-electron oxidation, the 1-electron product of AA is a free radical that becomes more common in the senescent leaves of plants pdf.) 

The authors hypothesize that the increased PMRS in erythrocytes during aging may be a protective mechanism of the system for efficient extracellular DHA reduction and ascorbate recycling under condition of increased oxidative stress.

And, perhaps, increased cycling of ascorbate is also a response to reductive stress. Oxidative stress, when not adaptive, as in the immune response, or toxic in nature, is the product of reductive stress, and both are increased by hypercaloric intakes of micronutrient deficient diets.