Are Chronic Inflammation and its Metabolic Counterpart, Insulin Resistance, the Common Denominators for All Chronic Behavioral and Neurodegenerative Disorders? – A Review Of the Evidence – Part III


Assuming that inflammation plays an important role in all neurodegenerative and mood disorders, as I suggested in the first two installments of this series, another important question must now be asked: Even though efficacy of antidepressant medication is highly variable with depression, when it is effective could the reason be its impact on inflammation as opposed to the generally accepted mechanism of optimization of neurotransmitter activity, principally serotonin?  If this is true, it would certainly provide powerful proof that inflammation is as much of a cause of depression as is neurotransmitter imbalanceand maybe more so.  However, even if antidepressants are not proven to have an anti-inflammatory effect, this does not conclusively document that inflammation has no role in depression.  Why?  As I mentioned above, given the fact that antidepressants demonstrate poor efficacy so often, the presence of chronic central nervous system (CNS) inflammation could also prove why antidepressants are not effective.  With this complexity in mind, I would like to examine both sides of this controversy.

For me, the paper that best dissects the complicated relationship between depression, antidepressant medication, and inflammation is the paper “Association of depressive disorders, depression characteristics and antidepressant medication with inflammation” by Vogelzangs et al (1).  The authors begin their paper by suggesting that the reason antidepressant medications have limited effectiveness is because of the inflammatory component of depression, which is not addressed by the medications:

“Depression is a complex heterogeneous disorder, which may need a similarly heterogeneous offer of treatment possibilities.  Currently available antidepressant medications largely target monoamine pathways, but treatment of depression is only effective in about a third to a half of patients.  Identification of additional pathophysiological pathways involved in depression (subtypes) is needed to guide the development of alternative treatment strategies.  Increasing interest has been directed to immune dysregulation in depression.  Recently, two meta-analyses have shown that inflammatory marker levels such as C-reactive protein (CRP), interleukin (IL)-6 and tumor necrosis factor (TNF-α) and increased in depressed persons compared with non-depressed subjects.”

In the next quote Vogelzangs et al (1) suggest that the variable response seen with antidepressants is possibly due to the fact that only certain subsets of depressed patients are inflamed:

“It is plausible that immune dysregulation is not generally present in depression, but restricted to particular subgroups of depressed persons.”

What subgroups need to be considered?  First, there may be gender differences:

“First, sex differences might exist, but results are thus far inconsistent.  Stronger, weaker or similar effects have been found for women compared with men for different inflammatory markers.  Inflammation levels fluctuate throughout female life according to hormonal changes owing to phase of menstrual cycle, use of hormonal contraceptives, menopause and use of estrogens, which might influence the relationship between depression and inflammation.”

In addition, significant inflammation with depressed individuals may only appear with more severe depression:

“In line with a dose-response assumption, more severe and/or more chronic disorders can be hypothesized to show the most inflammation.”

Finally, the variable response seen with antidepressant use in highly inflamed depressed patients may be due to the fact that different medications have a different impact on inflammation:

“…antidepressant medication might influence inflammation levels and this effect might differ across type of medication.  As summarized by Miller et al., several studies showed that antidepressant treatment, mainly selective serotonin reuptake inhibitors, was associated with decreases in inflammatory markers.  In contrast, recent results of two large studies suggest that use of antidepressants, mainly tricyclic antidepressants (TCA), is associated with elevated inflammation levels.”

Therefore, as you can see, it is frustratingly clear that, even though there is a clear relationship between depression, inflammation and variable outcomes from the use of antidepressant medication, the nature of this relationship may take many different directions depending on the patient.  These directions could be any of the following:

  1. Success is due to the fact that the antidepressant positively impacted either neurotransmitter activity or inflammatory mediator levels and maybe both.
  2. Failure was due to the fact that the antidepressant medication positively affected neurotransmitter activity but failed to address significant inflammatory activity or the reverse.
  3. Failure was due to the fact that the particular medication used lacked either sufficient neurotransmitter or anti-inflammatory activity or both.
  4. Failure was due to the fact that the particular medication used actually increased inflammatory activity.

The next few quotes I would like to feature from the Vogelzangs et al (1) paper discuss results of their research on depressed patients as the results relate to efficacy and inflammation.  The first quote points out that, in this particular study, only the depressed men demonstrated significant inflammatory activity:

“After taking a large set of possibly confounding factors into account, it was found that men with current depressive disorders had higher levels of CRP, and marginally higher levels of IL-6, but not of TNF-α.  No overall associations were found in women.  Increased inflammation was in particular found in depressed men with an older age of onset (CRP, TNF-α).”

The next quote provides fascinating information on the differing inflammatory effects of different medications in the study participants for whom inflammation was a significant factor, the men.  The medications used by the study participants were selective serotonin reuptake inhibitors (SSRI), serotonin-norepinephrine reuptake inhibitors (SNRI), tricyclic antidepressants (TCA), and tetracyclic antidepressants (TeCA).  As you will see from the quote below, inflammatory activity varied greatly depending on the medication used:

“The highest inflammation levels were found in men using SNRI, TCA or TeCA, while IL-6 levels in men using SSRI were lower compared with medication-free depressed men.”

Could this difference in inflammatory activity be due to the fact that the first three types of medication have an impact on adrenaline production?  The authors state:

“Interesting to note is that the first three classes of medication have a combined serotonergic/noradrenergic effect, whereas SSRI act purely serotonergic.”

What is the relationship between noradrenergic activity and inflammation?  The authors suggest:

“Possibly noradrenergic effects are driving increased inflammation mechanisms.  Noradrenaline is part of the human stress response and has been suggested to potentiate cytokine production.”

Furthermore, SNRI, TCA, and TeCA can adversely affect other metabolic pathways that relate to inflammation:

“Use of SNRI, TCA and TeCA has also been observed to disturb functioning of autonomic nervous system, blood pressure, and the metabolic syndrome.”

Why were only depressed men found to be inflamed in this study?  Vogelzangs et al (1) suggest:

Instead of inflammatory or vascular, the true etiology of depression in this subgroup of men with late-onset depression might have a metabolic nature.  Several studies have confirmed an association between CRP, IL-6 and TNF-α with the metabolic syndrome and visceral fat deposits releasing cytokines.  The metabolic syndrome and in particular visceral fat have been bidirectionally associated with depression late in life, specifically in men.  Men possess higher amounts of visceral fat compared with women and are therefore more likely to experience related inflammation and depression.  Involvement of metabolic processes is further supported by our finding that the association between depression and inflammation in particular decreased after adjustment for BMI.”

Basically, what this quote is pointing out is that the men had a higher allostatic load or, in simpler terms, they were more chronically ill than the women, probably demonstrating a wide variety of metabolic imbalances besides neurotransmitter metabolism disturbances due to, most likely, several environmental stressors.

The next quote I would like to feature provides a very concise overview of all the ways inflammation can affect mood.  Even though I discussed many of these in the first two installments of this series, I feel they warrant the excellent summary provided by Vogelzangs et al (1):

“Several biological mechanisms could further explain the relationship between depression and inflammation.  Depression has been associated with dysregulation of important stress systems of the human body, that is, the hypothalamus-pituitary-adrenal-axis and the autonomic nervous system.  Although the hypothalamus-pituitary-adrenal axis in normal situations should temper inflammatory reactions, prolonged hyperactivity of the hypothalamus-pituitary-adrenal-axis could result in blunted anti-inflammatory responses to glucocorticoids resulting in increased inflammation.  In addition, both decreased parasympathetic as well as increased sympathetic nervous system activity have been associated with increased inflammation.  Furthermore, proinflammatory cytokines might inhibit hippocampal neurogenesis, which could lead to a reduced hippocampal volume, which is also seen in depression.  Also, several inflammatory markers have been shown to promote indoleamine-2,3-dioxygenase activation, which catalyzes tryptophan, the precursor of serotonin, to kynurenine, thereby indirectly reducing the availability of serotonin.”

What is the solution for depressed patients demonstrating high inflammatory activity?  The obvious choice is the use of anti-inflammatory agents:

“Preliminary evidence from studies among patients treated with anti-inflammatory agents for other indications suggests that these agents may have beneficial effects on mood.  One study found positive effects on mood in medically healthy, major depressed patients.”

(Consider use of EPA/DHA HP Select, InflammaSelect, and CurcuSelect from the Moss Nutrition Select professional line).

Another option would be to switch inflamed depressed patients to a mood medication that does not increase inflammatory activity.  Vogelzangs et al (1) state:

“…we found, like others, that SSRI might have a beneficial effect on inflammation, suggesting that SSRI could be effective in depressed patients with immune dysregulation through this anti-inflammatory effect.”

Finally, consider lifestyle changes:

“…behavioral interventions, such as exercise, have been shown to normalize immune and metabolic dysregulation, as well as to improve depressive symptoms to some degree, and might therefore be an indicated treatment for an immune/metabolic depression subgroup.”

Still another paper that discusses why inflammation is an important reason explaining the high failure rate of antidepressant medication is “Inflammation and its discontents: The role of cytokines in the pathophysiology of major depression” by Miller et al (2).  The authors state:

“Relevant to the potential clinical applications of the association between inflammation and depression, data indicate that inflammatory biomarkers may identify depressed patients who are less likely to respond to conventional antidepressant treatment and may provide an indicator of treatment response.  For example, patients with evidence of increased inflammatory activity prior to treatment have been reported to be less responsive to antidepressants, lithium, or sleep deprivation (a potent short-term mood elevator).  Moreover, patients with a history of nonresponse to antidepressants have been found to demonstrate increased plasma concentrations of IL-6 and acute phase reactants when compared to treatment-responsive patients.”

What are the best laboratory markers of inflammation to measure in your patient to determine which medications might be effective or not for depression and which might actually exacerbate the problem?  This question was addressed in the study “Cumulative meta-analysis of interleukins 6 and 1β, tumour necrosis factor α and C-reactive protein in patients with major depressive disorder. by Haapakoski R et al (3).  In this paper the authors state:

“In conclusion, this cumulative meta-analysis confirmed higher mean levels of interleukin-6 and C-reactive protein in patients with major depression compared to non-depressed controls.  No consistent association between tumour necrosis factor-α, interleukin-1β and major depression was observed.”


Now that I have, hopefully, established a firm connection between inflammation and mood and neurodegenerative disorders, another question needs to be asked – What exactly is the source of the inflammatory mediators that are affecting the CNS?  While many valid answers could legitimately be provided, I would now like to discuss one that may have received little or no attention by many in both the alternative and allopathic medical community: the gut.

In “Toward a better understanding of the central consequences of intestinal inflammation” by Nyuyki and Pittman (4) this connection is explored.  The first quote I would like to present demonstrates that individuals experiencing GI dysfunction appear to have a higher incidence of neurologic issues that what might be seen in the general population:

“…there are numerous reports of a higher incidence of neurologic problems among patients with inflammatory bowel diseases (IBD); these neurological complications include seizure disorders, cerebrovascular accidents, peripheral neuropathy, and myopathy.  However, behavioral problems such as depression, fatigue, anxiety, and deficits in executive function are common comorbid symptoms associated with IBD.”

Furthermore, as I suggested above, these associations are all too often poorly understood and, therefore, overlooked:

“Nonetheless, these comorbidities are often overlooked, poorly understood, and poorly controlled.”

The next quote specifically discusses the connection between gut dysfunction and depression:

“There are multiple reports that individuals with IBD show high rates of depression (reaching 25% in adolescents) that are significantly higher compared to those with other chronic diseases.  In Canada, for example, individuals with IBD have depression rates triple those of the general population, and rates of mood disorders and psychological distress are also elevated, often even in periods of remission.  Patients also suffer from high levels of fatigue and lower emotional well-being as well as reduced cognitive function.”

What is the metabolic basis of this association?  As you might expect, one aspect of the metabolic basis would be inflammation:

“Inflammatory processes in the gut may trigger central changes that lead to altered brain function, presumably underlying these behavioral comorbidities.”

Another aspect of the metabolic basis suggested by Nyuyki and Pittman (4) is one you also might expect, disturbances in the hypothalamic-pituitary-adrenal axis:

“Previous data show that autonomic and hypothalamic-pituitary-adrenal (HPA) functions are disturbed in patients with IBD, even during inactive disease states, and similar alterations have also been seen in experimental gut inflammation.”

It is also interesting to note, according to the authors, that, in models of parasitic gut inflammation, altered neuronal excitability and synaptic function has been seen along with accompanying anxiety-like behavior.  In addition, altered glutamatergic transmission has been seen with experimental colitis.

The last two quotes from this paper I would like to present give an overview of how IBD specifically affects neuronal activity:

“Although central cytokine release and alterations in neuronal excitability and synaptic plasticity are a possible mechanism for behavioral comorbidities in IBD, there could be other contributing factors.  For example, cytokines stimulate the hypothalamus to release corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP) to cause downstream HPA axis activation.  It has been known for many years that elevated glucocorticoid levels are found in depressed patients, and it is possible that dysregulated glucocorticoid receptors may be causal for depression.”

With the above in mind, the authors point out:

“Under chronic peripheral inflammatory situations such as IBD, there could be an enhanced activation of the CRH/AVP system, which could potentially modulate comorbid psychopathologies in IBD.”


In the research discussed above I briefly discussed gender differences in the relationship between depression and inflammation.  In “Sex differences in depression: Does inflammation play a role?” by Derry et al (5) these differences are explored in more detail.

To begin, recall from the Vogelzangs et al (1) study discussed above that, in their study, men were more prone to demonstrate a connection between depression and inflammation.  In contrast, it is the position of Derry et al (5) that, worldwide, women are much more likely to demonstrate this relationship.  The authors begin their discussion along this train of thought by discussing worldwide trends in relationship to gender and depression:

“Women experience higher rates of depression than men, a consistent pattern across cultures.  From puberty through their reproductive years, women have a roughly twofold higher risk for major depressive disorder (MDD) compared to men, with 21.3% of women and 12.9% of men experiencing major depressive episodes in their lifetimes.”

It is also interesting to note that women experience inflammatory disorders more often than men, thus providing one explanation for the gender difference in depression:

“Inflammation may be a key contributor to depression for women.  Women have higher rates of autoimmune diseases compared to men, including a twofold to ninefold greater risk for lupus, Hashimoto’s thyroiditis, and rheumatoid arthritis.  Additionally, more women have clinically relevant elevations in CRP, an inflammatory marker indicating cardiovascular risk, compared to men.”

Derry et al (5) than mention a significant vicious circle between inflammation and depression that may more prevalent in women.  Because women generally become more depressed by relationship issues than men, this can create inflammation which, in turn, leads to more depression.  In addition, as will be discussed in more detail, inflammation derived from more direct somatic causes (rheumatoid arthritis, etc.) can lead to depression.  Therefore, depression that may have been created by somatic inflammation can be exacerbated by the development of relationship issues.  Then, the increased inflammation caused by the increased depression can lead to even more depression.  As I hope you can see, this is certainly a very damaging, very clinically relevant vicious circle.  A similar effect has been seen with obesity, which tends to affect women more than men worldwide, where obesity can lead to inflammation which can exacerbate depression caused by negative self-perceptions created by the obesity.  The authors state:

“…multiple factors that elevate MDD risk can also promote inflammation, suggesting overlapping pathways.  Several of these factors, including relationship distress, childhood adversity, and obesity, appear to affect women to a greater extent than men, which may help to explain sex differences in MDD risk.”

The major point brought up by Derry et al (5) is that women exposed to the same inflammatory stimuli as men appear to be more likely to have an adverse mood response than men:

“Women may also be more vulnerable to inflammation-induced mood and behavioral changes compared to men.  In a recent study, women and men had a similar IL-6 and TNF-α responses following a low-dose endotoxin administration.  However, women reported greater increases in depressed mood than men.  Compared to men, women also reported feeling more lonely and disconnected from others in response to endotoxin administration.  Furthermore, women who had larger IL-6 and TNF-α responses also reported feeling more lonely and socially disconnected than those with smaller cytokine responses; the magnitude of inflammatory responses was not related to social disconnection for men.”

In the next few quotes Derry et al (5) explore the vicious circle I described above in more detail:

“Depression enhances inflammation, which may fuel further depressive symptoms over time.  For example, those with more depressive symptoms had larger increases in CRP and IL-6 over several years compared to those with fewer depressive symptoms.  Accordingly, depression and inflammation influence each other bidirectionally.”

The next quote highlights the fact that depression can enhance the inflammatory response that comes with stressors, keeping in mind that the term “stressors” is not limited to psychological stress but refers to any type of stress such as dietary stress, toxicologic stress, exercise stress, etc.:

“Depression also magnifies individuals’ inflammatory responses to stressors.  People with MDD had larger IL-6 responses to a laboratory stressor than those without MDD…”

How does this apply to women?  Consider the following:

“…childbirth evoked greater IL-6 increases in pregnant women with a prior history of depression compared to those without a history of depression.  Following influenza vaccination, pregnant women with more depressive symptoms had greater inflammatory responses than those with fewer depressive symptoms.”

Next Derry et al (5) explore the vicious circle where somatic issues can create inflammation and inflammation can create additional somatic issues.  In turn the additional somatic issues created by the initial inflammation can lead to additional depression:

“Elevated inflammation can induce sickness behaviors such as fatigue, anhedonia, pain, and sleep changes.  In laboratory models of elevated inflammation, these sickness behaviors often precede the onset of depressive symptoms and closely resemble the neurovegetative symptoms of major depression.”

Finally, the authors point out that women are more likely to experience inflammation after the development of somatic symptoms, which can preferentially predispose them to depression:

“Women may be more vulnerable to the inflammatory effects of somatic symptoms.  Following sleep deprivation, women experienced more persistent rises in IL-6 and TNF-α compared to men.  Furthermore, women reported more fatigue on average than men.  In addition, fatigue corresponded to higher CRP among women, but men’s fatigue was not associated with elevated inflammation.  Accordingly, women tend to experience somatic symptoms to a greater extent than men, which may have inflammatory consequences.”

The next few quotes discuss what I mentioned above that relationship difficulties tend to be more proinflammatory in women than men, thus predisposing women to depression more than men:

“Some evidence suggests that marital quality may be closely related to inflammation among women than men.  In the MIDUS study, women who reported lower spousal support had higher levels of IL-6 and CRP than those who reported higher support; the relationship between spousal support and inflammation was not significant for men.”

Similar effects are seen when individuals are lacking in quality friendships and social relationships:

“Social disconnection also contributes to heightened inflammation.  Healthy individuals who reported receiving less support from friends, families, and spouses had higher levels of several inflammatory markers over the course of 5 years than those who reported more support.  In the MESA study, adults with lower social support had higher levels of CRP than those with higher social support.  Similarly, breast cancer patients who reported higher levels of social support before starting treatment had lower levels of inflammation 6 months after their treatment ended compared to those who reported lower social support.”

With the above in mind, it would certainly be expected that adults who experienced childhood adversity would be more likely to demonstrate depression and inflammation.  Furthermore, research has suggested that childhood adversity is more common in women than men.  Derry et al (5) point out:

“A history of adverse childhood events may be more common in women than men.  In a large epidemiological study, women were approximately 1.5 times more likely to have experienced verbal abuse or severe physical abuse and 7.5 times more likely to experience inappropriate sexual contact as a child compared to their male counterparts.”

While we would expect that adults who have experienced childhood adversity are more likely to be depressed as adults, can we also expect that they are more likely to be inflamed?  Consider the following:

“In addition to heightened risk for depression, childhood adversity also has strong links to inflammation across the lifespan.  In prospective studies, children who experienced adverse life events had elevated levels of IL-6 and CRP in later childhood and adolescence.  People with a history of childhood adversity had elevated risk for heightened CRP and major depression at age 32 than those without a similar history.  Among older adults, people with a history of multiple adverse childhood events had higher levels of IL-6 than those without such a history, and childhood abuse was specifically related to higher levels of IL-6 and TNF-α.”

What about obesity, which tends to be more prevalent in women than men worldwide?

“Across the world, more women are obese than men, a characteristic that promotes both depression and inflammation.”

How are inflammation, obesity and depression related?  Derry et al (5) state:

“Inflammation plays a key role in linking obesity and depression.  Adipose tissue releases proinflammatory cytokines and foster chronic low-grade inflammation.  Indeed, obese people have higher concentrations of proinflammatory cytokines, such as IL-6, CRP, and TNF-α, compared to normal weight individuals.  In addition, waist circumference and physical inactivity predict higher inflammation and increased risk for heart disease.”

The authors next point out another vicious circle where obesity can increase inflammation and inflammation can promote obesity:

“Just as obesity contributes to inflammation, elevated inflammation can promote obesity by stimulating the HPA axis and contributing to accumulation of fat tissue.  Importantly, the relationship between body fat and CRP is stronger for women than for men, as demonstrated in a recent meta-analysis.”

Similar to how depression can make individuals more susceptible to increased inflammation and depression from other stressors, obesity will do the same:

“Obesity also primes exaggerated inflammatory responses to stressors.  For example, young women with greater abdominal obesity had larger stress-induced inflammatory responses compared to those with less abdominal obesity.  Furthermore, obesity’s impact on reactivity appears to persist for subsequent stressors.  Following an initial stressor, overweight healthy adults had larger IL-6 responses to a second stressful task compared to normal weight individuals.  Accordingly, repeated stressors may persistently elevate inflammation for obese women, a pathway that could increase their depression and health risks over time.”

Can the cycling of reproductive hormones unique to women predispose them to inflammation and depression?

The authors address this important question with the following:

“…women’s levels of reproductive hormones fluctuate throughout the life span, which has implications for inflammation.  Estrogen levels increase during puberty, one factor that may contribute to increases in MDD among girls during adolescence.  As reproductive hormones rise and fall during the menstrual cycle in pre-menopausal women, corresponding changes in CRP occur.”

What about menopause?  Derry et al (5) note:

“Due partially to low estrogen levels, postmenopausal women experience elevated inflammation, which may affect women’s health during aging.  Indeed, estrogen typically inhibits inflammation, and inflammatory processes play a role in atherosclerosis, cardiovascular disease, and other age-related diseases later in life.  These complex relationships affect other hormone mediators and neurotransmitters, providing promising targets for explaining sex differences in mood disorders.”

Of course, female reproductive physiology, in and of itself, will not create inflammation and depression.  However, as suggested above, when combined with other issues such as obesity, adverse childhood experiences, somatic dysfunction, etc., female reproductive physiology can certainly provide a partial explanation for the gender differences in depression.

Final thoughts from Derry et al (5)

To conclude their paper Derry et al (5) provided this summary of why it is vital to consider inflammation when dealing with depressed women:

“Inflammation may be a key pathway to consider in women’s depression for several reasons.  Women have higher levels of inflammation and higher rates of autoimmune diseases compared to men, both of which elevate subsequent depression risk.  Transient increases in inflammation also affect women’s mood and social disconnection to a greater extent than their male counterparts.  Relationship distress and obesity, both of which elevate depression risk, are more strongly tied to inflammation for women than for men.  Furthermore, women experience several risk factors for inflammation at higher rates than men, including obesity, physical inactivity, and childhood adversity.  Taken together, these findings suggest that women’s susceptibility to inflammation and its mood effects can contribute to sex differences in depression.” 

Moss Nutrition Report #268 – 04/01/2016 – PDF Version


  1. Vogelzangs N et alAssociation of depressive disorders, depression characteristics and antidepressant medication with inflammation. Translational Psychiatry. 2012;2(e79)
  2. Miller AH et alInflammation and its discontents: The role of cytokines in the pathophysiology of major depresson. Biol Psychiatry. 2009;65(9):732-41.
  3. Haapakoski R et alCumulative meta-analysis of interleukins 6 and 1-beta, tumour necrosis factor alpha and C-reactive protein in patients with major depressive disorder. Brain Behav Immun. 2015;49:206-15.
  4. Nyuyki KD & PIttmanToward a better understanding of the central consequences of intestinal inflammation. Annals NY Acad Sci. 2015;1351:149-54.
  5. Derry HM et alSex differences in depression: Does inflammation play a role? Curr Psychiatry Rep. 2015;17(78)