Yet as popular as this theory has become, it is riddled with problems. For example:
- Reducing levels of norepinephrine, serotonin and dopamine does not produce depression in humans, even though it appears to do so in animals.
- Although some depressed patients have low levels of serotonin and norepinephrine, the majority do not. Several studies indicate that only 25 percent of depressed patients have low levels of these neurotransmitters.
- Some depressed patients have abnormally high levels of serotonin and norepinephrine, and some patients with no history of depression have low levels of them. (2)
That is exactly what the most recent research on depression is telling us. A new theory called the "Immune Cytokine Model of Depression" holds that depression is not a disease itself, but instead a "multifaceted sign of chronic immune system activation." (3)
To put it plainly: depression may be a symptom of chronic inflammation.
The connection between depression and inflammation
A large body of research now suggests that depression is associated with a low-grade, chronic inflammatory response and is accompanied by increased oxidative stress.
In an excellent review paper by Berk et al, the authors presented several lines of evidence supporting the connection between depression and inflammation: (4)
- Depression is often present in acute, inflammatory illnesses. (5)
- Higher levels of inflammation increase the risk of developing depression. (6)
- Administering endotoxins that provoke inflammation to healthy people triggers classic depressive symptoms. (7)
- One-quarter of patients who take interferon, a medication used to treat hepatitis C that causes significant inflammation, develop major depression. (8)
- Remission of clinical depression is often associated with a normalization of inflammatory markers. (9)
Interestingly enough, antidepressants (particularly SSRIs) have been shown to reduce the production of pro-inflammatory cytokines like TNF-α, IL-1, interferon IFN-ɣ and increase the production of anti-inflammatory cytokines like IL-10. (11, 12) They also change the gene expression of some immune cells that are involved in inflammatory processes. This suggests that SSRIs are anti-inflammatory, which would explain their mechanism of action if inflammation is a primary driver of depression.
The research on this topic is robust, and the connection between depression and inflammation is now well-established. But if depression is primarily caused by inflammation, the obvious question that arises is, "what is causing the inflammation?"
Common causes of inflammation and depression
If you've been following my blog for any length of time, you know that inflammation is at the root of nearly all modern disease, including diabetes, Alzheimer's, cardiovascular disease, autoimmune disease, allergies, asthma, and arthritis. So perhaps it shouldn't come as much of a surprise that depression is also caused by inflammation.
The downside of this connection is that our modern diet and lifestyle are full of factors that provoke inflammation—and thus cause disease. The upside is that if we address these factors and reduce inflammation, we can prevent and even reverse the chronic, inflammatory diseases that have become such a fixture of industrial civilization.
According to the authors of the Berk et al review paper I referenced above, the following are the most common causes of inflammation that are associated with depression.
There are several problems with the modern diet. It is high in foods that provoke inflammation, such as refined flour, excess sugar, oxidized (rancid) fats, trans fats, and a wide range of chemicals and preservatives. And it is low in foods that reduce inflammation, like long-chain omega-3 fats, fermented foods, and fermentable fiber. Numerous studies have associated the Western diet with major depressive disorder. (13)
One of the most harmful consequences of the modern diet has been the dramatic increase in obesity. Obesity is an inflammatory state. Studies have shown higher levels of inflammatory cytokines in obese people, and weight loss is associated with a decrease in those cytokines. (14) Obesity is closely linked with depression, and while that relationship is likely multi-factorial and complex, inflammation appears to play a significant role. (15)
Disruptions in the gut microbiome and leaky gut (i.e. intestinal permeability) have both been shown to contribute to inflammation and correlate with depression. For example, a leaky gut permits endotoxins called lipopolysaccharide (LPS) to escape the gut and enter the bloodstream, where they provoke the release of inflammatory cytokines such as TNF-α, IL-1 and COX-2. (16) And numerous studies have linked unfavorable changes to the bacteria inhabiting our gut with major depressive disorder. (17)
Stress may be one of the most obvious causes of depression, but the link between stress and inflammation is less well-known. Research has shown that psychosocial stress stimulates the pro-inflammatory cytokine network, including increases in TNF-α and IL-1. (18) These increases in inflammatory cytokines are in turn closely related to depressive symptoms, as described above.
There's a huge amount of evidence indicating that exercise is an effective treatment for depression—in many cases as effective or more so than antidepressant drugs. It has also been shown to prevent depression in healthy people with no pre-existing symptoms. (19) Interestingly enough, while exercise initially produces the same inflammatory cytokines that are associated with depression, that is quickly followed by induction of anti-inflammatory substances. (20) This is known as a hormetic effect, where an initial stressor provokes a compensatory response in the body that has positive, long-term consequences.
Chronic sleep loss has been shown to increase inflammatory markers even in people that are otherwise healthy. (21) And although temporary sleep deprivation has been used to therapeutically improve depression, chronic sleep loss is a well-known contributing factor to developing depression in the first place. (22)
Chronic infections produce ongoing inflammation, so it's no surprise to see that depression is associated with Toxoplasma gondii, West Nile virus, Clostridium difficile, and other pathogens. (23, 24, 25)
Dental caries and periodontal disease
Dental caries and periodontal disease are another source of chronic inflammation, and thus a potential cause of depression. According to one large study of over 80,000 adults, researchers found that people with depression were more likely to have tooth loss even after controlling for several demographic and health factors. (26)
Vitamin D deficiency
Low levels of vitamin D are common in Western populations, and there is growing evidence linking vitamin D deficiency to depression. Vitamin D modulates immune responses to infection, including reducing inflammatory markers like TNF-α and IL-1 that are associated with depression. (27) Supplementation with vitamin D to normalize serum 25D levels has been shown to to reduce inflammatory markers in some, but not all cases. (28)
Final thoughts and recommendations
The early 1980s discovery that inflammatory cytokines produce all of the characteristic signs and symptoms of depression should have made a big splash. For the first time ever, scientists had discovered a class of molecules that were tightly and consistently associated with depression, and, when administered to healthy volunteers, produced all of the symptoms necessary for the diagnosis of depression.
Unfortunately, the "chemical imbalance" theory continues to be the dominant paradigm for understanding depression nearly 30 years after this profound discovery, despite the weak correlation between serotonin, norepinephrine, and dopamine and depressive symptoms. There are probably several reasons for this—and you'd be correct if you guessed that some of them are financial—but I'll leave that discussion for another time.
The significance of this finding is huge—both for patients and clinicians. It shifts our focus from viewing depression as being a disease caused by a chemical imbalance, which often requires medication to correct, to being a symptom of a deeper, underlying problem. It also leads to entirely new avenues of treatment—many of them more effective and safer than antidepressant drugs.
Understanding the physical roots of depression can have a profound effect on people who are suffering from it. Although the stigma surrounding depression has decreased in recent years, many who are depressed still carry the burden of thinking that there's something wrong with them, and the depression they experience is "their fault". When my patients with depression learn that there's an underlying physiological cause of their symptoms, they often feel a tremendous sense of relief and empowerment. What's more, when we address this underlying cause, their mood improves dramatically and they quickly realize that the self-judgment and shame they felt about being depressed was misplaced and unwarranted.
I don't mean to suggest that emotional and psychological factors don't play an important role in depression. In many cases they do, and I've written on that topic before. However, the assumption in mainstream medicine that depression is exclusively caused by those factors is obviously not true, and too often these other potential underlying causes go unexplored. The doctor prescribes an antidepressant, the patient takes it, and that's the end of the discussion.
With this in mind, what can you do if you're suffering from depression? Follow these two steps:
- Adopt an anti-inflammatory diet and lifestyle. This means eating a nutrient-dense, whole foods diet, getting enough sleep, managing stress, engaging in appropriate (not too little or too much) physical activity, and nourishing your gut. For more on how to do this, see my book Your Personal Paleo Code (published in paperback as The Paleo Cure in December 2014).
- Investigate other underlying causes of inflammation. On your own or with the help of a good functional medicine practitioner, explore other possible causes of inflammation that could be contributing to depression. These include gut issues (SIBO, leaky gut, dysbiosis, infections, etc.), chronic infections (viral, bacterial, fungal), low vitamin D levels, dental caries and periodontal disease, exposure to heavy metals and mold or other biotoxins, obstructive sleep apnea, and more.