Here’s what Wikipedia says about the link between inflammation and depression:
There is evidence for a link between inflammation and depression. Inflammatory processes can be triggered by negative cognitions or their consequences, such as stress, violence, or deprivation. Thus, negative cognitions can cause inflammation that can, in turn, lead to depression.
In addition there is increasing evidence that inflammation can cause depression because of the increase of cytokines, setting the brain into a “sickness mode”. Classical symptoms of being physically sick like lethargy show a large overlap in behaviors that characterize depression. Levels of cytokines tend to increase sharply during depressive episodes in manics and drop off during remission.
Furthermore, it has been shown in clinical trials that anti-inflammatory medicines taken in addition to antidepressants not only significantly improves symptoms but also increases the proportion of subjects positively responding to treatment.
Inflammations that lead to serious depression could be caused by common infections such as those caused by a virus, bacteria or even parasites.
There does appear to be a link.
Mind And Body: Scientists Identify Immune System Link To Mental Illness, University of Cambridge, Research, 2014
When we are exposed to an infection, for example influenza or a stomach bug, our immune system fights back to control and remove the infection. During this process, immune cells flood the blood stream with proteins such as interleukin-6 (IL-6), a tell-tale marker of infection. However, even when we are healthy, our bodies carry trace levels of these proteins – known as ‘inflammatory markers’ – which rise exponentially in response to infection.
Professor Peter Jones, Head of the Department of Psychiatry and senior author of the study, says: “Inflammation may be a common mechanism that influences both our physical and mental health. It is possible that early life adversity and stress lead to persistent increase in levels of IL-6 and other inflammatory markers in our body, which, in turn, increase the risk of a number of chronic physical and mental illness.”
This potential common mechanism could help explain why physical exercise and diet, classic ways of reducing risk of heart disease, for example, are also thought to improve mood and help depression.
Is Depression A Kind Of Allergic Reaction? A Growing Number Of Scientists Are Suggesting That Depression Is A Result Of Inflammation Caused By The Body’s Immune System, The Guardian, January 2015
Add this to the fact that stress, particularly the kind that follows social rejection or loneliness, also causes inflammation, and it starts to look as if depression is a kind of allergy to modern life – which might explain its spiralling prevalence all over the world as we increasingly eat, sloth and isolate ourselves into a state of chronic inflammation.
I wouldn’t say that social rejection, loneliness, or sloth are unique to modern life. So maybe depression isn’t unique to modern life either, we just medicate it more. In its non-extreme from, depression may be beneficial. There has to be a reason why we continued to select for it as we evolved.
What follows from this, and what excites pharmaceutical companies, is that if you can reduce inflammation, you may be able to treat depression.
Anti-Inflammatory Drugs ‘Could Fight Depression’, The Guardian, December 2014
“Obviously if you get a disease – like rheumatoid arthritis – that causes inflammation, you are likely to get depressed,” he added. “However, recent research shows the link is actually causal.” One piece of evidence is provided by studies of patients with hepatitis. They are often treated with interferon. The drug triggers an inflammatory response that drives out the hepatitis virus. The treatment is very effective – with one major drawback: 30-40% of patients become very depressed.
The link [between inflammation and depression] strongly suggests that it might possible to adapt current drugs that tackle inflammation so that they could help those affected by depression.
Depression has many flavors.
Sadness is only one small part of depression and some people with depression may not feel sadness at all. Different people have different symptoms. Some symptoms of depression include:
- Persistent sad, anxious, or “empty” mood
- Feelings of hopelessness or pessimism
- Feelings of guilt, worthlessness, or helplessness
- Loss of interest or pleasure in hobbies or activities
- Decreased energy, fatigue, or being “slowed down”
- Difficulty concentrating, remembering, or making decisions
- Difficulty sleeping, early-morning awakening, or oversleeping
- Appetite and/or weight changes
- Thoughts of death or suicide or suicide attempts
- Restlessness, irritability, anger
- Aches or pains, headaches, cramps, or digestive problems without a clear physical cause and/or that do not ease even with treatment
Not just depression, but anxiety, irritability, body aches, insomnia, irritable bowel … any of these may have a link to inflammation.