Inflammation can play a massive role in our long-term health. Diabetes, cardiovascular diseases, allergies and asthma, arthritis, and joint diseases are all chronic inflammatory diseases and they are the most significant cause of death across the world (1). Unfortunately, the damage doesn’t stop there, as chronic inflammation can cause inflammation of the brain, leading to neurodegeneration in the form of neurological and mental health issues.
What is inflammation?
Inflammation is an immune system response to any type of foreign object or irritant, which can come in form of a pathogen (virus, bacteria, fungi), an injury (scrape, wound), a foreign object (splinter, thorn), a chemical, or even radiation (2). As soon as the body recognizes there is a threat, the immune system gets to work. Immune cells release various substances, called inflammatory mediators, which begin fighting the irritant (2). Two of these inflammatory mediators are histamine and bradykinin, which serve multiple purposes. They cause blood vessels to dilate, increasing blood flow and allowing more immune cells to enter the area (2). They also irritate nerves, which send pain signals to the brain (2). Each of these responses allows the body to do its job and heal, and alert your mind through many senses (pain, sight, and touch) that the area is injured and requires special care.
How can inflammation be bad for you?
While inflammation is a necessary and important response to harm and irritants, the body can at times mistake normal, healthy cells as foreign objects it needs to attack. This can be found in cases of chronic inflammatory diseases, such as rheumatoid arthritis, psoriasis, and bowel diseases like Crohn’s disease and ulcerative colitis (2). However, widespread inflammation occurs concurrently in the most common conditions, such as diabetes, cardiovascular disease, obesity, depression, and dementia (3). Indeed, more studies are showing that chronic inflammation anywhere in the body can cause brain inflammation.
How can inflammation in one part of the body spread to the brain?
Chronic inflammation that begins in one area (from injury, illness, or microbiota disturbances, especially in the gut) can cause a breakdown in barriers, resulting in escaped pathogens or other harmful substances reaching the bloodstream (1, 4, 5). Once they reach the bloodstream, they can begin to disrupt the blood-brain barrier, increasing permeability and allowing substances that are toxic to enter the brain (1, 6). The immune system is then triggered, causing inflammation in the brain and potentially causing harm. For those with other types of conditions or disorders that cause inflammation, such as rheumatoid arthritis, system-wide inflammation can cause the same pro-inflammatory effect in the brain as well (1, 7, 8). Additionally, consistent levels of environmental toxins, stress, or even multiple episodes of acute inflammation from recurring infection or injury can also cause neuroinflammation (1).
What are the effects of brain inflammation?
While inflammation in any part of the body can cause harm, inflammation in the brain is especially worrisome. Inflammation can affect the physical structure of the brain, creating problems with brain metabolism and tissue structure and function (1). High levels of inflammatory markers can also disrupt or break down the blood-brain barrier. This may allow neurotoxins to enter the brain or lead to abnormal tissue structure, which can cause the immune system to continually respond and begin a cycle of chronic brain inflammation (1). This leads to problems with brain function, including (3, 5, 9, 10, 11):
General cognitive impairment
Memory decline and disorders, including dementia and Alzheimer’s Disease
Neurological disorders such as Parkinson’s Disease
Neuropsychiatric problems and disorders, including low mood, depression, and anxiety
What are the symptoms of chronic inflammation?
While the symptoms of neuroinflammation specifically will vary based on each person and may manifest as any of the disorders listed above, symptoms of chronic peripheral or neuro-inflammation may include (1):
General low mood, depression, anxiety, or other mood disorders
Gastrointestinal complications like diarrhea or constipation
Weight gain or loss
Frequent or recurring infections
If neuroinflammation is so often caused by peripheral inflammation, what are the most common causes of peripheral inflammation? There are four main categories to consider.
1. High Levels of Stress
Stress is something that all of us are familiar with and, especially in our modern world, suffering from chronic stress is almost a given. While it may be easy to write it off as normal, elevated stress levels can be incredibly harmful to the body and brain. Too much chronic stress is a precursor to the development of the most common inflammatory diseases, as well as all of the effects on brain function listed above, because it is directly linked to both peripheral inflammation and neuroinflammation (5, 12). Stress and inflammation are being more commonly recognized as playing a role in the risk for and development of mood and neurodegenerative disorders (12).
2. Arthritis, Gout, and Other Autoimmune Diseases
An autoimmune disease is, as its most basic, an overreaction of the immune system resulting in the body mistakenly attacking its own cells and causing inflammation. Gout, rheumatoid arthritis (RA), and lupus are three such autoimmune diseases that are also directly tied to neurological inflammation and changes (7, 13, 14). Rheumatoid arthritis activates pro-inflammatory pathways in the body that lead to both joint and general peripheral inflammation and can also inflame the brain (7). One therapy for RA is actually so successful at reducing neuroinflammation that it is also protective for Alzheimer’s disease (7).
Gout is another type of arthritis that is caused by too much uric acid in the body (13). Uric acid can trigger the release of pro-inflammatory proteins in joints and in the brain as well. Thus, gout is related to an increase in the risk of neurological disorders such as Alzheimer’s, Parkinson’s, Huntington’s, and other inflammatory diseases. Similarly, common symptoms of lupus, another inflammation-causing autoimmune disease, include cognitive decline, mood disturbances, and mood disorders (14).
3. Allergic Reactions to Environmental Factors
Allergies are chronic inflammatory conditions that affect 20-30% of the world’s population and can occur alongside other conditions such as asthma, sleep disturbances, and sleep apnea (15). They are also associated with neuroinflammation, especially when the reaction is caused by exposure to toxic environmental factors such as smoke, exhaust fumes, environmental compounds, and other chemicals commonly found in thousands of our everyday products (16). The allergic reaction that occurs when the body encounters these allergens is also associated with a release of pro-inflammatory proteins in the brain. As a result, there is an association with dementia and Alzheimer’s disease, as well as mood disorders such as anxiety and depression (15, 17).