Chronic low-grade inflammation (or chronic silent inflammation)

Highlight – Why this is a central topic
Although intermittent increases in inflammation are essential for survival during physical injury and infection, recent research has revealed that certain social, environmental, and lifestyle-related factors can promote systemic chronic inflammation (SCI), which in turn may lead to a variety of diseases that collectively represent the leading causes of disability and mortality worldwide, such as cardiovascular disease, cancer, diabetes mellitus, chronic kidney disease, non-alcoholic fatty liver disease, and autoimmune and neurodegenerative diseases.

References
Furman et al., Science, 2019
Calder et al., Nutrients, 2017

What is inflammation
Inflammation is a central component of innate (nonspecific) immunity. In general terms, inflammation is a local response to cellular damage characterized by increased blood flow, capillary dilation, leukocyte infiltration, and localized production of a series of chemical mediators that contribute to the elimination of toxic agents and the repair of damaged tissues.

It is now clear that the termination (also known as resolution) of inflammation is an active process involving cytokines and other anti-inflammatory mediators, particularly lipid mediators, rather than a simple shutdown of pro-inflammatory pathways.

Inflammation acts both as a “friend and a foe”: it is an essential component of immune surveillance and host defense; however, a persistent inflammatory state over time is a pathological feature of a wide range of chronic conditions.

References
Medzhitov, Nature, 2008
Serhan et al., Nature, 2007

Acute inflammation
Acute inflammation is the body’s rapid, short-term response to injury or infection, characterized by redness, swelling, heat, and pain. It is a beneficial process that helps protect against pathogens and initiates tissue repair. Although it may last from a few hours to a few days, it differs from chronic inflammation, which persists for longer periods and can be harmful.

(Personal note: The classic signs of acute inflammation—heat, redness, swelling, pain—indicate that the body is fighting and healing.)

References
Abbas et al., Cellular and Molecular Immunology
Serhan et al., Nature, 2007

Chronic low-grade inflammation
Low-grade, or “silent,” inflammation is a chronic, non-infectious, low-intensity immune response that persists for months or years. It is often triggered by obesity, metabolic stress, and poor nutrition, which includes not only unhealthy food choices but also incomplete digestive processes and microbiota imbalances.

This condition is characterized by slightly elevated blood markers that are often technically within normal ranges (such as CRP), making clinical diagnosis extremely challenging. It acts as a “silent killer,” serving as a precursor to serious conditions such as diabetes, heart disease, and chronic pain.

Key aspects of low-grade inflammation include:
Multifactorial causes:

In addition to physical inactivity and environmental factors, metabolic disturbances and alterations of the intestinal barrier play a crucial role. When food is not properly digested, it can trigger a persistent immune reaction that fuels the inflammatory state.

Systemic impact:

This chronic state causes mild but continuous tissue damage, directly linked to diseases such as Alzheimer’s disease, type 2 diabetes, cardiovascular disorders, and certain cancers.

How to diagnose it:
A. First phase:

Because standard tests do not detect acute abnormalities, diagnosis must rely on analysis of persistent symptoms such as unexplained fatigue, chronic pain, and cognitive changes (brain fog).

B. Second phase:

High-sensitivity C-reactive protein (hs-CRP) blood test. Unlike standard CRP, hs-CRP can measure values below 0.3 mg/dL, allowing detection of minimal fluctuations that would otherwise remain invisible.

C. Third phase

Interlukin-6 (IL-6). This is a specialized test. In most laboratories, IL-6 is considered “normal” up to about 5–10 pg/mL. In acute infection, IL-6 can rise to 100 or 1000 pg/mL.
In low-grade inflammation, IL-6 may increase from 1 to 3 pg/mL.
Although tripled (and therefore abnormal), the laboratory result will still read “Below limit: NORMAL.” This is why it is an “elusive” marker for general practitioners, but an “advanced biomarker” for specialists who can interpret subtle variations. Specialists often evaluate IL-6 together with the Neutrophil-to-Lymphocyte Ratio (NLR), a simple calculation from the complete blood count that confirms whether the immune system is in a state of chronic alert.

References
Minihane et al., British Journal of Nutrition, 2015
Hotamisligil, Nature, 2006
Pearson et al., Circulation, 2003
Lucius, Integrative and Complementary Therapies, 2023

Low-grade chronic inflammation and systemic inflammation
When the inflammatory state simultaneously involves multiple body districts, it is referred to as systemic inflammation. This condition may arise either from the generalization of an acute inflammatory process or from the progressive extension of an initially localized low-grade chronic inflammatory state.

The intestine represents one of the main sites of origin due to its extensive surface area, intense immune activity, and interaction with the microbiota. However, the process affects numerous organs and tissues.

References
Furman et al., Science, 2019
Franceschi et al., Cell, 2018

Global prevalence
Chronic inflammatory diseases are the leading cause of death worldwide. It is estimated that about 3 out of 5 people globally die from diseases linked to chronic inflammatory processes.

“Chronic inflammatory diseases are the most significant cause of death in the world. The World Health Organization (WHO) ranks chronic diseases as the greatest threat to human health. The prevalence of diseases associated with chronic inflammation is anticipated to increase persistently for the next 30 years in the United States. in 2000, nearly 125 million Americans were living with chronic conditions and 61 million (21%) had more than one. In recent estimates by Rand Corporation, in 2014 nearly 60% of Americans had at least one chronic condition, 42% had more than one and 12% of adults had 5 or more chronic conditions. Worldwide, 3 of 5 people die due to chronic inflammatory diseases like stroke, chronic respiratory diseases, heart disorders, cancer, obesity, and diabetes. 2022”.

References
Furman et al., Science, 2019

Main causes and triggering factors
Gut dysbiosis: Alteration of the intestinal bacterial flora, which may be caused by an unbalanced diet, excessive use of antibiotics, or other toxic substances.

Unhealthy diet: Excessive consumption of processed foods rich in refined sugars and saturated fats, which can promote inflammation.

Stress: Chronic stress can negatively affect the immune system and increase susceptibility to inflammation.

Environmental pollution and toxins: Exposure to chemicals present in the environment or in food may contribute to oxidative stress and inflammation.

Smoking and alcohol: These factors can worsen oxidative stress and damage cells, thereby promoting inflammation.

References
Cani et al., Diabetes, 2007
Tilg & Moschen, Gut, 2014
Egger & Dixon, AJPM, 2014
Slavich & Irwin, Psychological Bulletin, 2014

Common symptoms
Digestive disorders: Bloating, abdominal cramps, diarrhea or constipation, which may vary in intensity and frequency.

Persistent fatigue: Chronic tiredness, lack of energy, and difficulty concentrating.

Joint pain: Widespread muscle and joint pain.

Skin alterations: Rashes, eczema, or other skin manifestations.

Sleep problems: Difficulty falling asleep or maintaining deep sleep.

Skin manifestations

References
Dantzer et al., Brain Behav Immun, 2008
Miller et al., Biol Psychiatry, 2009

Long-term consequences
If left untreated, low-grade intestinal inflammation may contribute to the development of chronic diseases such as:

Cardiovascular diseases: Increased risk of heart attack, stroke, and other cardiovascular conditions.

Type 2 diabetes: Higher likelihood of developing insulin resistance and diabetes.

Autoimmune diseases: Increased susceptibility to conditions such as rheumatoid arthritis, lupus, etc.

Neurodegenerative disorders: Increased risk of developing diseases such as Alzheimer’s or Parkinson’s.

Certain types of cancer: Increased risk of developing some cancers.

General measures that may help reduce inflammation
Follow a balanced diet: Rich in fiber, fruits, vegetables, and whole foods, with a low glycemic index.

Reduce intake of processed foods, refined sugars, and saturated fats.

Manage stress: Through relaxation techniques, meditation, yoga, or other stress-reducing activities.

Maintain a healthy weight: Obesity and overweight can increase inflammation.

Limit alcohol consumption and quit smoking.

Supplement with probiotics: They may help restore the balance of the intestinal bacterial flora.

References
Estruch et al., NEJM, 2018
Calder et al., Br J Nutr, 2011

Note
Low-grade chronic inflammation (or “silent” inflammation) is a key factor in the development and progression of cardiovascular diseases, including atherosclerosis, hypertension, and myocardial infarction. This often asymptomatic process causes endothelial dysfunction, stimulates the formation and rupture of atherosclerotic plaques, and may lead to acute coronary syndromes.

References
Ridker et al., NEJM, 2017
Libby, Nature, 2002

Topics covered in the in-depth study
1 – Paradigmatic cases (Obesity, Metabolic syndrome, Rheumatoid arthritis (autoimmune disease), Biomarkers.
2 – Appendix A: Generalized inflammation
3 – Appendix B: Undigested food
4 – A special case: the role of gluten

Bibliographic references