Chronic inflammation: a low fire that doesn’t go out

Acute and chronic inflammation

Cut your hand and the area turns red, swells and feels warm. When infection or tissue damage occurs, white blood cells and plasma proteins gather at the site to fight off microbes and clear up damage. That is acute inflammation. It flares briefly and then settles, and we could not survive without it.

Chronic inflammation is different: an inflammatory response that does not settle and continues at low intensity for a long time, often without the visible signs of acute inflammation. Researchers call it systemic chronic inflammation or low-grade inflammation. The immunologist Ruslan Medzhitov described an intermediate response triggered not by infection or injury but by tissues under stress or malfunctioning (“para-inflammation”), and suggested it is probably behind the chronic inflammation associated with modern diseases.

Why chronic inflammation draws attention

In a 2019 Perspective in Nature Medicine, researchers from many fields set out how social, environmental and lifestyle factors can promote systemic chronic inflammation, which may in turn lead to cardiovascular disease, cancer, diabetes, chronic kidney disease, non-alcoholic fatty liver disease, and autoimmune and neurodegenerative disorders, together the leading causes of disability and death worldwide.

Ageing is part of the picture. The persistent, low-grade inflammation that develops with age even without infection is called inflammaging; researchers see it sustained by cell debris, nutrient excess and products of the gut microbiota. Inflammation driven by overnutrition and metabolic disease is sometimes called metaflammation.

What research names as drivers

The Nature Medicine paper lists these risk factors for chronic inflammation:

• Chronic infections
• Physical inactivity
• Poor diet
• Environmental and industrial toxicants
• Psychological stress

The list summarises many studies; how much each factor raises inflammation varies by person and circumstance. Diet and the gut microbiome are covered in ultra-processed food and the gut microbiome, and movement and the body’s signals in myokines.

What a CRP test tells you

C-reactive protein (CRP) is a protein the liver releases into the blood when there is inflammation. As the US National Library of Medicine (MedlinePlus) explains, a CRP test shows how much inflammation there is, but not what is causing it or where it is. Besides infection, autoimmune disease and inflammatory bowel disease, levels can rise with smoking, obesity, insomnia, depression and menopausal hormone therapy.

A high-sensitivity CRP (hs-CRP) test picks up very small changes and is used to help estimate heart disease risk. Both tests measure the same protein for different purposes, so results need to be read by a doctor alongside other tests, symptoms and history.

Does lowering inflammation reduce disease?

Inflammation occurring alongside disease is not the same as inflammation causing it. The best-known test of the question is the CANTOS trial, published in 2017. It gave 10,061 people who had had a heart attack and an hs-CRP of 2 mg/L or more an antibody injection (canakinumab) that targets the inflammatory signal interleukin-1β.

At the 150 mg dose, recurrent cardiovascular events were about 15% lower than with placebo, with no change in cholesterol. That supports the idea that inflammation is one causal pathway in heart disease. But fatal infections were more common on treatment, and overall mortality did not differ. Inflammation is tied to the body’s defences; less is not simply better.

What is still unknown, and when to see a doctor

There is no single standard test that diagnoses chronic inflammation, and research is still working out how much inflammation matters for whom. Claims about “anti-inflammatory foods” or “clearing inflammation” mostly go beyond the evidence at this stage.

Unexplained fever or weight loss, joint pain and swelling lasting weeks, persistent diarrhoea or blood in the stool, or fatigue without a clear reason are reasons to see a doctor to check for an inflammatory condition. If a check-up shows a raised CRP, ask your doctor whether to repeat it or test further.