Chronic Pain

What Gut Bacteria Have to Do With Pain Without Injury

The evidence here has moved recently from association to something closer to cause. Chronic pain that persists without ongoing tissue damage is one of the hardest problems in medicine. Fibromyalgia is the clearest example, causing widespread pain, fatigue and poor sleep with nothing structurally wrong to point at. It affects 2% to 4% of people. Because there's no injury to blame, attention has turned to how pain signals are processed, and the gut sits inside that system.

A study designed to rule out the obvious objections

Researchers compared gut bacteria in 77 women with fibromyalgia against 79 women without it, using both broad sequencing and full genetic sequencing of what was present.

The design anticipated the standard criticisms. Some controls lived in the same house as a patient. Others were parents, children or siblings. That controls for shared diet, shared environment and shared genetics far better than recruiting strangers.

About 20 bacterial species were present at different levels between the groups.

The most striking result concerned how much of the variation was explained. Fibromyalgia and its symptoms accounted for more of the difference between people's gut communities than any other factor measured, including diet, age and exercise.

Symptom severity tracked with the abundance of particular species, so this wasn't simply having the diagnosis or not.

Source: Altered microbiome composition in individuals with fibromyalgia

Ruling out diet as the explanation

The obvious counter-argument is that people in chronic pain eat differently, and diet shapes gut bacteria.

The same team tested it directly, assessing dietary intake in detail across the cohort.

Diet didn't account for the bacterial differences or for symptom severity. The pattern remained after taking eating into account.

That doesn't prove direction on its own. Chronic pain also changes sleep, activity and medication use, all of which affect gut bacteria. What it removes is the simplest alternative explanation.

Transferring pain to animals

The step that changes the picture came more recently.

Researchers took stool from women with fibromyalgia and from healthy women, selecting donors whose bacterial composition was representative of each group, and transferred it into mice.

The mice receiving bacteria from women with fibromyalgia developed increased pain sensitivity. Animals receiving bacteria from healthy donors did not.

That's the design that distinguishes cause from consequence. The recipient animals had no illness, no altered diet and no changed behaviour. What they received was a bacterial community.

Source: The gut microbiota promotes pain in fibromyalgia

How a gut signal becomes a pain signal

Several routes are plausible, and they aren't mutually exclusive.

Gut bacteria influence inflammatory signalling throughout the body, and inflammatory molecules act directly on sensory nerves, lowering the threshold at which they fire.

They also shape how tryptophan is handled. Tryptophan is an amino acid from food that the body converts either into serotonin, involved in how pain signals are dampened, or into a family of compounds called kynurenines, some of which act on the same brain receptors involved in pain amplification. Which route dominates depends partly on bacteria.

A separate study found bile acid profiles altered in fibromyalgia, tracking with specific bacterial changes and with symptom severity. Bile acids are made by the liver and modified by gut bacteria, and they act as signals well beyond digestion.

None of these has been shown to be the mechanism in people. Each is measurable and testable, which is more than this field had a decade ago.

What this means for you

Chronic pain deserves proper assessment and a management plan, since exercise therapy, sleep treatment and specific medications all help and no dietary change substitutes for them. In addition, the research above found gut bacteria differing in widespread chronic pain beyond what diet explains, with transferring those bacteria into animals raising their pain sensitivity. A varied, plant-rich diet supports the same gut bacteria this research ties to lower symptom severity. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.