A Daily Fiber Supplement and Knee Pain: What’s the Link?

Key takeaways

• In a six-week randomized trial involving 117 adults with persistent knee pain and underlying joint changes, a daily inulin supplement reduced pain.

• Participants taking inulin also showed improved grip strength and reduced pain sensitivity—suggesting effects beyond the knee itself.

• Higher levels of the gut hormone GLP-1 were associated with better grip strength, but the study did not establish that the hormone caused the improvement.

• The findings support further investigation of a gut–muscle–pain connection, not replacing exercise or established care with fiber supplements.

Looking beyond the joint

When a knee hurts, the gut is not the obvious place to look. But researchers at the University of Nottingham tested whether feeding gut bacteria could influence pain and physical function. Their findings suggest that what happens in the digestive tract may affect more than digestion.

The INSPIRE trial assigned 117 adults to one of four groups: daily inulin, digitally supported physical therapy, both interventions, or placebo.

After six weeks, both inulin and physical therapy independently reduced knee pain. Inulin also produced improvements in grip strength and pain sensitivity that were not observed with physical therapy alone.

Less pain, a different response to pain

Pain intensity and pain sensitivity are related, but they are not interchangeable.

One reflects how much discomfort someone experiences. The other reflects how strongly their nervous system responds to potentially painful stimulation.

The improvement in sensitivity is therefore intriguing. It suggests that the supplement may have influenced pain processing—not simply something happening locally inside the knee.

That does not mean the trial demonstrated joint repair. It measured changes in symptoms and function, not the rebuilding of damaged tissue.

The gut connection

Inulin is a prebiotic: a fiber that feeds certain gut microbes rather than supplying live bacteria.

When microbes ferment it, they produce short-chain fatty acids, including butyrate. Participants taking inulin showed increased butyrate levels, indicating that the intervention was changing microbial activity.

Researchers also observed higher levels of GLP-1, a gut hormone involved in blood-sugar regulation and appetite, with potential roles in pain and muscle function.

Higher GLP-1 levels were associated with better grip strength. That offers a possible connection between gut activity and physical performance—but not proof of the mechanism.

An increase in the body’s own GLP-1 also should not be treated as equivalent to taking a medication that targets its receptor.

An unexpected strength signal

A knee-focused study finding a change in handgrip strength raises an interesting question: could the intervention have effects beyond the painful joint?

Possibly. But better grip strength does not establish increased muscle mass, stronger legs, or improved whole-body performance.

The result is a reason to investigate further, not evidence that fiber can replace strength training.

Easy to take is part of the story

Only 3.6% of participants assigned to inulin alone dropped out, compared with 21% assigned to physical therapy alone.

That suggests the supplement was relatively manageable during this short trial. It does not prove that it was more effective overall or would be easier to sustain for years.

The most useful future role may be complementary: an additional approach that helps people feel and function better alongside movement-based care.

The takeaway

This small trial suggests that a daily prebiotic supplement may influence knee pain, pain sensitivity, and grip strength within weeks.

The next step is to confirm the benefits in larger, longer studies—and determine whether changes in microbial activity and gut hormones actually explain them.

References:

1. Afroditi Kouraki, Susan Franks, Amrita Vijay, Thomas Kurien, Moira A. Taylor, Stephanie L. Smith, Benjamin Smith, Anthony Kelly, Ana M. Valdes. Effect of Prebiotic Supplementation With and Without Physiotherapy on Pain and Pain Sensitivity in People with Knee OA. Nutrients, 2026; 18 (5): 714 DOI: 10.3390/nu18050714