Probiotics vs fiber: what the research does and doesn't show

The evidence does not show that probiotics or fibre is better; no trial we found pitted them against each other for the same outcome. They are different things: probiotics are live microorganisms, fibre is a plant carbohydrate. In adults with chronic constipation, each beat control in separate reviews, and fibre also raised gas.

What the evidence doesn't support

• A ranking. The two bodies of evidence come from different trials, doses and populations, each against its own control. Setting 66% against 57% across them would be comparing unlike things.
• "A probiotic is a fibre supplement." Probiotics are live microorganisms. Whether a given capsule also contains a prebiotic fibre is printed on its label; it is not a property of probiotics.
• Any probiotic mixture as a constipation aid. In the review of probiotics in chronic constipation, the effect on stool frequency was significant for Bifidobacterium lactis but not for mixtures of probiotics.
• Fibre as free of downsides. Flatulence was higher with fibre than control in the pooled trials.

What the trials found

Fibre supplements in chronic constipation

A systematic review of 16 randomized trials in 1,251 adults with chronic constipation found 66% responded to fibre and 41% to control (relative risk 1.48, 95% CI 1.17 to 1.88). Psyllium and pectin had significant effects. Benefit showed only at doses above 10 g a day and over four weeks or more; stool frequency and consistency improved and flatulence increased (SMD 0.80). Heterogeneity was considerable, so the authors urged caution.

Probiotics in chronic constipation

A review of 30 randomized trials of probiotics in adults with chronic constipation found 57% responded versus 44% on control (relative risk 1.28, 95% CI 1.07 to 1.52). Stool frequency rose, driven by Bifidobacterium lactis and not by mixtures of probiotics, Bacillus coagulans Unique IS2 or Lactobacillus casei Shirota. Stool consistency did not change. The authors described high heterogeneity and risk of bias and "cautious optimism," not a recommendation. Doses varied by trial.

One trial that put them side by side

A four-week, double-blind, placebo-controlled trial in 250 adults with functional constipation compared polydextrose, psyllium husk, wheat bran plus psyllium, a two-strain probiotic (Bifidobacterium animalis subsp. lactis HN019 and Lacticaseibacillus rhamnosus HN001) and a placebo, each with added oligosaccharides. There was no group-by-time effect on bowel movement frequency or straining. Stool softness scores rose about one point in all four active groups and did not change significantly in placebo. The abstract does not state the probiotic dose. It was one trial of specific products and does not rank probiotics against fibre.

Questions people ask

Do probiotics have fibre?

Probiotics are live microorganisms, not fibre. Some products add a prebiotic fibre such as inulin, which is called a synbiotic; read the supplement facts. See probiotics vs prebiotics.

Should I take probiotics with fibre, or without?

We found no trial showing that adding fibre to a probiotic does more. In the constipation review, synbiotics did not improve stool output. If you try fibre, the trials that showed an effect used more than 10 g a day for at least four weeks, which is also the range where gas increased. Ask a doctor first if you have a bowel condition.

Probiotics or fibre supplements: which one?

The evidence cannot say. Both are studied for constipation, with low-to-moderate confidence. Your diet, symptoms and any medicine matter more to that decision than a supplement label, and a doctor can check for other causes. For the probiotic side, see probiotics benefits: what the research does and doesn't show and how to choose a good probiotic.

When to talk to a doctor

Neither probiotics nor fibre is a treatment for constipation or any other condition. Constipation that persists, blood in stool, severe pain, vomiting or unexplained weight loss need a medical assessment before any supplement. Ask a doctor or pharmacist before starting a probiotic if you are pregnant or nursing, have a weakened immune system or take prescription medicine.