Probiotics and Prebiotics: What Is the Difference?

Walk into any Irish pharmacy or health shop and you will find a long shelf of probiotics, with prebiotics and “synbiotics” next to them. The labels promise a lot and explain little. It helps to start with the basic definitions, then look at what the research does and does not support.

What is a probiotic?

The widely used definition, from an expert panel convened by the World Health Organization and the Food and Agriculture Organization, is live microorganisms that, given in adequate amounts, confer a health benefit on the person taking them. Two parts of that sentence matter. The microbes must be alive, and there must be evidence of a benefit for that particular strain.

Most probiotic products contain bacteria from the genera Lactobacillus and Bifidobacterium, along with the yeast Saccharomyces boulardii. The name has three parts: genus, species and strain, such as Lactobacillus rhamnosus GG. The strain code matters because benefits found for one strain cannot be assumed for another with the same species name.

What is a prebiotic?

A prebiotic is a substance that your gut bacteria use as food, and that benefits you as a result. In practice, these are mostly fibres, such as inulin, fructo-oligosaccharides and galacto-oligosaccharides. You do not take them in the hope of adding new bacteria. You take them to support the ones already living in your bowel.

Good food sources include:

• onions, garlic and leeks
• oats and barley
• bananas, especially slightly underripe ones
• chicory root, asparagus and artichokes
• beans, lentils and chickpeas

When bacteria ferment these fibres they make short-chain fatty acids such as butyrate, which the cells lining the bowel use for energy. That is one reason fibre keeps coming up in discussions of gut health.

Probiotic foods and supplements

Foods made by fermentation, such as live yoghurt, kefir, sauerkraut and kimchi, contain live microbes, and they are a pleasant way to add variety to your diet. Keep in mind that the bacteria in a food are not always the same as the studied strains in supplements, and heat-treated products such as many shelf-stable sauerkrauts no longer contain live cultures. Check for “live” or “unpasteurised” on the label.

Supplements are different from foods because the strain and the dose are listed, and you can match them to a purpose. That does not mean they all work.

What does the evidence say?

Honest answer: it depends on the strain and the reason. A few points come up repeatedly in reviews:

• Some strains have been shown to reduce the risk of diarrhoea linked to antibiotics.
• For irritable bowel syndrome, results are mixed. Some people report fewer symptoms with certain strains, while large analyses find the effects small and inconsistent from one study to the next.
• Benefits for general wellbeing in healthy adults are much less certain.
• Prebiotic fibres can increase bifidobacteria, but they can also cause gas and bloating, particularly at higher doses.

None of this says probiotics are useless. It says they are not a general fix, and the claim that any one product will “balance your gut” goes beyond what the research shows.

How to choose a probiotic with some care

If you decide to try one, these points can help you avoid wasting money:

1. Look for the full strain name. A label that says only “Lactobacillus” is not enough.
2. Match the strain to a goal. Search for research on that exact strain and your reason for taking it, rather than relying on a general claim.
3. Check the dose in colony-forming units (CFU) at the end of shelf life, not just at manufacture.
4. Read the storage instructions. Some products need refrigeration to stay viable.
5. Give it a fair trial. Four weeks is a reasonable period. If nothing has changed, stop and review.
6. Ask first if you have a health condition. People with weakened immune systems, central lines or serious illness should speak to their doctor before taking probiotics, since rare infections have been reported.

Introduce prebiotic fibre slowly too. A sudden jump can cause wind and bloating, which people then wrongly blame on the bacteria.

Why the starting point matters

Two people can take the same product and get very different results. Part of the reason is that each person’s microbiome is different from the start. Someone who already has plenty of bifidobacteria is not in the same position as someone with very few.

A gut microbiome test gives you a baseline. It cannot tell you which probiotic to buy, and it is not a diagnosis, but it can show how diverse your gut community is and whether groups such as short-chain fatty acid producers look low. A practitioner can use that with your symptoms and diet to decide whether to focus on food first, fibre first or a targeted supplement, and a repeat test later can show whether anything has shifted.

Looking at your own gut

Our GI Axis test reports on diversity, key bacteria and short-chain fatty acid producers from a stool swab you take at home. If you would like a fuller picture that also screens for fungi, parasites and pathogenic bacteria, see GI Axis Advanced. All of our kits are in the shop. If you have persistent digestive symptoms, speak to your GP before starting any supplement.