Prebiotic vs. Probiotic: What’s the Difference?

Pre- and probiotics both aim to improve your health, but they don't do it in quite the same way.
Your gut's microbiome can influence everything from digestion to immune health.
Your gut's microbiome can influence everything from digestion to immune health. / Peter Dazeley/The Image Bank via Getty Images

Walk into any major pharmacy or well-stocked health food store and you’re likely to encounter a shelf—or perhaps even an entire aisle—devoted to probiotic supplements. Culturelle may be the most widely known, but there are dozens and dozens of products that promise to boost or restore your gut health by offering beneficial bacteria. There are also dietary aids, dubbed “prebiotics,” that make many of the same claims.

So what’s the difference between the two, and do they even work? And if so, who should use them and when?

Prebiotics vs. Probiotics

Not all bacteria are bad. Our digestive system is full of microbes that make up our microbiome, which helps regulate everything from how we absorb nutrients to waste elimination and immune system function. According to Harvard Health, a typical large intestine may have 100 trillion bacteria in its microbiome—and they need to be nurtured.

Probiotics add good bacteria to your system, either to bolster the microbes that are already there or to replace those compromised by poor diet, health issues, or antibiotics. Many foods, such as yogurt, kefir, sauerkraut, kimchi, tempeh, and miso, contain probiotics as “active cultures.” While there’s an endless number of strains out there, with many appearing in different combinations depending on the product, some of the most common include Lactobacillus acidophilus and Bifidobacterium longum.

Probiotic supplements contain “live” bacteria, yeast, or fungi present in different amounts (measured in colony-forming units, or CFUs) and concentrations depending on the brand.

Prebiotics, on the other hand, contain fiber from unprocessed fruits, vegetables, and whole grains that feed healthy bacteria in the gut and encourage them to multiply. They can be obtained by eating a balanced, mostly plant-based diet or by taking supplements.

Put another way: probiotics are good bacteria, while prebiotics feed bacteria.

Do Probiotics and Prebiotics Work?

There’s no question we need the right balance and quantity of beneficial intestinal bacteria to remain healthy. But do probiotic supplements do the job?

The science is mixed. Some studies demonstrate that people with ailments like inflammatory bowel disease can benefit from taking probiotics in supplement form, while others show inconclusive or negative results. A 2018 meta-analysis of studies found that probiotics were “generally beneficial” to people with existing gut issues, but efficacy depended on the bacteria strains and the ailments in question.

Part of the problem in finding a definitive answer is the sheer variety of probiotic supplements as well as unique gut biomes. One person may benefit from probiotic use while another might wind up adversely disrupting their bacterial balance. The food we consume, particularly sugary and processed items, can also affect that harmony. It’s not as easy as simply giving someone a probiotic and reporting on the results. Complicating matters is the fact that dietary supplements are not regulated for safety and effectiveness by the Food and Drug Administration, so supplement makers can’t claim that their products treat or cure any particular health condition.

There’s also the issue of whether the probiotic supplement can successfully deliver live bacteria to the gut without being destroyed first, either by improper storage or by your stomach. Spore-based probiotics typically have “shells” that don’t dissolve until reaching the intestine. Certain probiotic brands use encapsulation techniques so the bacteria can go through the stomach without losing efficacy.

Many nutritionists suggest probiotics are best consumed naturally, via a healthy diet. Fermented foods like sauerkraut, apples, and kimchi can boost gut health. What you eat can also have a negative impact: Sugars, alcohol, and simple carbohydrates can damage good bacteria.

Prebiotics aren’t as widely embraced as probiotics. In supplement form, they’re simply purified fiber. Because fiber is important for digestive health, it’s possible prebiotics could help with bowel movements. Some people also opt to take a probiotic and prebiotic combination, called synbiotics. But if you’re getting enough fiber in your diet—the USDA’s Dietary Guidelines for Americans recommend 14 grams of fiber for every 1000 calories of food you eat—you probably don’t need a prebiotic.

Should I Take Probiotics with Antibiotics?

People who wouldn’t normally consider taking a probiotic are sometimes advised to start a regimen either during or after a round of antibiotics. The thinking is that they could “restore” gut flora decimated by a drug that kills off all bacteria, thus lessening unpleasant antibiotic side effects like diarrhea.

One 2018 study looking at probiotic usage in humans and mice after antibiotic use found that the biome was largely restored without probiotic supplementation after a few weeks. Those taking a probiotic demonstrated a disrupted biome and a less diverse bacterial colony than those who skipped them. (You can analyze gut biome in stool, in case you were wondering.)

It’s possible probiotic use with antibiotics could interrupt normal re-colonization of the gut. But again, the sheer variety of probiotic formulations make any definitive conclusion difficult. Some health experts recommend supplementing antibiotic ingestion with S. boulardii, which is a yeast that antibiotics don’t kill and which may help reduce the chances of experiencing antibiotic-related diarrhea.

Do Probiotics Need to Be Refrigerated?

You may find that some probiotic formulations are stocked in the refrigerated section of health food stores. Others are kept at room temperature. Whether a probiotic needs refrigeration depends mainly on how the bacteria is protected or encapsulated. Often, these products will use freeze-dried powders that remain shelf-stable. But certain bacteria, like the Bifidobacterium genus, are more susceptible to warmer temperatures and might lose efficacy if not kept cooled.

Some manufacturers know that not all bacteria will survive owing to temperature and may ultimately offer the colony-forming unit amount taking that into account. The CFU is what’s expected to be left in the supplement after reaching the end of its shelf life.

Some people advise putting shelf-stable probiotics in the refrigerator after purchase to help maintain their potency.

Are Probiotics or Prebiotics Dangerous?

No supplement is guaranteed safe, but both probiotics and prebiotics carry relatively little risk to healthy adults beyond some potential stomach discomfort.

However, premature infants can have dangerous and potentially fatal reactions if they ingest probiotics and come down with infections. People with compromised immune systems or existing bowel disease may also experience side effects from altering an already-dysfunctional gut biome. And remember that the Food and Drug Administration doesn’t approve supplements, so pre- and probiotics aren’t under any regulatory oversight to ensure safety and effectiveness. They may work for you, for someone else, or not at all.

If you’re taking probiotics or prebiotics for overall health or for a specific condition, it’s best to consult with a physician before starting them. If one particular strain or blend doesn’t work, another might. But one thing experts agree on is that no probiotic or prebiotic can help you stomach a bad diet.

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