By Emily Reich, RD at Princeton Longevity Center
If you walk into any store and walk on over to the gut health section, you will find a litany of digestive health products. Probiotic capsules, prebiotic powders, fiber supplements, synbiotics, postbiotics, etc… There is such a wide variety of products, but most people are guessing as to what these items actually do!
We hear from patients regularly who are purchasing a multitude of these products without a clear understanding of what they are actually buying or whether it is helping. So let’s slow down and sort through it together, because the answer to what actually matters might be simpler than you think!
Let’s Start With the Basics
Probiotics are live bacteria and yeasts that, when consumed in adequate amounts, can provide health benefits depending on your unique needs. You find them in fermented foods like yogurt, kefir, kimchi, and sauerkraut, and in supplement form. [1]
Prebiotics are non-digestible carbohydrates that feed the beneficial bacteria already living in your gut. Think of them as fertilizer for your microbiome. Common sources include garlic, onions, leeks, asparagus, oats, and underripe bananas. [2]
Dietary fiber is a broader category when compared to the other two. All prebiotic compounds are technically a type of fiber, but not all fiber works as a prebiotic. Some fiber passes through your system largely unchanged, providing other benefits like lowering cholesterol and adding bulk to stool. [3]
A simple way to think about it: fiber is the umbrella, prebiotics are a specific type of fiber that feeds gut bacteria, and probiotics are the live bacteria themselves.
What Probiotics Can (and Cannot) Do
Probiotics have great marketing behind them, but the research on whether or not everyone should be taking probiotic supplements is a bit more nuanced.
The strongest evidence for probiotic use is in specific, targeted situations: reducing diarrhea associated with antibiotic use, managing symptoms of irritable bowel syndrome, and supporting gut recovery after illness. Research shows that across multiple gastrointestinal conditions and their symptoms, probiotics can be found to significantly reduce symptoms of bloating, nausea, abdominal pain, and diarrhea [4].
However, there is an important detail that determines the effectiveness of probiotics. Any and all benefits of probiotics are highly specific to the strain being consumed. There are thousands of strains that have been researched, and likely many more that have not even been discovered yet. Each probiotic supplement that you see in the store, likely has different strains of probiotics in them (for example, Lactobacillus acidophilus, or Bifidobacterium longum) and therefore have different benefits that they provide for various symptoms or medical conditions.
Research has shown that whether a probiotic strain even colonizes your gut depends on your individual microbiome. Every person has a varied microbiome, including what bacteria are present, and therefore consuming any strain may not necessarily be effective for everyone [5]. Taking a random probiotic off the shelf hoping it will improve your overall gut health, without a specific reason, has limited scientific support. That does not mean probiotics are useless. It means they work best as a targeted, intentional tool, not a daily habit without purpose.
Let’s Talk Prebiotics
Prebiotics tend to fly under the radar compared to their probiotic counterparts. Rather than introducing new bacteria into your gut, prebiotics nourish the bacteria you already have!
When beneficial bacteria ferment prebiotic fibers in your colon, they produce short-chain fatty acids (SCFAs), particularly butyrate, propionate, and acetate. These are not just byproducts. They serve as fuel for the cells lining your colon, help regulate inflammation, support immune function, and even play a role in gut-brain communication. [6]
Studies consistently show that prebiotic compounds like inulin and galactooligosaccharides (GOS) promote growth of beneficial Bifidobacterium species, which is one of the most well-studied markers of a healthy microbiome. [7]
The best sources of prebiotics are whole foods you can find at any grocery store: garlic, onions, leeks, asparagus, oats, barley, legumes, and apples. Most people are not eating nearly enough of these on a daily basis, and that gap matters. There are supplement prebiotics available if necessary depending on your unique situation, but for the general population, whole food sources are the best way to go for prebiotics.
Most Importantly: Fiber!
When we start talking about fiber, this is where there is so much comprehensive evidence backing up the benefits for everyone. A large research study that included over 3.5 million participants found that higher dietary fiber intake was associated with a 23% lower risk of dying from any cause, a 26% lower risk of cardiovascular disease mortality, and a 22% lower risk of cancer mortality. [8]
Even further research confirms similar findings, showing that fiber from cereals, vegetables, and fruits was consistently linked to lower all-cause and cardiovascular mortality. Each additional 7 grams of fiber per day was associated with roughly a 9% reduction in cardiovascular risk. [9]
Beyond just mortality, fiber has been shown to assist in regulating blood sugar, supporting healthy cholesterol levels, feeding a diverse microbiome, and promoting satiety. Incorporating adequate fiber is one of the most well-supported dietary interventions available for long-term health. [10] Even with all of that being said, most Americans consume only about half of the recommended 25 to 38 grams of fiber per day!
What Should You Actually Prioritize?
Eat more fiber-rich whole foods. This is the foundation and it has the most consistent evidence behind it. Vegetables, fruits, legumes, and whole grains should be prioritized. Variety matters too, a diverse diet supports a more diverse microbiome, and that diversity is associated with better health outcomes overall.
Include prebiotic-rich foods regularly. Garlic, onions, asparagus, oats, and legumes are easy additions to most eating patterns. If your diet is consistently low in these, a targeted prebiotic supplement may offer benefit, but prioritizing food first is always the best option.
Use probiotics strategically. If you are on antibiotics, managing IBS, or dealing with a specific GI condition, a targeted, evidence-backed probiotic strain is worth discussing with your provider. For general wellness without a specific indication, fermented foods like yogurt, kefir, and miso are a better everyday choice than supplements.
The Bottom Line
Gut health does not have to be complicated, and it should not require a monthly supplement budget. The most powerful thing the majority of people can do is simply eat more fiber from a variety of whole plant foods. That single shift supports your microbiome, your cardiovascular health, your blood sugar, and your longevity in a multitude of ways.
Prebiotics and probiotics both have a place, but they work best when they are supporting an already solid dietary foundation, not replacing one. At Princeton Longevity Center, we believe that the most effective health strategies are the ones you can actually sustain. Sometimes the most impactful changes are also the most straightforward ones!
References
[4] Probiotics and gastrointestinal disorders: an umbrella meta-analysis. PMC. 2025.
[7] Holscher HD. Dietary fiber and prebiotics and the gastrointestinal microbiota. Gut Microbes. 2017.
[9] Yao F, et al. Dietary intake of fiber and risk of mortality. Front Nutr. 2023;10:1153165.