Fermented foods and your gut

Are fermented foods good for your gut? Do fermented foods help with symptoms of IBS? Do fermented foods help with weight loss? What are the health benefits of kombucha? Are fermented foods good for the gut microbiome?

Since launching the site, I've been surprised by how many companies are out there making solely fermented products that are in large part marketed on the premise of improving gastrointestinal health. What is the data behind these claims? I thought it was important to release this post the same week that we explore probiotics, because there's quite a bit of overlap in the literature between the two. Let's start with definitions.


What are fermented foods?

Fermented foods are made through controlled microbial growth that produces a natural preservative to bacteria, which would normally cause food to spoil. The "natural preservative" varies based on the raw materials and can range from yeast, acid (most commonly lactic, acetic, or propionic) to even molds or fatty acids. The process of fermentation is a critical part of foods ranging from yogurt, cheese, kimchi, sauerkraut, tempeh, and many more.


How do fermented foods differ from probiotics? 

Preserving bacteria give fermented foods "live microbes" similar to those found probiotics, but different enough to warrant experts to classify it as an entirely different entity and recommend against the use of the term "probiotic" to describe fermented foods. Why? Put simply, probiotics are typically high volume, low diversity: they are chalked full of very specific strains of individual bacteria. In contrast, fermented foods tend to have a diverse mix of live microbes but at a much lower concentration.

If you're too lazy to read any more, the long story short is that the scope of published research is limited for fermented foods compared to probiotics, but let's take a deeper dive and look at the data behind each proposed gut health benefit.


Do fermented foods help with weight loss? 

If you've read our article on fad dieting, you know that you can pretty much throw a dart and it'll land on a study that showed weight loss for any intervention. In a large study, yogurt intake was associated with lower weight gain (not weight loss) than all other foods compared, even including vegetables, nuts, fruits, and whole grains. Another even showed yogurt (and yet another showed kimchi) as helping to reduce diabetes, but the effect was modest. Remember that not all yogurts are created equal, and checking for sugar content is an important first step. While it's certainly possible the live microbes in fermented foods interact with colonic bacteria (aka the gut microbiome) to influence weight loss, it's way too early to jump to that conclusion based on the available data.


Do fermented foods help symptoms of irritable bowel syndrome (IBS)?

Many fermented foods are marketed to the IBS community by piggybacking on the data for probiotics, however there is really no research to support these claims. Fermented foods contain a wide array of food types, and dairy-containing fermented foods may cause more harm than good. The only study we could find is one that looked at IBS symptoms with white versus rye bread, which is technically a fermented food, and found no difference in symptom severity unless many other FODMAP-dietary changes were also made. Could fermented foods improve your IBS symptoms? Sure, but there is likely a contribution of increased fiber intake, a proportional decreased intake of processed foods, and many other variables at play rather than the microbial content of the food group itself. 


Does kombucha have health benefits?

This is probably the most frequent question we've received on the topic of fermented foods, and the answer is simple. There is no scientific evidence suggesting kombucha has any proven health benefits. Companies claim that kombucha can lower blood pressure, stimulate the immune system, treat diabetes, and prevent cancer, but there is literally no data to support these claims. This makes logical sense when you think of what kombucha really is: black tea with so much added sugar that the sugar aides in the fermentation process. We've touched on how added sugar affects the gut in our posts on the microbiome and the FODMAP diet, and knowing how the colon works should be enough to convince you against any gut benefits. Ironically, kombucha is often advertised as probiotic, but it is so acidic that it has actually been shown to instead have antimicrobial activity


The bottom line on fermented foods

Based on the available research, it's still too early to tell if fermented foods improve your gut health in the ways many companies claim that it does. What we do know is that not all fermented foods have the same health benefits, and the best studied is yogurt. If prepared safely, research suggests fermented foods are harmless. Will it cure IBS or cause rapid weight loss? Almost certainly not, but there are definitely more harmful fads out there to explore.

-The Gut Doc


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