There’s a great deal of evidence to suggest that consuming polyphenol-rich foods has beneficial health effects. When we hear people talk about foods that are high in antioxidants, they are usually referring to the polyphenol content. Polyphenols are the compounds within plants that protect the plants from things like pests and disease. Those same compounds are thought to have beneficial effects in people. Of the proposed benefits of polyphenols, some appear to be the result of their antioxidant effects, in addition to other unique mechanisms. The reason for the health promoting-effects of polyphenol-rich foods is unclear, but there is research to suggest that gut bacteria may play a major role.
A (Very Brief) Polyphenol Primer
Polyphenols are plant chemicals that serve several important functions within plants. These include regulation of growth hormones and signaling molecules, protection from the sun, and resistance to insects and animals. Polyphenols occur in almost all plants.
There are many compounds within the polyphenol class. Some of the most commonly talked about are anthocyanins, flavonols, and phenolic acids. For the purposes of this post, I am going to talk about polyphenols as a group, and won’t go into specific details about which polyphenols apply to which plants. Part of the reason for this is that many plants have multiple types of polyphenols and it would be nearly impossible to separate out which specific compounds have specific effects. Additionally, there is little value in separating out and identifying individual polyphenols for the purpose of application to lifestyle. We don’t consume any of them in isolation, so to go into specifics about each isn’t particularly useful beyond scientific curiosity (which is extremely valid! But not the point of this post).
There is a great deal of speculation that in humans (and probably in other animals), polyphenols provide a number of health benefits, including bone, gut, and cardiovascular health, as well as cancer prevention, and decreased risk of diabetes and neurodegenerative diseases. Some of the proposed benefits seem to be the result of intact polyphenol compounds (the ones that are absorbed directly by the gut before reaching gut microbes), while others may be the result of interactions with gut bacteria.
Polyphenols and Your Gut Bugs
Consumption of probiotic- and prebiotic-rich foods is generally known to promote the growth of beneficial bacteria in our guts. In addition, it appears that foods and beverages rich in polyphenols exert positive effects on our health through activity in the gut microbiome. The gut microbiome, with its vast diversity of organisms, it is capable of processes that humans have not evolved to perform. Bacteria can do more than ferment non-digestible food particles, and are able to interact metabolically with compounds on a chemical level. Meaning that polyphenol compounds actually feed some gut microbes and improve the population of beneficial bacteria. After consuming the polyphenols, those microbes then produce other compounds that improve the health of the host (that’s us!). Pretty darn cool if you ask me.
When we consume polyphenols, some of the compounds are absorbed into the body before they reach the large intestine. These intact compounds seem to have some health benefits on the body. However, many polyphenols commonly found in foods can’t make it through the gut barrier to be used directly by the body. Those that aren’t absorbed or excreted make it to the colon where they feed gut bacteria. When this happens, the gut microbes metabolize the compounds into what are called aromatic acids. The aromatic acids are simply new compounds created by the gut bacteria that are easily absorbed into the body and can provide more health benefits.
Evidence From Specific Foods
In the human diet, polyphenols are found most commonly in fruits, vegetables, and some beverages such as tea and coffee. Much of the research on polyphenols to date addresses the effects of particular foods or extracts on the gut and overall health. Many different foods have been analyzed, and I would like to take a quick look at the results of some studies of a few of those foods.
Consumption of black tea has been associated with reduced risk of cardiovascular disease. Researchers have found that polyphenols in tea can act as prebiotics for beneficial microbes, while suppressing the growth of pathogenic (“bad”) bacteria. On top of that, when the beneficial microbes metabolize, or “eat”, the polyphenols, the byproducts of metabolism may help to improve gut and overall health. Specific to heart health, when microbes metabolize tea polyphenols, they produce compounds that have anti-inflammatory and blood pressure-lowering properties.
Pomegranates are rich in a particular type of polyphenols called ellagitannins, which have been credited with the health benefits associated with eating pomegranates. These include antimicrobial, antioxidant, anticancer, and anti-inflammatory effects. As with many other types of polyphenols, ellagitannins build up in the colon after passing undigested through the rest of the digestive tract. Researchers have found that when these compounds make it to the colon, they prevent the growth of pathogenic bacteria and leave beneficial bacteria almost completely unharmed. It is very likely that at least some of the health benefits associated with pomegranate consumption can be attributed to the beneficial effects on the gut microbiome. Many berries, especially raspberries, are also high in ellagitannins and likely have similar beneficial effects.
[Side note: Pomegranate extract has been shown to improve the effectiveness of antibiotics in treating MRSA. Very encouraging at a time when cases of antibiotic-resistant infections are rapidly increasing in number.]
Red Wine and Grapes
Research into the effects of red wine extracts on the gut microbiome of rats showed that gut microbes broke down wine polyphenols into compounds that are known antioxidants. These compounds were better at preventing cell damage than were the original intact polyphenols. In this case, a beneficial microbial environment may need to be present in order for the benefits of wine polyphenols to be seen. However, the researchers did not explore the effects of wine polyphenols on the composition of the gut microbiome. Based on other research, I would suspect that the polyphenols also promote a beneficial microbiome composition.
I would be curious to see whether the effects of red wine are exclusive to wine itself or if other grape products have a similar beneficial effect. If grapes are equally or similarly beneficial, it seems to me like a better strategy to consume non-alcoholic food and beverages. Much of the existing research explores the effects of red wine and grape juice together, but there are some studies that look at grapes alone.
Chicks (of the baby bird variety) fed various grape products had higher biodiversity in their intestinal microbiome than those fed the same diet without grape products. The polyphenols also impaired the growth of pathogenic bacteria without harming populations of beneficial bacteria. Very similar effects have been reported in rats treated with grape products.
Cranberry extract rich in polyphenols appears to lead to increases in specific beneficial microbes (Akkermansia spp.) and protected against intestinal inflammation, insulin resistance and diet-induced obesity. Cranberry juice is also well-known as a tool to prevent urinary tract infections. Gut bacteria may metabolize the polyphenols in cranberries and create byproducts that prevent the problematic bacteria from attaching to cells in the bladder, which stops the bacteria from being able to reproduce and cause a UTI.
This is just a small snapshot of some high-polyphenol foods and their bacterial benefits. But it gives you an idea of the ways polyphenol-rich foods can affect your gut microbiome and influence your overall health.
What Does It All Mean for Us?
Every person has a unique microbiome that respond in a unique way to the things we eat and drink. When it comes to polyphenols, however, it seems that just about anyone can benefit from consuming a diet high in polyphenol-rich foods. Foods such as tea and colorful fruits and vegetables have numerous beneficial effects on our microbiome and overall health. There are certainly some caveats and considerations. For example, I don’t eat nightshades, which means polyphenol-rich bell peppers and tomatoes aren’t an option for me. Instead, I stock up on things like berries, beets, green leafy veggies, and green and black tea.
Some other foods that are notably high in polyphenols are blueberries, dark chocolate (yay!), cherries, artichokes, cloves, and peppermint. There are way more than this, and it’s always a good idea to go for as much variety as possible. If you have multiple color options for the same fruit or vegetable, you’re pretty safe if you go with anything that is dark blue or purple. Purple sweet potatoes are some of my favorites and I think they taste way better than the orange ones. Purple carrots are also very good. This strategy also has the added bonus of providing us with lots of micronutrients. And that’s never a bad thing :).
This is total speculation, but I am really interested in the potential implications for people who suffer from digestive issues like SIBO and are unable to consume much in the way of traditional prebiotics such as inulin. If polyphenols act as a different kind of prebiotic (a chemical compound rather than a fiber) and preferentially suppress bad bacteria, this may be a way to feed the good bacteria and promote a healthy and diverse gut microbiome without the need to consume irritating foods.
Anhe et al. (2014). A polyphenol-rich cranberry extract protects from diet-induced obesity, insulin resistance and intestinal inflammation in association with increased Akkermansia spp. population in the gut microbiota of mice. Gut.
Bialonska et al. (2009). The effect of pomegranate (Punica granatum L.) byproducts and ellagitannins on the growth of human gut bacteria. Journal of Agricultural and Food Chemistry, 57, 8344-8349.
Bolca et al. (2013). Gut metabotypes govern health effects of dietary polyphenols. Current Opinion in Biotechnolgy, 24(220-225).
Gonthier et al. (2003). Microbial aromatic acid metabolites formed in the gut account for a major fraction of the polyphenols excreted in urine of rats fed red wine polyphenols. Nutrient Metabolism, 133(2), 461-467.
Lee et al. (2006). Effect of tea phenolics and their aromatic fecal bacterial metabolites on intestinal microbiota. Research in Microbiology, 157(9), 876-884.
Moco et al. (2014). Metabolomics view on gut microbiome modulation by polyphenol-rich foods. Journal of Proteome Research, 11, 4781-4790.
Pandey & Rizvi (2009). Plant polyphenols as dietary antioxidants in human health and disease. Oxidative Medicine and Cellular Longevity, 2(5), 270-278.
Selma et al. (2009). Interaction between phenolics and gut microbiota: Role in human health. Journal of Agricultural and Food Chemistry, 57(15), 6485-6501.
van Duynhoven et al. (2013). Interactions of black tea polyphenols with human gut microbiota: Implications for gut and cardiovascular health. American Journal of Clinical Nutrition, 98(6), 16315-16415.
Viveros et al. (2011). Effects of dietary polyphenol-rich grape products on intestinal microflora and gut morphology in broiler chicks. Poultry Science, 90(3), 566-578.