Are Bananas Making Your Smoothies Less Gut-Friendly?

We’ve often talked about polyphenols and their remarkable health benefits made possible through their metabolism by our gut microbiome. I recently came across a study which talks about a subgroup of flavonoids (the largest family of polyphenols), the flavan-3-ols, and the impact that the enzyme polyphenol oxidase has on its bioavailability.

The study by Ottaviani et al. was a controlled, single-blind cross-over study conducted in 8 healthy, male subjects between the ages of 25 and 60 and was published in August 2023 in the journal Food & Function. The research was performed at the Ragle Human Nutrition Research Center at the University of California, Davis.

Like other flavonoids, flavan-3-ols are large molecular bioactive compounds found in many fruits and vegetables which have been linked to beneficial effects on brain and cardiovascular health. Polyphenol oxidase (PPO) has a well-documented role in enzymatic browning of fruits and vegetables, and has been shown to impact nutritional quality through this breakdown.

Prior to this study’s results, it was hypothesized that fruits with high PPO levels can potentially reduce or eliminate the content and bioavailability of the health promoting polyphenol flavan-3-ols, if blended together with other fruits in a smoothie. I add bananas to my smoothies most mornings, and bananas happen to be one of the fruits highest in PPO. So I read this study with some personal interest.

In the first part of the study, subjects were given two types of fruit smoothies with varying PPO content: a banana smoothie with high PPO content, and a mixed berry smoothie with low PPO activity. A different control group consumed a (-)-epicatechin-containing cocoa extract capsule with cow’s milk (epicatechin is a type of flavan-3-ol). In the second part of the study, the drinks resembled the composition of the banana-based smoothie used in part 1, but prevented the exposure of (-)-epicatechin with banana PPO prior to intake. The drinks were the flavan-3-ol drink and a banana blend (FD + BD) and was consumed simultaneously by alternating sips from each; as well as a flavan-3-ol drink (FD) without any banana that served as the control.

As shown in figure 1, the intake of the mixed berry smoothie and the control intervention mixed with the flavan-3-ol-containing extract significantly increased the levels of structurally-related (-)-epicatechin metabolites (SREMs) in the blood. However, when the subjects ingested the banana smoothie, it yielded SREM levels that were significantly lower to those after intake of the other drinks. In other words, SREM levels remained lower when consuming the high PPO drink. For some further clarification, after ingestion, flavan-3-ols are extensively metabolized and rapidly absorbed. Other studies have shown that the intake of (-)-epicatechin results in more than 20 different metabolites, also known as SREMs.

In order to figure out if banana-PPO could also affect the bioavailability of flavan-3-ols after ingestion, another study was conducted where flavan-3-ols contained in the mixed berry smoothie and the PPO contained in the banana-containing smoothie were ingested separately but concomitantly, preventing any direct contact between flavan-3-ols and PPO before ingestion.

As shown in figure 2, the concomitant consumption of the flavanol and banana (FD + BD) drink did not significantly impact the peak plasma concentration, however it did significantly impact the area under the curve values that were about 37% lower than those obtained after ingestion of FD alone, suggesting a significant decrease in SREMs when the drink included the banana blend (high PPO)

The results of this study showed three things:

First, the intake of flavan-3-ols from a high PPO banana smoothie, but not a low PPO mixed berry smoothie, significantly reduced the levels of flavan-3-ol metabolites in blood. This implies that high-PPO containing foods, including bananas, can degrade flavan-3-ols into other molecules, potentially limiting some of the health-promoting aspects of them.

Secondly, after the banana smoothie preparation, flavan-3-ol levels rapidly declined in vitro, and the decrease was prevented by PPO inhibition. This shows that the degradation of flavan-3-ols can occur pre- and post-ingestion, the stomach isn’t doing the degrading.

Lastly, while preventing direct PPO contact in the food preparation did lessen some of the flavan-3-ol loss, it was insufficient to fully prevent the loss of flavan-3-ols when co-ingested (as shown in figure 2).

Together, these results show that PPO plays a direct role in influencing the systemic bioavailability of flavan-3-ols in foods prior to consuming and afterwards.

When I first found out about this study, I came away advising people to keep bananas out of their berry smoothies. I’ve changed my mind or at least changed my sense of how much it matters.

Here’s the context I left out: roughly 9 in 10 American adults fall short of the recommended intake of fruits and vegetables, and only about 12% meet the fruit recommendation at all. Set against that, a 37% reduction in the plasma levels of one class of metabolites, measured in eight men in a single acute study, is a small problem. The large problem is all the fruit people aren’t eating in the first place.

I also think about how findings like this travel. A careful study of enzyme kinetics becomes “bananas cancel out your berries,” and someone decides the smoothie isn’t worth making. That’s a poor trade. A banana-and-berry smoothie still gives you fiber, potassium, vitamin C, and plenty of polyphenols that PPO never touches. Partial flavan-3-ol degradation still leaves you well ahead of no smoothie at all.

So the practical advice is short. If you love bananas in your smoothie, keep adding them. If you want to get the most out of your flavan-3-ols specifically, you have easy options that don’t require giving anything up: drink your green tea or cocoa separately from the smoothie or build a berry blend with yogurt or milk on the days you skip the banana.