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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.”

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. If you’re like me, I love adding bananas to my smoothies, and unfortunately, one of the fruits with the highest levels of PPO are bananas.

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.

Fig. 1 Concentrion of structurally related (-)-epicatechin metabolites (SREMs) in plasma after the intake of flavan-3-ols in capsule format (n=8) and flavan-3-ols mixed in different fruit smoothies (n=6).
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.

“…the concomitant consumption of the flavanol and banana drink significantly impacted the area under the curve values that were about 37% lower than those obtained after ingestion of the flavanol drink alone.”

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).

Fig 2. (A) Concentration of structurally related (-)-epicatechin metabolites (SREM) in plasma after the intake of a flavanol drink (FD) and a flavanol drink consumed together with a blended banana drink (FD + BD). (B) Amount of SREMs and gVLM excreted in urine 0-24h after the intake of FD and FD + BD drinks.

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 bioavailability 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-3ols 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.

While bananas are delicious and a great quick-energy snack, and a good source of fiber, based on the results of this study they are not the food that helps you increase your health promoting polyphenol intake. In contrast, it may be best to not mix them in with your berries or other high flavonoid foods, to receive the maximum benefits from them. For those interested, below is a table listing foods with high PPO content: