Smoothies are not only a delicious fruit soup, but can also be a quick and easy way to get in some vitamins and minerals. It turns out, however, that the key to their nutrition-boosting abilities could be in the combination of fruits used – and depending on what benefit you’re looking for, bananas might be best left in the fruit bowl.

One such beneficial group of compounds is flavanols, which are naturally found in many popular smoothie components – strawberries, blueberries, and raspberries, for example – and are thought to have benefits for heart and cognitive health.

Researchers set out to test how different levels of polyphenol oxidase (PPO), an enzyme found in many fruits, including bananas, could impact both the level of flavanols in smoothies and absorbed by the body. “We sought to understand, on a very practical level, how a common food and food preparation like a banana-based smoothie could affect the availability of flavanols to be absorbed after intake,” explained lead author Javier Ottaviani in a statement.

The study, which involved eight healthy adult male participants who were required to fast for 12 hours beforehand, was carried out over three days, separated by a minimum of six days. On each study day, volunteers were given a different smoothie: a banana-based smoothie with naturally high PPO activity, a mixed-berry smoothie with naturally low PPO activity, or a flavanol capsule as a control. Blood tests were then carried out to determine the level of flavanols in the body.

Compared to taking the flavanol capsule, those who drank the banana-based smoothie were found to have 84 percent lower levels of flavanols in their body. “We were really surprised to see how quickly adding a single banana decreased the level of flavanols in the smoothie and the levels of flavanol absorbed in the body,” said Ottaviani. “This highlights how food preparation and combinations can affect the absorption of dietary compounds in foods.”

It's a particularly relevant finding after the Academy of Nutrition and Dietetics issued a recommendation in late 2022 suggesting that consuming 400 to 600 milligrams of flavanols per day could help to reduce the risk of many health conditions. For those trying to meet that recommendation, Ottaviani suggested keeping bananas out of berry-based smoothies and vice versa.

The researchers hope the study will encourage further research into how different methods of storage, preparation, and consumption could affect flavanol levels. “This is certainly an area that deserves more attention in the field of polyphenols and bioactive compounds in general,” Ottaviani concluded.

And if smoothies aren’t your thing, you might be pleased to know that flavanols can also be found in chocolate – you’d better go and thank your grandma for keeping you healthy with all those selection boxes over Christmas.

The study is published in Food & Function.