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Smoothies are one of the easiest ways to pack more fruit into your day. Toss in a banana, add some berries, blend, and you have what looks like a perfectly healthy drink. But research from the University of California, Davis suggests that this popular combination may have an unexpected downside.
The issue is not that bananas are unhealthy. Instead, it comes down to how certain ingredients interact after they are blended together. In a study published in the Royal Society of Chemistry journal Food & Function, researchers found that fruits with high levels of an enzyme called polyphenol oxidase, or PPO, can sharply reduce the amount of flavanols your body absorbs from a smoothie.
Flavanols are natural plant compounds linked to heart and cognitive health. They are found in foods such as apples, pears, blueberries, blackberries, grapes, cocoa, and other common smoothie ingredients.
The Enzyme Behind Browning Fruit
“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,” said lead author Javier Ottaviani, director of the Core Laboratory of Mars Edge, which is part of Mars, Inc., and an adjunct researcher with the UC Davis Department of Nutrition.
Anyone who has sliced an apple or peeled a banana has seen PPO in action. When the fruit is cut, bruised, or exposed to air, the enzyme helps trigger the browning reaction. The UC Davis team wanted to know whether that same process could also affect the nutrients people hope to get from smoothies.
To test the idea, the researchers used freshly prepared smoothies made with ingredients that naturally contain different amounts of PPO. Bananas have high PPO activity, while mixed berries have low PPO activity.
Bananas Versus Berries
Participants drank a banana based smoothie, a mixed berry smoothie, and a flavanol capsule used as a control. The researchers then analyzed blood and urine samples to see how much of the flavanols became available in the body.
The difference was striking. People who drank the banana smoothie had 84% lower flavanol levels compared with the control. In contrast, the low PPO mixed berry smoothie produced flavanol levels similar to the capsule control.
“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,” Ottaviani said. “This highlights how food preparation and combinations can affect the absorption of dietary compounds in foods.”
The study also included a second test in which participants consumed flavanols along with a high PPO banana drink, but the ingredients were kept from contacting each other before intake. Flavanol levels were still reduced, which suggests PPO activity may continue to matter after consumption, possibly in the stomach.
What This Means for Your Smoothie
The findings do not mean bananas are bad for you. Bananas provide fiber, potassium, and other nutrients, and they can still be part of a healthy diet. The more specific lesson is that bananas may not be the best choice when the goal is to maximize flavanol intake from berries, grapes, cocoa, or other flavanol rich foods.
The Academy of Nutrition and Dietetics has issued a dietary recommendation suggesting 400 to 600 milligrams of flavanols per day for cardiometabolic health. Those compounds are found in foods such as tea, apples, berries, grapes, and cocoa.
For people trying to boost flavanols through smoothies, Ottaviani recommends pairing flavanol rich fruits like berries with ingredients that have low PPO activity. Good options include pineapple, oranges, mango, or yogurt.
Bananas can still be eaten on their own or used in smoothies where flavanol intake is not the main goal. But if your smoothie is built around berries, grapes, or cocoa, the better strategy may be to leave the banana out or enjoy it separately.
A Small Study With a Practical Message
The original study was controlled and carefully designed, but it was also small. The first part included eight healthy men, and a second test included 11 participants. That means the results are useful and intriguing, but they should not be treated as the final word for every person or every diet.
Nutrition experts commenting on the research have also urged people not to overreact. Smoothies with bananas can still be nutritious, especially as part of a varied diet. Individual digestion, food patterns, and overall nutrient intake all matter.
The best takeaway is simple: ingredient combinations can change what your body gets from food. A smoothie is not just a pile of nutrients in a glass. How the ingredients interact can affect the final nutritional payoff.
Why Flavanols Remain a Hot Research Topic
The smoothie finding fits into a larger area of nutrition research focused on flavanols and other plant bioactives. These compounds are being studied for possible benefits related to blood flow, blood pressure, cholesterol, glucose regulation, and brain health. The Academy of Nutrition and Dietetics guideline described moderate evidence for 400 to 600 milligrams per day of flavanols to support cardiometabolic health, while emphasizing food sources rather than supplements.
Recent cocoa flavanol research has produced a more nuanced picture for cognition. In the COSMOS related research program, cocoa extract containing 500 milligrams of flavanols per day did not show broad cognitive benefits for everyone, but some analyses suggested potential benefit among older adults with lower habitual diet quality.
That makes the smoothie study especially practical. If people are choosing berries, cocoa, or grapes for their flavanols, then preparation and pairing may matter. More research is still needed, but the idea is easy to apply at home.
Better Smoothie Combos for Flavanols
If the goal is a flavanol friendly smoothie, try combining berries with low PPO ingredients such as mango, pineapple, orange, or yogurt. These options can keep the drink sweet and creamy without adding the high PPO activity found in bananas.
For banana lovers, there is no need to give them up. Just consider separating your smoothie goals. Use bananas when you want creaminess, potassium, and sweetness. Use berries, cocoa, grapes, or apples with lower PPO partners when you want to preserve more flavanols.
The research may also point beyond smoothies. Ottaviani said tea, another major source of flavanols, could be affected by preparation methods that change how many flavanols are available for absorption.
“This is certainly an area that deserves more attention in the field of polyphenols and bioactive compounds in general,” said Ottaviani.
Jodi Ensunsa, Reedmond Fong, Jennifer Kimball and Alan Crozier, all affiliated with the UC Davis Department of Nutrition and researchers affiliated with the UC Davis Department of Internal Medicine, University of Reading, King Saud University and Mars, Inc. contributed to the research.
The study was funded by a research grant from Mars, Inc., which collaborates with researchers to study potential benefits of cocoa flavanols for human health.