If your morning routine involves tossing a banana into a blender with frozen berries, UC Davis researchers want you to know that combination is likely canceling out most of the cardiovascular benefit you're reaching for — and a simple ingredient swap can fix it.
A study published in the Royal Society of Chemistry journal Food & Function found that people who drank a banana-based smoothie had 84% lower levels of flavanols in their blood and urine compared to a control group — a striking difference that points directly at a common enzyme in bananas called polyphenol oxidase, or PPO. The research was conducted at the University of California, Davis in collaboration with the University of Reading, and received renewed attention this week when UC Davis issued a new institutional press release highlighting the findings.
Flavanols are bioactive compounds found in berries, apples, grapes, cocoa, and tea. They are the reason berry smoothies have a reputation as functional health foods: the Academy of Nutrition and Dietetics recommends consuming 400 to 600 milligrams of them daily specifically to support cardiometabolic health — blood pressure, cholesterol, and blood sugar regulation. What the research shows is that pairing those berry-derived flavanols with a banana may neutralize the bulk of that benefit before it ever reaches the bloodstream.
Polyphenol Oxidase: Banana's Browning Enzyme Doubles as Nutrient Blocker
PPO is the enzyme responsible for the brown color of a cut apple or a bruised banana — it oxidizes phenolic compounds when exposed to air. In a banana, PPO is present at unusually high concentrations relative to other common smoothie ingredients. When a banana is blended with flavanol-rich berries, the enzyme comes into direct contact with those compounds and rapidly degrades them into quinones, which the body does not absorb the same way.
"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, an adjunct researcher at UC Davis and director of the Core Laboratory at Mars Edge, which is part of Mars, Inc.
In the controlled crossover study, eight healthy men each consumed a banana-almond milk smoothie, a mixed-berry smoothie, and a flavanol capsule used as a control. Researchers then analyzed blood and urine samples for flavanol metabolites. Those who drank the banana smoothie showed an 84% reduction in flavanol levels compared to the capsule control. The mixed-berry smoothie, by contrast, produced flavanol levels comparable to the capsule.
A second test, in which participants consumed flavanols and a banana drink at the same time but without blending them together, still showed a reduction in absorption — suggesting PPO remains enzymatically active in the stomach after ingestion and not only during blending. The magnitude of that effect was smaller but present.
Prof. Gunter Kuhnle, a professor of nutrition and food science at the University of Reading who contributed to the study, described the scale of the finding as unexpected: "The extent of the effect from adding a single banana was still very surprising — it had enough polyphenol oxidase to destroy the vast majority of flavanols found in the berries."
What Flavanol Absorption Means for Heart and Brain Health
Flavanols are among the most studied plant bioactives in nutrition science. The evidence base for their cardiovascular effects includes data on blood vessel dilation, blood pressure reduction, platelet function, and blood sugar regulation. The Academy of Nutrition and Dietetics, reviewing 157 randomized controlled trials and 15 cohort studies, found moderate evidence for the 400–600 mg daily recommendation and described the guidance as the first-ever dietary recommendation for a non-essential bioactive food compound.
The cognitive evidence is more nuanced. Research under the COSMOS-Web program found that cocoa flavanol supplementation did not produce broad cognitive benefits across all participants, though some analyses suggested potential benefit among older adults with lower baseline dietary quality.
What the smoothie study adds to this picture is a preparation variable that most consumers would never think to account for: the same daily habit intended to deliver flavanols may be systematically undermining its own purpose, depending entirely on which fruits share the blender.
What the Researchers Actually Recommend
Ottaviani's recommendation is not to boil or microwave the banana before blending — the study does not test or endorse that approach, and the enzyme's activity during digestion suggests heating the banana alone would not solve the stomach-phase effect. Instead, he advises anyone trying to boost flavanol intake to pair flavanol-rich fruits like berries with ingredients that have naturally low PPO activity: pineapple, oranges, mango, or yogurt all qualify and can provide creaminess and sweetness comparable to banana without the enzymatic interference.
Bananas are not the enemy of a healthy diet. They supply potassium, vitamin B6, fiber, and vitamin C, and Ottaviani is explicit that bananas remain a nutritious fruit and an appropriate smoothie base — just not if maximizing flavanol absorption is the specific goal. The practical takeaway is to separate the objectives: use banana when you want texture and potassium; use berries with a low-PPO base when you want the cardiovascular benefit.
Other fruits and vegetables also carry PPO, including beet greens and, to a lesser degree, red delicious apples. The study's implications extend to any smoothie that mixes high-PPO ingredients with flavanol-rich ones.
Study Limitations and Funding Disclosure Worth Knowing
The research was conducted in a small and homogeneous group. The primary experiment included eight healthy men; a second test involved 11 participants. Both trials measured acute flavanol metabolite levels in blood and urine — a pharmacokinetic outcome — rather than long-term health endpoints. Whether the 84% reduction in acute absorption translates directly to a measurable difference in long-term cardiovascular outcomes has not been tested.
The study was funded by a research grant from Mars, Inc., which has a financial interest in promoting flavanol science through its Mars Edge nutrition division. The lead author, Javier Ottaviani, holds his primary appointment at Mars Edge. The funding does not invalidate the findings — the underlying enzyme kinetics are well-established biochemistry, and the study was published in a peer-reviewed journal — but it is a relevant disclosure that readers assessing the research should have.
How to Build Flavanol-Focused Smoothies: Ingredient Swap Guide
For anyone making the change, the framework is simple. Flavanol-rich base ingredients to include: blueberries, blackberries, raspberries, strawberries, dark grapes, cocoa powder (in appropriate formulations), and apples. Low-PPO mixing ingredients to pair them with: pineapple, oranges, mango, yogurt, oat milk, and almond milk.
Ingredients to separate from the flavanol-focused blend: banana, beet greens, and high-PPO apples like red delicious varieties.
If bananas remain a preferred ingredient for their other nutritional qualities, enjoy them alongside a separate berry smoothie rather than blended into it.
Frequently Asked Questions
Why does banana reduce flavanol absorption in a smoothie?
Bananas contain unusually high levels of polyphenol oxidase (PPO), an enzyme that causes fruit to brown when cut or bruised. When blended with flavanol-rich berries, PPO chemically degrades the flavanols into compounds the body cannot absorb as effectively. UC Davis researchers found this reduced flavanol levels in the bloodstream by 84% compared to a berry-only smoothie.
What should I add to a smoothie instead of banana to preserve heart-health benefits?
UC Davis researchers recommend pairing berries with low-PPO ingredients such as pineapple, mango, oranges, or yogurt. These provide sweetness and texture without the enzymatic interference that banana introduces. Bananas are still nutritious and can be eaten on their own or blended separately when flavanol intake is not the priority.
Does blending fruit reduce its nutritional value generally?
For most nutrients, blending has minimal effect on nutritional content. The banana-flavanol interaction is a specific case involving an active enzyme, not a general consequence of blending. The issue is the combination of a high-PPO ingredient like banana with flavanol-rich fruits — not the blending process itself.
How many flavanols should I consume per day?
The Academy of Nutrition and Dietetics recommends 400 to 600 milligrams of flavanols daily for cardiometabolic health. That target is achievable through food sources including berries, tea, apples, dark grapes, and cocoa-based ingredients — but preparation choices, including which fruits share the blender, affect how much of that intake the body can actually absorb.
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