- A UC Davis study found that adding banana to a berry smoothie sharply reduced the amount of flavanols absorbed by the body.
- The likely reason is an enzyme called polyphenol oxidase, which is high in bananas and can break down flavanol compounds.
- Bananas are still healthy, but they may not be the best partner if the goal is to maximise flavanols from berries, cocoa or grapes.
A banana and berry smoothie looks like an obvious health win.
But a UC Davis study suggests that combination may reduce how much of one group of beneficial plant compounds the body absorbs.
The compounds are flavanols.
They are found in foods such as berries, apples, grapes, cocoa and tea, and have been linked with heart and brain health.
The issue is not that bananas are unhealthy.
The problem is an enzyme called polyphenol oxidase, or PPO.
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This enzyme is involved in the browning you see when fruit is cut or bruised.
Bananas have high PPO activity, while berries have much lower activity.
In the study, participants drank a banana-based smoothie, a mixed berry smoothie and a flavanol capsule as a control.
Blood and urine tests showed that the banana smoothie led to 84% lower flavanol levels compared with the control.
The berry smoothie produced flavanol levels much closer to the capsule.
That suggests the banana was interfering with flavanol availability.
The researchers also tested a setup where flavanols and the high-PPO banana drink did not contact each other before being consumed.
Flavanol levels were still reduced, suggesting PPO activity may continue to matter after swallowing.
This does not mean bananas should be avoided.
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They provide fibre, potassium and other useful nutrients.
But if the point of a smoothie is to maximise flavanols from berries, cocoa or grapes, banana may not be the best ingredient.
Better options may include mango, pineapple, orange or yoghurt.
The wider point is useful too.
A smoothie is not just a list of ingredients blended together.
Food combinations can change what the body actually absorbs.
