“Bananas ruin the antioxidants in berry smoothies.”
Limited confidence because one small industry-linked crossover study found a large reduction in absorption of one polyphenol class when flavan-3-ols were consumed with a high-polyphenol-oxidase banana preparation. It did not show that all antioxidants, total nutrition or health benefits were ruined.
Banana can sharply reduce the amount of one antioxidant family, flavan-3-ols, available from a smoothie. It does not erase all antioxidants or make a banana-berry smoothie unhealthy.
In eight healthy men, a high-polyphenol-oxidase banana smoothie produced much lower blood levels of flavan-3-ol metabolites than a capsule or low-enzyme berry smoothie. This is a narrow bioavailability finding, not a trial of disease outcomes or the overall nutritional value of fruit.
Safety note: This food combination is not known to be dangerous. People with kidney disease, diabetes, food allergy or other nutrition restrictions may need individualized advice about smoothie ingredients and portions for separate reasons.
Talk to a healthcare professional if: Routine medical advice is not needed for combining bananas and berries. Ask a dietitian or clinician if a medical condition requires limits on potassium, carbohydrate or specific foods.
A banana-berry smoothie still provides fruit, fiber, vitamins and other plant compounds. If maximizing flavan-3-ol absorption is a specific goal, use a lower-enzyme base or eat the foods separately.
Keep the smoothie if you enjoy it. For variety, sometimes pair berries with yogurt, citrus, pineapple or another lower-polyphenol-oxidase ingredient.