July 17, 2026, 10:59 am | Read time: 5 minutes
A bit of leafy greens, a handful of berries, and finally a banana–your smoothie is ready. But next time, you might want to skip the banana. FITBOOK nutrition expert Sophie Brünke reveals the negative effects bananas can have and how to make smoothies creamy without the yellow fruit.
Are Bananas Unhealthy?
Here’s a straightforward answer: No! They provide valuable vitamin C, which is important for the immune system. The fiber they contain aids digestion. Since they offer a large amount of potassium and magnesium, bananas are also a perfect snack for athletes. However, there’s a catch when using them in smoothies.
Secondary Plant Compounds for Heart and Brain
A research team from the University of California-Davis wondered in 2023 how people could consume more flavanols.1 These secondary plant compounds, which are abundant in berries, offer numerous health benefits. Flavanols positively affect the cardiovascular system and cognitive functions.
The Academy of Nutrition and Dietetics, the world’s largest organization of nutrition professionals, recommends 400 to 600 milligrams of flavanols per day to prevent heart disease and diabetes.2 It cites the following as particularly good sources:
- a cup of tea (120 to 320 milligrams)
- 150 grams of blackberries (65 milligrams of flavanols)
- an apple (15 milligrams)
- 3 pieces of dark chocolate with 80 percent cocoa content (20 milligrams)
However, many people do not eat enough fruits and vegetables to enjoy these protective properties. In Germany, women eat an average of 3.1 and men 2.4 portions of the recommended five servings of fruits and vegetables per day.3
Smoothies for More Flavanols
The California research team examined whether berry-containing smoothies could be a suitable measure for higher flavanol intake. They conducted two experiments for this purpose.
Bananas Inhibit Secondary Plant Compounds
First, the researchers wanted to test whether the ingredients in bananas affect the bioavailability of flavanols in the smoothie. They suspected the enzyme polyphenol oxidase (PPO), which is involved in the oxidation process that turns bananas brown, as the cause of bananas’ effect on flavanols. The experiment, in which a banana-berry smoothie with high PPO content and a pure berry smoothie were left at room temperature, provides insight: After an hour, the flavanol content in the banana variant was significantly lower than in the berry smoothie. However, if the PPO enzyme in the banana was inactivated beforehand, the flavanol content remained unchanged. These results show that PPO can already limit the availability of flavanols before they enter the human body.
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This Effect Also Occurs in Humans
In a small controlled, blinded crossover study with eight participants, the researchers gave each subject:
- a flavanol-rich berry smoothie
- a flavanol-rich banana smoothie
- a flavanol capsule (control)
On the days when participants consumed the berry smoothie or the capsule, the concentration of flavanol metabolites in their blood increased. However, when they consumed a banana-berry smoothie, the metabolite concentration decreased by a remarkable 84 percent. “We were really surprised at how quickly the addition of a single banana reduced the flavanol content in the smoothie and the amount of flavanols absorbed by the body,” said Javier Ottaviani, the study’s lead author.4
Can the Enzyme Be Outwitted?
To find out if bananas can still reduce the bioavailability of secondary plant compounds in the stomach, the researchers had participants drink both a banana smoothie and a berry smoothie. This was intended to prevent the PPO from interacting with the flavanols before consumption. However, the result was disappointing: The flavanol concentration in the participants’ blood was lower after drinking both smoothies than when they did not consume any part of the banana smoothie.
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How Long Does the Banana Effect Last?
The question remains: How long should you wait after eating a banana to consume berries–since the enzyme still worked in the stomach? The authors do not answer this question. However, they explain that the pH level in the stomach temporarily changes after eating. It only returns to a very acidic state after more than an hour, which slows down PPO. It would be plausible to wait at least an hour after eating a banana before snacking on berries.
Conclusion of the Study
In short: If you want to increase your flavanol intake with smoothies, you should avoid bananas. However, it should also be noted that while the study has a strong design (crossover and control), eight participants are still quite few. Nevertheless, Ottaviani emphasizes that the results illustrate “how the preparation and combination of foods can affect the absorption of nutrients from foods.”