If you’re anything like me, then your go-to post-workout smoothie will involve a stack of berries (redcurrants, blackberries, blueberries and raspberries), flaxseeds, vegan protein, oat milk and half a banana. Without that banana, it’s slightly too runny and too tart. And bananas are full of potassium and gut-loving fibre. It’s the ultimate vitamin bomb… right?

Well, maybe not. According to a new study, adding banana to a berry smoothie may actually decrease the nutrient value of the drink. 

Published in the journal Food and Function, researchers from the University of California, Davis, have been looking at how a certain enzyme affects the levels of flavanols (heart-healthy compounds) in foods that can be absorbed by the body.

Blueberries and blackberries are particularly rich in flavanols, as are grapes, pears and cocoa (the nibs of which make for a brilliantly crunchy smoothie topping).

But high levels of polyphenol oxidase (PPO) – an enzyme found in many fruits and veg – can reduce the level of flavanols available. When you peel a banana, the fruit turns brown quickly – and that’s because of PPO. When the enzyme is exposed to air, the food turns brown.

Researchers got participants to drink a banana-based smoothie with naturally high PPO activity, and a smoothie made with mixed berries – which have naturally low PPO activity. They also took a flavanol capsule as a control.

After analysing participants’ blood and urine samples, scientists were able to measure how much flavanols were present in the body after consuming the smoothie samples and capsules. The result? Those who drank the banana smoothie had 84% lower levels of flavanols in their body. 

“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,” said lead author Javier Ottaviani. “This highlights how food preparation and combinations can affect the absorption of dietary compounds in foods.”

He went on to stress that bananas can be a really healthy fruit but that you’re better off eating bananas and other high PPO activity fruits away from flavanol-rich fruits like berries, grapes and cocoa. Tea is also super-rich in flavanols, so if your elevenses tend to involve a cup of tea and a banana, it might be worth choosing a different snack or eating your banana at a slightly different time.

Mixed berry smoothie (banana-free)

Blitz together:

• ½ cup milk (dairy or plant-based) 
• 1 ½ cups of mixed berries (blueberries, blackberries, currants etc)
• Handful of spinach
• 1 scoop of vegan protein (berry flavour or neutral)
• 1 tbsp Greek yoghurt

If you’re making a smoothie bowl, top with lashings of almond butter, cacao nibs, desiccated coconut and a spoon of granola.

Peanut butter and banana smoothie

Blitz together:

• 3 pitted Medjool dates
• 1 small banana
• 1 cup milk
• 1 tsp cinnamon
• 1 tbsp roasted peanut butter 

It’ll come out quite thick, so add more milk or a splash of water if you like it to be more runny. Makes for a delicious dessert!

Images: Getty