Maximizing flavanol content: Researchers suggest avoiding bananas in smoothies

28 Aug 2023 --- Adding a banana to fruit smoothies reduces the bioavailability of flavanols naturally found in fruits and vegetables, according to US-based researchers. The study suggests that high levels of polyphenol oxidase (PPO) – an enzyme in fruits and vegetables – affect the food flavanols levels to be absorbed in the body.

The researchers created smoothies to test how various levels of PPO affected the bioavailability of flavanols. Fruits high in PPO quickly brown after they are exposed to air, cut or bruised.

The authors state that more research is needed on the types of foods recommended to increase flavanol intake and how they are prepared, stored and consumed to maximize their potential to support health. 

“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,” says lead author Javier Ottaviani, director of the Core Laboratory of Mars Edge, which is part of Mars, Inc., and an adjunct researcher with the UC Davis Department of Nutrition.

“This is certainly an area that deserves more attention in the field of polyphenols and bioactive compounds in general.” 

Flavanols have been linked to heart and cognitive health. The compounds are naturally found in apples, pears, blueberries, blackberries, grapes and cocoa.

Clinical trials  
In a single-blinded, cross-over study, eight healthy men consumed a flavanol-containing banana-based smoothie with high PPO activity, a flavanol-containing berry smoothie (low-PPO activity) and flavanols in capsule format as a control measure. 

After consumption, the researchers analyzed blood and urine samples to measure how much flavanols were present. 

Flavanols are found in apples, pears, blueberries, blackberries, grapes and cocoa.The highest plasma concentration of flavanol metabolites was similar after consumption of the capsules and the low PPO drink. However, consumption of the banana-based smoothie decreased the flavanol metabolite concentration in plasma by 84%. 

“We were astonished 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,” says Ottaviani. “This highlights how food preparation and combinations can affect the absorption of dietary compounds in foods.”

In a follow-up study, eleven participants consumed a high-PPO banana drink with flavanol capsules without mixing the two before consumption. Still, plasma flavanol levels were reduced, which the researchers note points to an effect related to post-ingestion PPO activity degrading flavanols in the stomach. 

They explain that PPO activity in the banana smoothies remained active under simulated gastric digestion conditions and could thus affect the bioavailability of flavanol, even after consumption. 

The research was funded with a grant from Mars, Inc., which collaborates with researchers to study the potential health benefits of cocoa flavanols. 

Mixing smoothies for health  
In 2022, the Academy of Nutrition and Dietetics recommended a 400-600 mg daily consumption of flavanols for cardiometabolic protection. Researchers linked this intake, for example, by consuming two cups of brewed tea daily, to improvements in blood pressure, cholesterol and blood sugar levels. 

Ottaviani suggests that people trying to consume more flavanols should consider preparing smoothies that combine flavanol-rich fruits like berries with low-PPO activity, like pineapple, oranges, mango or yogurt.

The researchers found a substantial range in the PPO activity of 18 fruits, vegetables and plant-derived dietary products. Foods high in PPO include bananas, beet greens and apples, while those low in PPO activity include spirulina, kale and cocoa powder. 

Ottaviani sees bananas as a good fruit for consumption or to add to smoothies but recommends not to combine them with flavanol-rich fruits, such as berries, grapes and cocoa. 

He hopes the study’s findings spur future research into how food preparation affects flavanols. For example, he explains that the preparation of tea, an essential source of flavanols, involves how much flavanols would be available for absorption. 

Similarly, researchers found that specific milk products in a spinach smoothie affected the content of the antioxidant lutein in the leafy green. Adding cow or coconut milk maximized lutein content, while soy milk harmed available levels. 

Flavanol research  
US-based researchers suggest that flavanol deficiency is linked to age-related memory loss in a clinical trial. However, experts pointed to limitations of the study, adding that the relevance of the findings was overstated. 

Earlier this year, the FDA allowed a qualified health claim on cocoa flavanols reducing the risk of cardiovascular diseases for conventional foods. This claim only applies to high-flavanol powder – containing at least four percent of naturally conserved cocoa flavanols – or foods with high-flavanol powder. 

Meanwhile, researchers found that the health benefits of cocoa are resilient to processing, such as fermentation and roasting. Cocoa’s compounds drive these health benefits, such as reduced risk of stroke, coronary heart disease and heart attack. 

By Jolanda van Hal