Blending a fruit smoothie for breakfast or a midday snack is a convenient (and tasty) way to boost your nutritional intake.
Whole fruits and vegetables supply vitamins, minerals and fibre, along with a myriad of antioxidant and anti-inflammatory phytochemicals.
If your go-to smoothie pairs up banana and berries, however, you might not be getting the nutritional lift you think you are.
That’s according to new research from the University of California, Davis. Certain fruits (and vegetables), it seems, can diminish your smoothie’s flavanol content and the amount your body gets to absorb.
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Flavanol’s natural enemies
Flavanols belong to a category of plant compounds called polyphenols, the largest and most studied group of phytochemicals.
You’ll find flavanols in apples, blackberries, blueberries, sweet cherries, grapes, plums, strawberries, raspberries, nectarines, peaches and pears. They’re also plentiful in green and black tea, cocoa, pinto beans, kidney beans, hazelnuts and pecans.
A higher intake of flavanols has been linked to better memory and cardiovascular health. The U.S. Academy of Nutrition and Dietetics recommends a daily flavanol intake of 400 to 600 mg, largely based on evidence that flavanols help improve cholesterol, triglyceride and glucose levels and blood pressure.
Turns out, some fruits and vegetables are high in an enzyme called polyphenol oxidase (PPO) which breaks down flavanols. The activity of PPO can vary considerably between different types and varieties of fruits and vegetables.
PPO is the enzyme responsible for the browning of certain produce when it’s exposed to air (e.g., sliced apples, peeled bananas), cut or bruised.
The new findings
The study, published last month in the journal Food and Function, looked at whether consuming smoothies made with different PPO-containing fruits affected the amount of flavanols available for the body to use.
To do so, the researchers first asked eight healthy males, ages 25 to 60, to drink two different freshly prepared smoothies.
One smoothie was made with bananas, which have a very high PPO activity; the other was made with mixed berries (blueberries, blackberries, strawberries and raspberries) which are high in flavanols and have low PPO activity.
A standardized flavanol extract was added to each smoothie to even out their flavanol contents.
In addition to the smoothie tests, participants took the same standardized flavanol extract as a supplement with a glass of milk (the control intervention).
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The three tests were performed at least six days apart at the research facility. Blood samples were taken every hour for six hours after consuming the smoothies and the flavanol supplement.
Consuming the banana smoothie with high-PPO activity – but not the mixed berry smoothie with low-PPO-activity – led to significantly reduced plasma (blood) flavanol levels.
After the volunteers drank the banana smoothie, they had an 84 per cent reduction in plasma flavanols compared to after taking the flavanol supplement with milk (e.g., no banana).
The researchers found that adding one banana to the smoothie caused a rapid decline in its flavanol content during the hour after it was prepared.
In a second study, 11 volunteers consumed a flavanol-containing drink and a blended banana drink separately but at the same time by taking alternating sips from each drink. This way, the flavanols wouldn’t come in contact with the banana PPO enzyme before participants drank the smoothies.
The researchers observed that flavanol breakdown by PPO also occurred during digestion.
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Dietary implications, take-aways
These new findings suggest that making a smoothie with high-PPO activity fruits can diminish its flavanol content as it sits in your glass and digests in your stomach.
This controlled study is not without limitations, including its small sample size.
Even so, the reaction of the enzyme PPO with flavanols – and other polyphenols – has been well known for decades.
According to the study’s lead author Javier Ottaviani, adjunct researcher with the University of California, Davis Nutrition Department, “while it will be nice to see these results repeated in a larger population, we believe our findings are generalizable and consistent with the current scientific knowledge in this area.”
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The findings highlight the need to consider not only which types of foods to eat to increase flavanol intake, but also how these foods are prepared and consumed in order to maximize their nutritional value.
So, if you’re trying to hit your daily 400 to 600 mg flavanol target, you might need to rethink the combination of fruit and vegetables you blend into your smoothie.
Consider pairing flavanol-rich fruit (e.g., berries, cherries, grapes, peaches) with ingredients that have low PPO activity such as pineapple, mango, oranges and kale.
Save your potassium-packed banana for smoothies that don’t contain flavanol-rich fruits.
Leslie Beck, a Toronto-based private practice dietitian, is director of food and nutrition at Medcan. Follow her on Twitter @LeslieBeckRD
Leslie Beck