In September 2024, Dr. Michael Greger, a respected voice in the vegan community, raised an important question about a breakfast staple for many of us: should we include bananas in our berry smoothies? In his videos (Video 1, Video 2, Video 3), Dr. Greger highlighted research suggesting that bananas’ polyphenol oxidase (PPO) enzyme could degrade flavan-3-ols (compounds found in berries that are great for heart and brain health) so that you get less goodness from your expensive berries. His conclusion? You should skip bananas in your berry smoothies. Use mango or pineapple instead. While Dr. Greger is a great guy and his work at NutritionFacts.org has brought us many great research suggests over the years, there’s more to this story. I didn’t like the conclusion too much myself since I, too, have been drinking daily banana and berry smoothies in the morning since about 1997. So, to follow Dr. Greger’s advice would throw a real kink in my daily meal routine, both for me and my family. We have thrived on these smoothies.
This effect is one that you can see with your own two eyes. You let a warm banana-strawberry shake sit on the counter. At first it is all nice and pink. Come back 30 minutes later, and you wonder where all the color went. It is strikingly pale, especially with commercial strawberries. So, quite a while back we developed some ways to make the color last longer, which indicates that we have limited the PPO enzyme degradation quite a bit.
Anyways, I’ve dug into the research and found practical ways to mitigate PPO’s effects while keeping my favorite smoothies on the menu. Let’s take a closer look at the science, find out what the study really found about berries in smoothies, and then I’ll share some tips you can use, too, to maximize flavan-3-ols in your diet.
The Study: Where are the Flavan-3-ols Really From?
The debate stems from a 2023 study by Ottaviani and coworkers in Food & Function, which found that blending bananas with flavan-3-ol-rich ingredients in smoothies reduced flavan-3-ol bioavailability by a whopping 84%. The Figure 1 from the article is rather astounding and convincing (see below). Flavan-3-ols, like catechins and proanthocyanidins, are linked to reduced frailty and better mental health as we age. But here’s the key detail: the berries in the smoothies didn’t provide enough flavan-3-ols to measure well, so the study’s smoothies were spiked with cocoa-derived flavan-3-ols (70–80 mg (-)-epicatechin), while berries contributed less than 20 mg. The total flavan-3-ol content was high (484 mg for the berry smoothie, 638 mg for the banana smoothie, and 541 for the capsule control) far more than a typical berry smoothie (2–30 mg from 150 g berries, per USDA Database, 2018).
The 84% loss occurred because bananas’ PPO enzyme oxidized flavan-3-ols during blending, where oxygen and tissue disruption activate PPO, and then continued working. A second experiment in the study showed that consuming a banana drink and a flavan-3-ol-rich beverage separately (mixing only in the stomach) led to a much milder 37% loss (shown in the second figure, below). This suggests PPO is less active in the stomach’s acidic, low-oxygen environment (pH ~2–3) compared to inside the blender or on the countertop. So, the study’s big takeaway isn’t just about berries—it’s about avoiding blending high-PPO fruits like bananas with flavan-3-ol-rich ingredients, especially cocoa or tea extracts.
Why I’m Keeping My Banana-Berry Smoothie
As a long-time smoothie drinker who loves my morning mixed berry/banana/flax smoothie, I was initially concerned by Dr. Greger’s videos. But the science shows that for a typical smoothie (150 g berries, 1 banana), the flavan-3-ol loss from berries alone is small—likely no more than 50% with proper mitigation, and closer to 37% in the stomach if I blend smartly. Berries also offer fiber, vitamins, minerals and other phytonutrients. These benefits, plus bananas’ potassium, magnesium, and fiber, make my smoothie worth keeping. And blending pulverizes the berries so well, including the blackberry seeds, so I know I am getting as much as possible from my berries, much more than if I just chewed them. So, here’s how I mitigate PPO’s effects to preserve flavan-3-ols.
Here’s How to Mitigate PPO in Your Smoothie
You don’t need to ditch bananas to enjoy berry smoothies. These science-backed approaches reduce PPO activity before the smoothie reaches your stomach, limiting losses to the ~37% seen in the study’s second experiment:
Keep It Cold
PPO works fastest at 25–40°C but like all other enzymes it really slows down when you get it down to 4°C (39°F). We always use frozen berries, frozen pineapple, and ice in our morning smoothies. They are cold, and this keeps the color longer and quenches the PPO activity of bananas quite a bit.
Add Vitamin C
Ascorbic acid inhibits PPO by reducing oxidized flavan-3-ols and lowering pH. You can include a tablespoon of lemon juice or 50 g strawberries for a good bit of food-based vitamin C. We always put in about 1,000 mg of sodium ascorbate per person into our morning smoothies, because it is generally good for you, and it does help with inhibiting the PPO enzyme. Sodium ascorbate works for this, too, not just ascorbic acid, based on personal experience.
Use Pineapple
Pineapple’s acidity (pH ~3.5), from malic and citric acid, also reduces PPO enzyme activity. Add 100 g (~3 ounces) pineapple chunks for flavor and protection. We almost always put pineapple in our morning smoothies. The proteolytic enzymes in pineapple theoretically could help break down the PPO enzyme in bananas, too, but I don’t have any proof for this. Our smoothies are cold, so the main protection will be in your stomach, not in your glass.
Drink Immediately
PPO needs oxygen and time to degrade flavan-3-ols. Consume your smoothie right after blending to limit pre-digestion losses. You will still lose some flavan-3-ols in your stomach, but the losses beforehand will be minimized.
These steps can reduce flavan-3-ol losses in the glass to near zero, leaving only the ~37% stomach-based loss when bananas and berries mix during digestion. For a smoothie with 20 mg flavan-3-ols (e.g., 150 g berries), you’d retain ~12–16 mg (60–80% bioavailability) vs. ~3 mg without mitigation.
But this next step is really what will get you the flavan-3-ols you are looking for.
Boost Flavan-3-ols with Green Tea
At the beginning of this article I pointed out that berries don’t have a lot of these flavan-3-ols to start with. The real champions are green tea and natural cacao, (found in minimally processed very high cacao products). To ensure you get enough flavan-3-ols, you could add a cup of green tea later in the day, away from your banana smoothie. Green tea is a flavan-3-ol powerhouse, delivering 240–430 mg per 240 mL cup (mostly catechins like EGCG), with 50–260 mg absorbed . This is about 10X as much as you get from berries and easily covers any smoothie losses. Brew 2–3 g tea leaves at 80°C for 2–3 minutes, and drink 1–2 hours before or after bananas to avoid mixing them together in your stomach. (I’m not much of a tea person, but I might take up drinking some green tea just for the health benefits.)
If you don’t like green tea, but already take Hallelujah Diet’s Joint Health supplement, then you are set. You are already getting a health-boosting amount of procyanidins, which are flavan-3-ols as well. Just don’t take your Joint Health supplement with anything that has bananas in it.
Addressing Dr. Greger’s Concerns
Dr. Greger’s videos rightly highlight PPO’s impact, but his call to avoid bananas in berry smoothies is a bit extreme and premature, because it overlooks mitigation strategies and the study’s context. The 84% loss was driven by cocoa-derived flavan-3-ols, not berries, and real-world smoothies have lower flavan-3-ol content (2–30 mg vs. 484–638 mg in the study). With cold temperatures, vitamin C, pineapple, and immediate consumption, losses drop significantly. Plus, separating flavan-3-ol-rich foods (like green tea, cacao products, and grape seed extracts) from bananas ensures high intake. These steps take the punch out of the “no bananas in smoothies” advice, letting us keep our berry-banana smoothies and our green smoothies with bananas and berries in them while maximizing health benefits.
Dr. Greger is right, in one sense. To be a purist and to get the very most out of your berries, you can remove the bananas. But, for me, that just takes the joy and delight and a lot of nutrition right out of my smoothie, for a very small gain. Not worth it for me. I’d rather have a cup of green tea and get 10X as many flavan-3-ols, thank you very much.
Final Thoughts
Thanks to Dr. Greger for sparking this conversation, but the science gives us tools to have our smoothies and flavan-3-ols too. By mitigating PPO with cold temperatures, vitamin C, and pineapple, and adding green tea later, we can keep bananas in our berry smoothies without worry. The study’s findings are more about cocoa than berries, and real-world losses are manageable. So, blend up your berry-banana smoothies, sip some green tea, and enjoy the best of both worlds!
