Thanks for your comment and question. While I still would like you to point me to the article you mentioned, I guess you are referring to the bananas’ high PPO content (polyphenol oxidase) which is known to inhibit the bioavailability of some fruit/berry polyphenols.
That is absolutely correct. However, you need to consider the wider context.
PPO is an enzyme that drives fruit ripening and browning (actually, it produces the colours black and red aside from brown). PPO is widely present in different fruits, not just bananas.
In the context of fruit smoothies that do or don’t contain bananas, it is important to consider several issues. I’ll address them with reference to a recent study that investigated the bioavailability of certain blueberry polyphenols/flavonoids when served either together with or without banana in a smoothie (https://pubs.rsc.org/en/content/articlelanding/2023/fo/d3fo01599h).
First, the bioavailability of polyphenols is generally very poor. That is, the proportion of intact polyphenol molecules that make it through the digestive tract and into the blood is very small. The aforementioned study confirms this. When you convert the banana-free smoothie’s content of roughly 600 mg of flavan-3-ols into what showed up in the blood you’ll notice that it was only roughly 1%.
That’s because, within our digestive system, the overwhelming majority of polyphenols are metabolized into other compounds that exert their beneficial health effects through other pathways. One of those pathways is the composition of the microbiome. This is a subject that I address in my next post which I will release very soon.
I’m emphasizing the generally low bioavailability to help you understand that a large portion of polyphenols’ health benefits cannot be adequately described by measuring their blood plasma concentration.
Second, the dramatic drop in bioavailability was only seen when the banana smoothie wasn’t consumed immediately after preparation. Banana PPO reduces polyphenols already before ingestion when you let the smoothie sit for a while.
When the smoothie was consumed directly after preparation, the bioavailability was only reduced by roughly 10-15%.
Finally, PPO activity can be abolished or substantially reduced by enzyme inhibitors. Ascorbic acid (Vitamin C) is one of them. That’s why I add it into our breakfast smoothie.
