 I hear from lots of people every day who are concerned about how their diet is affecting their health. They need answers based on facts, in other words, from the peer-reviewed medical literature, and that is what I'm here for. Welcome to the Nutrition Facts Podcast. I'm your host, Dr. Michael Greger. You may have heard about a study suggesting that an enzyme in bananas that turns bananas brown can partially destroy a class of nutrients found in foods like berries. So should you stop putting bananas in your smoothies? Here are some answers. We all know there are recommended daily intakes of essential nutrients like vitamins to prevent deficiencies, but recently, the first dietary recommendation for a bioactive food compound was published. Bioactive food compounds are not necessary for basic human needs, but may have health benefits. For example, plant metabolites known as flavonoids. Flavonoids are a type of polyphenol, of which the most commonly consumed one is these flavon-3-ols, also known as just flavonols. A review of the literature suggests that intake of 400 to 600 mg a day of these flavon-3-ols will be beneficial for cardiovascular protection, such as potential for improving blood pressure, cholesterol, and blood sugars. Now note that this is specifically a food guideline, not a supplement guideline, though really it's actually more like a beverage guideline, since if you look at the primary sources, it's really a tea. Green or black is the easiest way to reach those levels, though there's certainly some in a variety of berries and cocoa. However, there's an enzyme called polyphenol oxidase that can break down these wonderful polyphenols like flavonoids. That is what makes certain fruits of vegetables brown when you cut them, such as bananas and potatoes. Now why would a plant do that to itself? Well, apparently it is a defense mechanism. It's part of the plant's immune system, and when exposed to oxygen, what happens is that polyphenol oxidase can oxidize polyphenols into these breakdown products which have antimicrobial activity. So when a banana gets bruised, for example, you cut open a avocado where some of the polyphenol oxidase can start oxidizing polyphenols into these defensive compounds that can glom together, forming a dark compound called melanin. The same class of compounds that darkens the skin of a banana, also darkens our skin too. So no problem, we can just not eat bananas that have turned brown, because that's visual evidence that many of the polyphenols have been lost. Ah, but what if you mix that banana, which you know is filled with that polyphenol oxidase enzyme, in a smoothie with polyphenol-rich foods like berries or cocoa, and why you end up with less nutrition? Well, big chocolate-funded researchers sought to find out, and so what they did is they put about a half cup of cocoa's worth of cocoa flavonols into a banana-containing smoothie compared to the same amount in a berry smoothie with no banana, compared to the same amount of cocoa blended into a banana smoothie. So the same amount of those cocoa goodies, but it's almost as if there was no cocoa in there at all, presumably because the banana enzymes chopped up all the chocolate goodies. Let's not presume, though. They blended up that chocolatey banana smoothie again, which was just banana, almond milk, cocoa flavonols, and then they just measured the level of flavonols as the smoothie sat for an hour. And the cocoa flavonols plummeted more than 90% a half-life of about 10 minutes. So every 10 minutes, the levels of polyphenols drop in half. Now, how do we know it was for sure it was this polyphenol oxidase enzyme? Because then if you add various polyphenol inhibitors, then they block the effect. Okay, so no problem if you add bananas to your smoothies, just you have to chug them down before the enzyme does its job, right? I mean, presumably the enzyme would be deactivated when it hit the acid bath of our stomach, but you don't know until you put it to the test. And I love that the researchers did all this, or what they did is this time they had studied participants alternate sips between a straight banana almond milk smoothie and like chocolate milk, chocolate flavonoids plus the almond milk, right? So there was no premixing in the blender between the banana enzyme and the flavonols, just mixing in the stomach. Such an elegant research move. Okay, so you get 37% less in your bloodstream, showing that the banana enzyme can still do some polyphenol damage in your stomach. And indeed, in a simulated stomach acid conditions, yes, the polyphenol oxidase is indeed down, but not out. Okay, so yeah, if you drink some smoothie with a banana, and it's better not to let it sit around, but even if you drink it immediately, you can get less polyphenols in your system than if you had to skip the banana. But bananas make smoothies so rich and creamy. Anything we can add to the smoothie to counter the banana enzyme's effects, there's one food that has even more polyphenol oxidase activity than banana, and it's a vegetable that's not a potato. What vegetable turns brown? That's not a white potato. White mushrooms! White button mushrooms have more polyphenol oxidase activity than even bananas. Now, for those of you thinking, don't worry, I don't think I expect to be adding any mushrooms into my smoothies anytime soon. Now, some people do add avocados to smoothies, or make like a chocolate avocado pudding. And here's the question, what if you eat mushrooms with a meal packed with polyphenol rich foods, right? Might it mix in your stomach and decrease the absorption like the anthocyanins in red cabbage or berries for dessert, right? And look, you could also be having some potatoes with your meal, or eggplant with your meal. Now, note though, however, this is for fresh produce, right? So, you know, we're not worrying about the eggplant potatoes. And the enzyme is utterly destroyed by cooking. And look, remember, we should be cooking our mushrooms anyway, right? Because of the guarantee, most of our mushrooms, you don't have to cook oyster mushrooms, but the white and criminy portobello mushrooms, right? So, problem solved, right? So, you don't have to worry about cooked potatoes, cooked eggplant, cooked mushrooms, okay. But who wants to cook their bananas though, right? What else could we put in a smoothie instead? My personal favorite is mango, which you buy frozen when it's not in season. Do we have to worry about that? Having a polyphenol eating enzymes? Well, what happens when you cut open a mango? Does it go brown? No, it doesn't go brown. Mango has, oh, maybe 500 times less than that banana. Now, the apple certainly doesn't surprise me, right? Because what happens with apples turns brown. But boy, the smoothie study raises so many interesting questions, right? So, forget smoothies. Should you not add bananas to your oatmeal, right? Because the bananas in your stomach would mix with like the berries or the cocoa that you put in your oatmeal. And indeed, that is what we should expect. So, I no longer add bananas to my Cran chocolate pomegranate breakfast bowl. That was one of my cooking videos, if you missed it. And since, you know, I don't want to lose any of those polyphenols. There's so many wonderful ingredients in that. I don't want to lose any. I don't want any of those destroyed, that 37% decrease in your stomach when it all mixes with that enzyme. Okay, but what if you're not making something chocolatey? This study only tested the effects of the enzyme on cocoa flavono. Like they didn't check to see if like mixing bananas with berries would affect the berry phytonutrients. But the presumption is that it would. And so, what does that mean for like ready-to-drink bottled smoothie drinks that you might find in a store, right, that may be sitting on the shelf? Well, if it's been pasteurized, then it should be okay, right? Because the heat destroys that enzyme. That's why vegetables are blanched, you know, before they're frozen to destroy the enzymes. Even just heating bananas to like 70 degrees C, which is like hot tea, 70 degrees Celsius, for two minutes in the presence of vitamin C and citric acid, which might come from lemon juice, for example, would suppress the enzyme by 80%. I'm not suggesting drinking hot smoothies, but rather that those pasteurized smoothie drinks, depending on the ingredients, may not have the same problem as fresh-made smoothies. What about just the adding the vitamin C and citric acid alone, like putting lemon juice in your smoothie, and only cuts the enzyme activity by about 13% without the heating step? However, it does work wonders for apples. Cut apple, you just add some lemon juice, and you can cut that activity nearly in half, explaining why adding lemon juice to your fruit salad, keeps the apples and your fruit salad from turning brown. Okay, anything we can add to a banana smoothie to inhibit the enzyme, so we can still use bananas, but somehow block the enzyme? Well, they use to put sulfites in fresh fruits and vegetables to block that enzyme until they were banned in 1986. Following cases of sulfite-induced asthma, they are still used in dried fruit, though, to prevent browning. Okay, sulfites are kind of off the table, but what about, what can we do for natural anti-browning agents to block this puppy? Well, onion extracts can prevent the browning of pears, but there has got to be something better for your smoothies. And indeed, pineapple juice does seem to help keep apples from browning, and bananas too, for that matter, but that was after soaking in pineapple juice for three days. It's not clear if it would work right away. There was a study comparing lemon juice and white wine to prevent browning in pastry dough. You've heard of hard cider, hard seltzer? Well, the lemon juice, it appears, beat out the wine, so better lemon squeeze than drinking a hard smoothie. That's the story on bananas. Should you not eat bananas? No, you can eat bananas anytime. You just don't want to eat bananas. You just don't want to have the bananas in your stomach. At the same time, you have other healthy foods like berries, cocoa, tea. We would love it if you could share with us your stories about reinventing your health through evidence-based nutrition. Go to nutritionfacts.org slash testimonials. 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