00:01I hear from lots of
people every day
00:03who are concerned about how
their diet is affecting their health.
00:07They need answers
based on facts.
00:11in the peer-reviewed
medical literature,
00:13and that is what
I’m here for.
00:16Welcome to the
Nutrition Facts Podcast.
00:19I’m your host,
Dr. Michael Greger.
00:21You may have heard
about a study
00:23suggesting that an enzyme in bananas
that turns bananas brown
00:27can partially destroy a class of nutrients
found in foods like berries.
00:32So, should you stop putting
bananas in your smoothies?
00:36Here are some answers.
00:38We all know there are recommended daily intakes
of essential nutrients like vitamins to prevent
00:45deficiencies, but recently, the first dietary
recommendation for a bioactive food compound
00:54was published. Bioactive food compounds are not
necessary for basic human needs but may have
01:03health benefits. For example, plant metabolites
known as flavonoids. Flavonoids are a type of
01:13polyphenol, of which the most commonly consumed
one is flavan-3-ols, also known as just flavanol.
01:25A review of the literature suggests that 400 to
600 milligrams a day of these flavanols would be
01:32beneficial for cardiovascular protection, such
as the potential for improving blood pressure,
01:37cholesterol, blood sugars, etc. Note that
this is specifically a food guideline,
01:45not a supplement guideline, though really, it's
actually more like a beverage guideline since,
01:53if you look at the primary sources, it's really
tea, green or black, that is the easiest way to
02:00reach those levels. Though there are certainly
some in a variety of berries and cocoa. However,
02:07there's an enzyme called polyphenol oxidase
that can break down these wonderful polyphenols
02:14like flavonoids. That is what makes certain
fruits and vegetables brown when you cut them,
02:21such as bananas and potatoes. Now, why would
a plant do that to itself? Well, apparently,
02:29it is a defense mechanism, part of the plant's
immune system, and when exposed to oxygen, what
02:42happens is that polyphenol oxidase can oxidize
polyphenols into these breakdown products, which
02:48have antimicrobial activity. So when a banana gets
bruised, for example, or you cut open an avocado,
02:58the polyphenol oxidase can start oxidizing
polyphenols into these defensive compounds that
03:04can glom together, forming a dark compound called
melanin, the same class of compounds that darkens
03:10the skin of a banana also darkens our skin too.
Okay, so no problem, we can just not eat bananas
03:18that have turned brown, right? Because that's
visual evidence that many of the polyphenols
03:23have been lost. Ah, but what if you mix that
banana, which is filled with that polyphenol
03:32oxidase enzyme, in a smoothie with polyphenol-rich
foods like berries or cocoa? And why you end up
03:38with less nutrition. Well, big chocolate-funded
researchers sought to find out, and so what they
03:45did: They put about a half cup of cocoa's worth of
cocoa flavanol into a banana-containing smoothie
03:54compared to the same amount in a berry smoothie
with no banana, compared to the same amount of
04:02cocoa blended into a banana smoothie. Right,
so the same amount of those cocoa goodies, but
04:12it's almost as if there was no cocoa in there at
all, presumably because the banana enzymes chopped
04:19up all the chocolate goodies. Let's not presume,
though. They blended up that chocy banana smoothie
04:28again, which was just banana, almond milk, cocoa
flavanol, and then they just measured the level
04:35of flavanols as the smoothie sat for an hour,
and the cocoa flavanols plummeted more than 90%,
04:44a half-life of about 10 minutes. So every
10 minutes, the levels of polyphenols drop
04:55in half. Now, how do we know it was for sure the
polyphenol oxidase enzyme? Because then, if you
05:02add various polyphenol inhibitors, then they block
the effect. Okay, so no problem if you add bananas
05:13to your smoothies, just you have to chug them down
before the enzyme does its job, right? I mean,
05:19presumably, the enzyme would be deactivated
when it hit the acid bath of our stomach,
05:26but you don't know until you put it to the test,
and I love that the researchers did all this. What
05:34they did is this time they had study participants
alternate sips between a straight banana almond
05:40milk smoothie and like chocolate milk, chocolate
flavonoids plus the almond note, right? So there
05:47was no premixing in the blender between the
banana enzyme and the flavanol, just mixing
05:53in the stomach, such an elegant research move.
Okay, so you get 37% less in your bloodstream,
06:05showing that the banana enzyme can still do some
polyphenol damage in your stomach. And indeed,
06:14in simulated stomach acid conditions, yes, the
polyphenol oxidase is indeed down but not out.
06:24Okay, so yeah, if you drink some smoothie with a
banana, it's better not to let it sit around, but
06:33even if you drink it immediately, you can get less
polyphenols in your system than if you had skipped
06:40the banana. But bananas make smoothies so rich
and creamy. I mean, anything we can add to the
06:49smoothie to counter the banana enzyme's effect?
There's one food that has even more polyphenol
06:57oxidase activity than banana, and it's a vegetable
that's not a potato. What vegetable turns brown
07:05that's not a white potato? Oh, white mushrooms.
White button mushrooms have more polyphenol
07:17oxidase activity than even bananas. Now, for
those of you thinking, don't worry, I don't
07:25think I'm expected to be adding any mushrooms to
my smoothies anytime soon. Now, some people do add
07:33avocados to smoothies or make like a chocolate
avocado pudding, and here's the question:
07:39What if you eat mushrooms with a meal packed with
polyphenol-rich foods? Right, might it mix in
07:46your stomach and decrease the absorption, like the
anthocyanin in red cabbage or berries for dessert?
07:54Right, and look, you could also be having some
potatoes with your meal, or eggplant with your
08:01meal. Now note, though, however, this is for fresh
produce, right? So, you know, we're not worrying
08:09about the eggplant, potatoes, and the enzyme is
utterly destroyed by cooking. And look, remember,
08:20we should be cooking our mushrooms anyway, right,
because of the agitoxin. Most of our mushrooms,
08:26you don't have to cook oyster mushrooms, but
the white and crimini and portobello mushrooms,
08:30right? So, problem solved, right? You don't have
to worry about cooked potatoes, cooked eggplant,
08:36cooked mushrooms. Okay, but who wants to cook
their bananas, though? Right, what else could
08:45we put in a smoothie instead? My personal favorite
is mango, which you can buy frozen when it's not
08:49in season. Do we have to worry about that
having a polyphenol-eating enzyme? Well,
08:55what happens when you cut open a mango? Does
it go brown? No, it doesn't go brown. Mango
09:02has maybe 500 times less than that banana. Now,
the apple certainly doesn't surprise me, right,
09:13because what happens with apples, turns brown. But
boy, the smoothie study raises so many interesting
09:20questions, right? So, what, forget smoothies,
should you not add bananas to your oatmeal,
09:26right? Because the bananas in your stomach would
mix with like the berries or the cocoa that you
09:31put in your oatmeal. And indeed, that is what
we should expect. So, I no longer add bananas
09:38to my cran-chocolate pomegranate breakfast bowl.
That was one of my cooking videos, if you missed
09:45it. And since, you know, I don't want to lose any
of those polyphenols, there are so many wonderful
09:51ingredients in that. I don't want to lose any,
don't want any of those destroyed, that 37%
09:56decrease in your stomach when it all mixes with
that enzyme. Okay, but what if you're not making
10:03something chocolatey? This study only tested the
effects of the enzyme on cocoa flavanol, like
10:12they didn't check to see if mixing bananas with
berries would affect the berry phytonutrients,
10:18but the presumption is that it would. And so,
hm, what does that mean for like ready-to-drink
10:27bottled smoothie drinkers that you might find
in the store, right, that may be sitting on
10:31the shelf? Well, if it's been pasteurized,
then it should be okay, right? Because the
10:37heat destroys that enzyme. That's why vegetables
are blanched, you know, before they're frozen,
10:43to destroy the enzymes. Even just heating bananas
to like 70 degrees C, which is like hot tea,
10:5370° C for 2 minutes in the presence of vitamin
C and citric acid, which might come from lemon
10:59juice, for example, would suppress the enzyme by
80%. I'm not suggesting drinking hot smoothies,
11:06but rather that those pasteurized smoothie drinks,
depending on the ingredients, may not have the
11:12same problem as fresh-made smoothies. What about
just adding the vitamin C and citric acid alone,
11:19like putting lemon juice in your smoothie? It only
cuts the enzyme activity by about 133% without the
11:24heating step. However, it does work wonders for
apples. Cut apple, you just add some lemon juice,
11:32and you can cut that activity nearly in half,
explaining why adding lemon juice to your fruit
11:37salad, you know, keeps the apples in your fruit
salad from turning brown. Okay, anything we can
11:43add to a banana smoothie to inhibit the enzyme
so we can still use bananas but somehow block
11:48the enzyme? Well, they used to put sulfites in
fresh fruits and vegetables to block that enzyme
11:58until they were banned in 1986, following cases
of sulfite-induced asthma. They are still used
12:06in dried fruit, though, to prevent browning.
What, so, okay, sulfites are kind of off the
12:13table. But what about what can we do for natural
anti-browning agents to block this puppy? Well,
12:22onion extracts can prevent the browning of
pears, but there has got to be something
12:31better for your smoothies. And indeed, pineapple
juice does seem to help keep apples from browning,
12:39and bananas too, for that matter. But that was
after soaking in pineapple juice for three days,
12:45so it's not clear if it would work right away.
There was a study comparing lemon juice and
12:53white wine to prevent browning in pastry dough.
You've heard of hard cider, hard seltzer, well,
13:01the lemon juice appears to beat out the wine.
So, better a lemon squeeze than drinking a
13:09hard smoothie. That's the story on bananas and
smoothies. So should you not eat bananas? No,
13:16you can eat bananas any time, you just don't want
to eat bananas. You just don't want to have the
13:19bananas in your stomach at the same time you have
other healthy foods like berries, cocoa, and tea.
13:27We would love it if you could
share with us your stories
13:30about reinventing your health
through evidence-based nutrition.
13:34Go to nutritionfacts.org/testimonials.
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13:41To see any graphs, charts, graphics,
images, or studies mentioned here,
13:45go to the Nutrition Facts
Podcast landing page.
13:49There you’ll find all the
detailed information you need –
13:51plus, links to all of the sources
we cite for each of these topics.
13:56My latest book, “How Not to Age”
is out now,
13:59premiering at #2 on the
New York Times Best Sellers list.
14:03Check it out at your
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14:34whose own life was saved
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