0:01

I hear from lots of people every day

0:03

who are concerned about how their diet is

0:06

affecting their health. They

0:08

need answers based on facts, in

0:10

other words, from the peer-reviewed medical

0:12

literature, and that is what I'm

0:15

here for. Welcome to

0:17

the Nutrition Facts Podcast. I'm

0:19

your host, Dr. Michael Greger. You

0:22

may have heard about a study

0:24

suggesting that an enzyme in bananas

0:26

that turns bananas brown can partially

0:28

destroy a class of nutrients found

0:30

in foods like berries. So should

0:33

you stop putting bananas in your

0:35

smoothies? Here are some answers.

0:38

We all know there are

0:40

recommended daily intakes of

0:43

essential nutrients like vitamins

0:45

to prevent deficiencies, but

0:48

recently, the

0:50

first dietary recommendation for a

0:52

bioactive food compound was

0:55

published. Bioactive

0:57

food compounds are not

0:59

necessary for basic human needs,

1:02

but may have health benefits. For

1:05

example, plant metabolites known as

1:08

flavonoids. Flavonoids

1:12

are a type of

1:14

polyphenol, of

1:18

which the most commonly consumed one is

1:20

these flavon3ols, also known

1:23

as just flavonols. A

1:26

review of the literature suggests

1:29

that intake of 400 to 600 mg a day of

1:31

these flavonols will be beneficial for

1:34

cardiovascular protection, such as potential for

1:36

improving blood pressure, cholesterol, blood sugars.

1:39

Now note that this

1:41

is specifically a food

1:44

guideline, not a supplement

1:46

guideline, so really it's

1:48

actually more like a

1:51

beverage guideline, since if

1:53

you look at the

1:56

primary sources, it's really a tea, green or

1:58

black. It's the easiest to use. way

2:00

to reach those levels, though

2:02

there's certainly some in a

2:04

variety of berries and cocoa.

2:06

However, dun dun dun dun,

2:08

there's an enzyme called polyphenol

2:10

oxidase that can break down

2:12

these wonderful polyphenols like flavonoids.

2:15

That is what makes certain fruits

2:18

and vegetables brown when you cut

2:20

them, such as bananas

2:22

and potatoes. Now why would a

2:25

plant do that to itself? Well,

2:27

apparently it is

2:29

a defense

2:32

mechanism. It's

2:35

part of the plant's immune

2:37

system and when

2:39

exposed to oxygen, what

2:42

happens is that polyphenol oxidase

2:44

can oxidize polyphenols into

2:47

these breakdown products which

2:49

have antimicrobial activity. So,

2:53

when a banana gets bruised,

2:55

for example, you cut open

2:57

an avocado or something, the

2:59

polyphenol oxidase can start oxidizing

3:01

polyphenols into these defensive compounds

3:03

that can glom together, forming

3:06

a dark compound called melanin.

3:08

Same classic compounds that darkens the

3:11

skin of banana, also darkens our

3:13

skin too. Okay, so no

3:15

problem. We can just

3:17

not eat bananas that have turned brown,

3:20

right? Because that's visual evidence that many

3:22

of the polyphenols have been lost. Ah,

3:24

but what if you

3:28

mix that banana, which

3:30

you know is filled with that polyphenol oxidase

3:32

enzyme, in a smoothie with

3:35

polyphenol-rich foods like berries or cocoa and

3:37

that why you end up with less

3:39

nutrition? Well, big chocolate-funded

3:41

researchers sought to find out

3:44

and so what they did is they

3:46

put about a half cup of cocoa's

3:48

worth of cocoa flavanols into

3:51

a banana-containing smoothie

3:54

compared to the same amount in a

3:57

berry smoothie with no banana. compared

4:00

to the same amount of cocoa blended

4:03

into a banana smoothie,

4:07

right? So the same

4:10

amount of those cocoa goodies, but

4:12

it's almost as if there

4:14

was no cocoa in there at all, presumably

4:17

because the banana enzymes chopped up

4:19

all the chocolate goodies. Let's

4:22

not presume though. They

4:24

blended up that chocolatey

4:26

banana smoothie again, which

4:30

was just banana, almond

4:32

milk, cocoa flavinalls, and then they

4:34

just measured the level of flavinalls

4:37

as the smoothie sat for an

4:39

hour. And the cocoa flavinalls plummeted

4:41

more than 90% a

4:44

half-life of

4:47

about 10

4:49

minutes. So every

4:51

10 minutes, the levels of

4:54

polyphenols drop in half. Now,

4:56

how do we know it was

4:59

for sure it was this polyphenol

5:01

oxidase enzyme? Because then if you

5:03

add various polyphenol

5:06

inhibitors, then they

5:08

block the effect. Okay. So

5:10

no problem. If you add bananas

5:13

to your smoothies, just you have to chug

5:15

them down before the enzyme does its job,

5:17

right? I mean, presumably

5:20

the enzyme would be deactivated

5:23

when it hit the acid bath of

5:25

our stomach, but you don't

5:27

know until you put it to

5:30

the test. And I love that the researchers did all this,

5:33

or what they did is this time they had study

5:35

participants alternate sips between

5:38

a straight banana almond milk smoothie

5:41

and like chocolate milk, chocolate flavonoids

5:44

plus the almond milk, right? So

5:47

there was no premixing in

5:50

the blender between the banana enzyme

5:52

and the flavinalls, just mixing in

5:54

the stomach, such

5:57

an elegant research move of Okay,

6:00

so you get 37% less in your

6:02

bloodstream showing

6:06

that the banana enzyme can still

6:08

do some polyphenol damage in your

6:11

stomach. And indeed, in

6:14

a simulated stomach acid

6:17

conditions, yes,

6:19

the polyphenol oxidase is indeed

6:21

down, but not out. Okay,

6:25

so yeah,

6:28

if you drink some water, smoothie

6:30

with a banana, and it's better not to let

6:32

it sit around, but even if you drink it

6:34

immediately, you can get less polyphenols

6:37

in your system than if you

6:39

had skipped the banana. But

6:42

bananas make smoothies so rich

6:44

and creamy. I mean, anything

6:47

we can add to

6:49

the smoothie to counter the banana

6:51

enzyme's effects, there's one

6:54

food that has even more

6:56

polyphenol oxidase activities in

6:58

banana, and it's a vegetable that's

7:01

not a potato. What

7:03

vegetable turns brown? That's

7:06

not a white potato. Oh,

7:09

white mushrooms. White

7:11

button mushrooms have

7:13

more polyphenols oxidase

7:17

activity than even bananas. Now,

7:22

for those of you thinking, don't

7:24

worry, I don't think I'm expecting

7:27

to be adding any mushrooms into

7:29

my smoothies anytime soon. Now,

7:32

some people do add avocados to

7:35

smoothies, or make like

7:37

a chocolate avocado pudding. And here's

7:39

the question, what if you eat mushrooms

7:41

with a meal packed with

7:43

polyphenol rich foods? Might

7:46

it mix in your stomach and

7:48

decrease the absorption like the anthocyanins

7:50

in red cabbage or berries for

7:53

dessert? And

7:56

look, you could also be having some

7:58

potatoes with your meal or eggplant. with

8:01

your meal. Now note though, however, this

8:03

is for fresh produce,

8:06

right? So, you know, we're not

8:08

worrying about the eggplant potatoes and

8:11

the enzyme is utterly

8:14

destroyed by

8:17

cooking and

8:19

look, remember, we should be

8:21

cooking our mushrooms anyway,

8:24

right? Because of the guarantee, most of

8:26

our mushrooms, you don't have to cook

8:28

oyster mushrooms but the white and criminy

8:30

and portobello mushrooms, right? So problem solved,

8:32

right? They, you know, so you don't

8:35

have to worry about cooked potatoes, cooked

8:37

eggplant, cooked mushrooms, okay. But

8:41

who wants to cook their bananas though, right?

8:44

What else could we put in a smoothie

8:46

instead? My personal favorite is mango, which

8:48

you buy frozen when it's not in season. Do we

8:51

have to worry about that having a

8:53

polyphenol eating enzymes? Well, what happens when

8:56

you cut open a mango? Does

8:58

it go brown? No, it doesn't go brown.

9:01

Mango has, oh, maybe

9:04

500 times less than

9:06

that banana. Now the

9:08

apple certainly doesn't, doesn't

9:10

surprise me, right? Because

9:12

what happens with apples

9:14

turns brown. But

9:17

boy, this smoothie study raises so

9:19

many interesting questions, right? So what

9:22

forget smoothies, should you not add

9:24

bananas to your oatmeal, right?

9:26

Because the bananas in your stomach would mix

9:28

with like the berries or the cocoa that

9:30

you put in your oatmeal. And

9:33

indeed, that is

9:35

what we should expect. So I

9:37

no longer add bananas to my

9:40

crin chocolate pomegranate breakfast bowl. That

9:43

was one of my cooking videos if

9:45

you missed it. And since, you know,

9:47

I don't want to lose any of

9:49

those polyphenols. There's so many wonderful ingredients

9:51

in that I don't want to lose any. Don't

9:54

want any of those destroyed that 37% decrease

9:56

in your stomach when

9:59

it all mixes. with that enzyme. Okay,

10:01

but what if you're not

10:03

making something chocolatey? This

10:07

study only tested

10:10

the effects of the enzyme on

10:12

coco flavonols. Like they didn't check

10:14

to see if mixing bananas with

10:16

berries would affect the berry phytonutrients,

10:18

but the presumption is that

10:20

it would. And

10:23

so, hmm, what does that mean

10:26

for ready-to-drink bottled smoothie drinks that

10:28

you might find in the store, right,

10:30

that may be sitting on the shelf?

10:33

Well, if it's been pasteurized, then it

10:35

should be okay, right, because the heat

10:38

destroys that enzyme. That's

10:40

why vegetables are blanched, you know, before

10:42

they're frozen to destroy the enzymes. Even

10:46

just heating bananas

10:49

to, say, like 70 degrees C, which

10:51

is like hot tea, 70 degrees Celsius,

10:53

for two minutes in the presence of

10:55

vitamin C and citric acid, which might

10:58

come from lemon juice, for example, would

11:00

suppress the enzyme by 80%. I'm

11:04

not suggesting drinking hot smoothies, but

11:06

rather that those pasteurized smoothie drinks,

11:09

depending on the ingredients, may

11:11

not have the same problem as

11:14

fresh-made smoothies. What

11:16

about just adding the vitamin C and

11:18

citric acid alone, like putting lemon juice

11:21

in your smoothie, and only cuts the

11:23

enzyme activity by about 13% without the

11:25

heating step? However, it does work wonders

11:28

for apples. A cut apple, you

11:30

just add some lemon juice, and you can

11:32

cut that activity nearly in half, explaining

11:35

why adding lemon juice to your fruit

11:37

salad, you know, keeps the apples and

11:39

fruit salad from turning brown. Okay, anything

11:42

we can add to a

11:44

banana smoothie to inhibit the enzyme, so

11:46

we can still use bananas to somehow block the

11:49

enzyme? Well, they used

11:51

to put sulfites

11:54

in fresh fruits and vegetables to

11:57

block that enzyme until they were

11:59

banned. in 1986 following cases of sulfite induced asthma.

12:01

They are still used in dried

12:07

fruit though to prevent browning.

12:10

What so okay sulfites are

12:12

kind of off the table but

12:15

what about what can we

12:17

do for natural anti-browning

12:19

agents to block this

12:21

puppy? Well onion extracts

12:24

can prevent the browning of pairs

12:26

but there

12:29

has got to be something better

12:32

for your smoothies and

12:34

indeed pineapple juice does seem

12:36

to help keep apples from browning and

12:38

bananas too for that matter

12:42

but that was after soaking in

12:44

pineapple juice for three days. It's

12:46

not clear if it would work

12:48

right away. There was a study

12:52

comparing lemon juice and white wine

12:54

to prevent browning in pastry dough.

12:57

You've heard of hard cider hard

12:59

seltzer? Well the lemon juice

13:02

it appears beat

13:04

out the wine so better

13:07

lemon squeeze than drinking

13:10

a hard smoothie. That's

13:12

the story on bananas.

13:14

Should you not eat bananas? No you can

13:17

eat bananas anytime. You just don't want to

13:19

eat bananas. You just want to have the

13:21

bananas in your stomach at the same time

13:23

you have other healthy foods like berries, cocoa,

13:25

tea. We

13:28

would love it if you could share

13:30

with us your stories about reinventing your

13:32

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13:34

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media to help inspire others. If

13:42

you see any graphs, charts, graphics, images

13:44

or studies mentioned here please go to

13:46

the nutrition facts podcast landing page. There

13:48

you'll find all the detailed information you

13:50

need plus links to all the sources

13:53

we cite for each of these topics.

13:56

My latest book How Not to Age

13:58

is out now premiering at number number

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