Welcome to Huberman Lab Essentials,
where we revisit past episodes for the
most potent and actionable science-based
tools for mental health, physical
health, and performance.
I'm Andrew Huberman, and I'm a professor
of neurobiology and opthalmology at
Stanford School of Medicine. Today we
are going to discuss sugar in
particular, how our nervous system
regulates our sugar intake and our
seeking of sugar. We are going to place
sugar into its proper context. The way I
want to start off by doing that is to
tell you a little bit of what happens
when we eat and a little bit of what the
brain does to respond to those events.
So what happens when we eat? Let's just
take a what I call top contour view of
the hormonal response to ingesting food.
Anytime we eat, that is the consequence
of a number of things that happened
before we ate. There's a hormone in our
brain and body called ghrein spelled g h
l i n. Ghrein is a hormone that
increases depending on how long it's
been since we ate last. Okay? So, the
longer it's been since we had a meal,
ghrein levels are going to be higher and
higher and higher. And it essentially
makes us hungry by interacting with
particular neurons in an area of the
brain called the arcuate nucleus of the
hypothalamus and some other areas as
well like the lateral hypothalamus. And
then when we eat typically what happens
is ghrein levels go down. So, it's a
very logical system. Now when we eat,
assuming that we eat carbohydrates, but
even if we just eat some protein and
some fats, we will experience a slight
or in some cases a large rise in blood
glucose. Blood glucose is simply blood
sugar. And the body and brain, we should
say particular the nervous system
doesn't function well if blood sugar is
too high or too low. So as a consequence
we have another hormone which is
released from the pancreas which is
called insulin which helps regulate the
amount of glucose in the bloodstream.
Now one of the chief organs for glucose
utilization is the brain. Neurons are
tremendously metabolically active and
their preferred mode of metabolism is
glucose metabolism. The same thing is
also true for the neurons in your body.
The way that you are able to move the
limbs of your body, the way you are able
to perform exercise or movement of any
kind for that matter is because neurons
called motor neurons send electrical
potentials to the muscle fibers. Those
neurons are also very metabolically
demanding, especially when you're doing
demanding types of physical work. But
also, deliberate thought, deliberately
controlling the way that your brain and
body is moving requires more glucose
uptake, more energy in those very
neurons. And this is also why after
doing a long bout of exercise, you might
be tired. But also, if you do a bout of
skill learning of any kind, or if you've
been reading and thinking about what
you're reading, or if you had a intense
conversation with somebody where you're
really forcing yourself to listen,
that's work. And that work requires
glucose uptake by neurons both in the
brain and in your body. Now that we've
established that glucose is the
preferred source of fuel for the nervous
system, I'd like to concentrate on a few
of the other types of sugars that we
ingest on a common basis and the impact
that those have on brain function and
body function. I'd particularly like to
focus on fructose. Fructose, of course,
is found in fruit. It's also found in
the infamous high fructose corn syrup,
which we will talk about today. It's
worth pointing out that the
concentrations of fructose in fruit is
quite low compared to the concentrations
of fructose in high fructose corn syrup.
Typically, the amount of fructose
fructose I I think is the proper
pronunciation that people are always
correcting me. Fructose
is anywhere from 1% to about 10%. Now,
high fructose corn syrup is a different
issue. And too much consumption of
anything, but fructose included, can be
a problem for the ways that it impacts
the neural circuits that process sugar.
Not just glucose, but fructose. One of
the key distinctions between glucose and
fructose is that fructose most likely
cannot directly access the brain. It
actually needs to be converted into
glucose in the liver. And the way that
conversion occurs feeds back to a set of
hormones and neural pathways that we
talked about earlier which have a lot to
do with appetite. And to just summarize
what is now a lot of very solid data,
fructose and specifically fructose has
the ability to reduce certain hormones
and peptides in our body whose main job
is to suppress ghrein. So although I and
I think pretty much everyone out there
say for a few individuals agrees that
calories in calories out is the
fundamental principle of weight loss m
weight maintenance or weight gain.
Ingesting fructose shifts our hormone
system and as a consequence our neural
pathways within our brain the
hypothalamus to be hungrier
regardless of how many calories we've
eaten. So, current recommendations for
most people are to eat more fruits and
vegetables, but for those of you that
are trying to control your hunger,
ingesting a lot of fructose is probably
not going to be a good idea. Certainly,
ingesting it from high fructose corn
syrup is not going to be a good idea
because of the enormous percentages of
fructose in high fructose corn syrup,
50% or sometimes even more. Fructose
provides a bridge for us between a
particular kind of sugar hormone
function
in this case ghrein and the hypothalamus
which leads us to the next question
which is what is it about sugar that
makes it such an attractive thing for
us? Why do we like it so much? And the
obvious answer that most people arrive
at is well it just tastes really really
good. But that's actually not the way it
works. The rewarding properties, as we
say, of sugar, whether or not they come
in the form of sucrose or fructose or
foods that increase glucose to a a very
high level, actually is not just related
to the taste of the foods that produce
that elevation in glucose, sucrossse, or
fructose. It is in part, but that's only
part of the story. And the rest of the
story once you understand it can
actually place you in a position to much
better control your sugar intake of all
kinds, but also your food intake in ways
that can allow you to make much better
choices about the foods you ingest. So
now I want to take us on a journey into
the nervous system to explain the
pathways in the brain and body that
regulate our appetite for sugar. Now,
keep in mind what I already told you
before, which is that when we ingest
foods, they're broken down into various
components, and glucose is going to be
shuttled to the brain and of course to
other neurons in our spinal cord and
elsewhere and to our muscles, etc., in
order for all of those cells and organs
and tissues to be able to function. The
fact that so many cells and organs and
tissues require glucose in order to
function has led to a situation where
you have dedicated neural machinery,
pieces of your brain that are almost
entirely, if not entirely devoted to
seeking out of sugar or foods that
contain sugars and to make sure that you
not only seek those out, but you know
where those foods are and that you
ingest more and more and more of them.
And there are two main ways that these
neural circuits work. In fact, we can
say that there are two neural circuits
entirely that work in parallel. In the
case of sugar consumption, the two
parallel pathways involve one pathway
related to the actual taste and the
perception of sweet tastes that lead not
just you but every animal that we're
aware of to seek more sweet containing
foods.
The other parallel pathway is related to
the nutrative component of sweet foods.
Meaning the degree to which a given food
will raise blood glucose. I want to
repeat that one pathway in your brain
and body is devoted to getting you to
seek out sweet tasting things that you
perceive as sweet and another parallel
pathway is devoted to getting you to
seek out foods that lead to increases in
blood glucose. It just so happens that
the foods that lead to big increases in
blood glucose typically are associated
with that sweet taste. Now, this is
distinctly different than the neural
pathways that control seeking of savory
foods or salty foods or spicy foods for
that matter or bitter foods. The sweet
pathway is what we would call hardwired.
It exists, as far as we know, in every
mammal. Basically, getting sweet stuff
into the body might seem like it has a
lot to do with the taste, but it has
just as much to do with the nutrative
components that sweet tasting foods
carry and the fact that your nervous
system and so many cells in your brain
and body run on glucose. If you recall
earlier, I said even if you ingest
fructose, fructose can be converted into
glucose in the liver. The fundamental
thing to understand here is that when
you think you want a piece of chocolate
or you think you want a piece of cake or
you're craving something sweet, you are
both craving the taste and your neurons
are literally craving the nutrative
components that arrive with with that
taste. Two parallel pathways. One of the
parallel pathways has to do with
conscious perception. So when you ingest
something sweet, very quickly there are
signals sent from those neurons in your
mouth to brain areas that cause you to
seek out or at least pay attention to
the source and the abundance of those
sweet things. They literally change your
perception. Does that mean that you
should never ingest anything sweet? No.
Certainly I'm not saying that. Everyone
has to decide for themselves what the
appropriate amount of sugar intake is.
But I find it remarkable when people
say, "Oh, you know, I need to get my
sugar fixed or I need to have my
chocolate or I need to have a little bit
of something to just kind of take care
of that sugar appetite." Because in
taking care of that sugar appetite,
maybe for the very disciplined of you,
you can just have that one piece of
chocolate and it's great and you can
relish in it. But it does shift the way
that you perceive other foods as well.
And the way it does that is through our
probably if you're a listener to this
podcast now old friend but incredible
neurom modulator dopamine. Dopamine is a
molecule that is released from several
places in the brain. There's a so-called
misolyic reward pathway which is a whole
set of places in the brain or circuits
designed to get us motivated and craving
and in pursuit of things. And then of
course there are areas of the brain that
are involved in movement that are linked
up with those areas involved in
motivation. And that makes perfect
sense. Why would you have a brain area
involved in motivation if you couldn't
actually do something with that
motivation? When we ingest something
sweet, the perception of that sweet
taste increases dopamine and the
misolytic reward pathways which then are
conveyed to pathways for motor behavior
and in general place us into modes of
focused action toward getting more of
whatever was sweet. But if you
understand the way that dopamine works,
what you'll realize is that when this
dopamine pathway is triggered, it tends
to create not the sensation or the
perception of satiety of feeling like
something is enough, but rather to
produce the sensation of wanting more.
In fact, we can say that the longer it's
been since you've indulged in something
that you really enjoy or would like, the
greater the dopamine you will experience
when you finally engage in that behavior
or indulge that thing, ingest that
thing. Now, I again, I'm not saying that
you shouldn't pursue pleasurable things.
These dopamine pathways are not evil.
They're not bad. But once you understand
the way they work, you can leverage them
to your advantage as opposed to them
leveraging you to their advantage. Now
there's the second pathway. The second
pathway is what's called the
postingestive
reinforcing properties of sugar, which
is really just a fancy nerdspeak way of
saying there are events that happen with
your within your stomach and below your
conscious detection that are also
driving you to seek out sweet tasting
things independent of their taste. and
foods that increase blood glucose
independent of their taste. And here's
how it works. We all have neurons within
our gut. These neurons have a name. They
are called neuropod cells. Neuropod
cells were famously discovered by
professor Dr. Diego Bahorquez at Duke
University. And these cells respond to,
among other things, to the presence of
sugar within the gut. These neuropod
cells send electrical signals through a
particular highway within the vagus to
the so-called noo's ganglen. This is a
cluster. A ganglen is just a cluster of
neurons. And then the nose ganglion
sends on information to the nucleus of
the solitary tract. The nucleus of the
solitary tract is very important for
understanding sugar preference. So we've
all heard of hidden sugars, meaning the
sugars that manufacturers have put into
foods and disguised them with other
flavors. The savory foods are often
laden with these hidden sugars that we
can't register as sweetness but trigger
the neuropod cells which then further
trigger dopamine which make us want more
of them. Now we may be able to resist
eating more of them but it makes us
crave more food in general. Now we will
talk about ways to regulate this pathway
to sort of intervene in this
subconscious pathway.
But for now, I'm hoping that just the
understanding that we all have this
pathway. This is hardwired into our body
could potentially allow people to better
understand why is it that their cravings
are so intense that it's not necessarily
just about the taste of that food. And
when you consider this, you start to
realize that there are multiple
mechanisms hardwired into us that make
it especially hard to not eat the sweet
thing or to not eat the food that we're
craving. And indeed that's the case. We
have two major accelerators. It's like a
car with two accelerators. And we will
talk about the brakes, but two ways that
really get us into forward motion toward
pursuing the consumption of sweet foods.
Now, some of you have probably heard of
the so-called glycemic index, which is
basically a measurement of how high and
to some extent how fast blood sugar
rises in response to ingesting
particular foods. And very broadly
speaking, we can say that there are low
glycemic index foods of less than 55
typically is the measurement or medium
glycemic index foods which go from about
55 to 69 and then so-called high
glycemic foods which are above 70. And
of course there's additional nuance
related to glycemic load and many more
features of the glycemic index. a couple
of things to understand about how the
glycemic index is measured. And then I'd
like to just briefly talk about how the
glycemic index can be leveraged to
shortcircuit some of the neural circuits
that would otherwise lead us to crave
and perhaps even ingest sugary foods.
First of all,
measurements of glycemic indices of food
are typically made by having people
ingest those foods in isolation. And in
general we can say that anytime we
ingest fiber and or fat lipids along
with a particular food it will reduce
the glycemic index of that particular
food. Either the absolute level of blood
glucose that a particular food causes or
the rate at which that elevation in
blood glucose occurs. Okay? And this is
why there are some seemingly paradoxical
aspects to sweet stuff in terms of
glycemic index. For instance, ice cream
has a lower glycemic index provided it's
ice cream that includes fat, which I
hope it would because that's the good
tasting ice cream in my opinion compared
to something like mangoes or table
sugar. The glycemic index is not
something to hold wholly in most cases
because most people are not ingesting
foods in isolation. Now, why am I
telling you about the glycemic index?
Well, if we zoom out and take our
perspective on all of this discussion
about the glycemic index through the
lens of the nervous system and we remind
ourselves that neurons prefer glucose
for energy and that all sweet things or
things that we perceive as sweet, but
also sweet things that are ingested and
registered by those neuropod cells in
our gut trigger the release of dopamine
and trigger these neural circuits to
make us want to eat more of these foods.
What we start to realize is that a sharp
rise in blood glucose or a very high
degree of elevation in blood glucose is
going to be a much more potent signal
than will a more moderate rise in blood
glucose or a slower rise in blood
glucose. And so for those of you that
are trying to reduce sugar intake and
you want to do that through an
understanding of how these neural
circuits work and you want to
shortcircuit some of the dopamine
release that's caused by ingesting
sugary foods, it can be advantageous to
ingest sweet foods in combination with
foods that reduce glycemic index or
reduce glycemic load. So that might mean
making different food choices. So paying
attention to sweet tasting foods that
can satisfy sugar cravings but do not
have as steep or I should say do not
cause a steep a rise in blood sugar. Or
it could mean consuming other foods
along with sweet foods in order to
reduce the glycemic index and thereby
slow or blunt the release of dopamine.
So, if you really wanted to adjust your
sugar cravings and you really still want
to inest some sugary foods, you probably
would be better off combining fiber with
that sugary or sweet food. So, what
we're really talking about here is
trying to reduce the dopamine signal
that is the consequence of ingesting
sweet foods. And we're talking about
doing that through these different
parallel pathways. Not just by
preventing sweet taste, but also by
preventing the post-ongestive effects of
sweet foods. And of course, the backdrop
to all of this is that most of us,
again, most of us, not all of us, should
probably be ingesting fewer refined
sugars. So, what are some ways that we
can reduce our sugar cravings and
ideally ways that we can do that that
also benefit us in other ways, both
nutritionally and from the neuroscience
standpoint. The fact that these neuropod
cells and I should say other neurons
within the gut respond very robustly to
the presence of particular amino acids
is also a potential lever by which one
could reduce sugar cravings. And there's
an interesting literature around the
amino acid glutamine in particular
supplementing with the amino acid
glutamine as it relates to sugar
cravings and certainly as it relates to
other aspects of the gut in particular
leaky gut. The use of supplemental
glutamine to try and treat leaky gut is
not a new phenomenon. There are other
approaches too of course but there are
many people who are experimenting with
supplementing with glutamine several
grams per day often even you know five
grams distributed through three or four
different servings throughout the day in
as a way to blunt their sugar cravings.
Now, there has not yet been a
large-scale clinical trial using
glutamine to reduce sugar cravings, but
the results of the few studies that I
looked at, as well as my understanding
of the logic of these neural circuits,
including the neuropod cells, brings us
to a conclusion that it makes sense why
if there's a population of neurons
within our gut that responds very
robustly to the presence of sugar, fatty
acids, or amino acids that the intake of
particular amino acids would allow the
dopamine pathways that might otherwise
be triggered by sugar to be triggered by
something like glutamine, which has very
few or no calories. I know some people
who actually take glutamine and mix it
with full fat cream and take it kind of
like a shot of full fat cream, which
sounds absolutely delicious. By the way,
glutamine is a little bit chalky, so
it's not that great tasting to ingest
with sugar. I should mention if you do
try and take this approach of ingesting
glutamine to reduce sugar cravings, you
want to increase the amount of glutamine
that you take somewhat gradually. It can
create some gastric distress if you
just, you know, I certainly wouldn't
take a big tablespoon of it, throw it in
water and chug it down three times a
day. Please also realize that there's an
entire literature devoted to the
potential hazards of increasing
glutamine if you have a pre-existing
cancer. So, if you have cancer or you're
cancerrone, I would really discourage
you from this approach. In any case, as
always, talk to your doctor. Now, there
are other ways to reduce sugar craving,
and there are certainly ways to reduce
the sharp rise in blood glucose that can
occur when we ingest sugary sweet foods
or even just an abundance of
carbohydrate foods. And there are a huge
number of these things. I'm going to
sort of layer up through the the ones
that you might find in your cupboard at
the grocery store and then get into some
of the more um extravagant or I should
say um esoteric ones. many of which
however um can be quite potent. The
first of which um is simple lemon juice
right or lime juice. Regardless,
there are now data pointing to the fact
that lemon juice and lime juice a couple
tablespoons or so if ingested before or
even during or even after consumption of
sugary foods or I should say foods that
sharply increase blood glucose or large
carbohydrate meals can actually blunt
the blood glucose response. And I did
see that when I did my own experiments
on myself with continuous glucose
monitor. It was kind of fun to do those
exper experiments. Um I preferred to do
those experiments by eating somewhat
larger meals of things that didn't
contain a lot of sugar. I saw some big
increases in blood glucose in certain
instances and then I would ingest some
some lemon juice or lime juice typically
mixed in with water and sure enough you
could see a blunting of the blood
glucose response. And of course this was
real time continuous hence continuous
blood glucose monitoring. When you
ingest lemon juice or lime juice, the
mechanism by which it blunts blood
glucose is probably twofold. One is
probably through the post-ongestive
effects of glucose in the gut, meaning
the way in which sugars are interacting
with neurons and other components of
your gut circuitry to impact things like
gastric emptying time, to impact things
like the firing of those neuropod cells
and their signaling to the brain. But
almost certainly it has something to do
also with the perception of sour taste
on the tongue. We didn't go into this
too much today, but you of course don't
just have sweet taste receptors in your
mouth. You also have bitter taste
receptors. You have salty taste
receptors. You have sour taste receptors
in your mouth and on your and of course
that means your tongue and pallet. And
those are interacting. If you ingest a
substance that's just sweet or mostly
sweet,
that causes a certain set of effects on
your blood glucose, but also your brain,
dopamine, and the other neural circuits
of your brain. If you also ingest
something that's sour, like lemon juice
or lime juice, it adjusts the output of
those neural circuits in your brain. So
again we have a situation where we have
two parallel pathways. One that's
post-ongestive coming from phenomenon
within our gut neurons but also things
like gastric emptying time the clearance
and the transfer of food and the
conversion of food into particular
nutrients and the circulation of glucose
in your bloodstream and how it gets into
the brain. But also simply by ingesting
something sour you are changing the way
that sweet things impact your brain. And
so I think it stands to reason that the
lemon juice, lime juice effect is not
going to be magic. It's going to have
everything to do with the way that
ingesting sour foods can adjust the
neural response to taste of sweet foods.
And in fact, we know based on the
beautiful work of Charles Zuker at
Colombia Medical School that that's
exactly what happens. Now, some of you
have probably heard that cinnamon can be
a useful tool for controlling blood
sugar. And indeed, that's the case. It's
very clear that cinnamon can adjust the
rate of glucose entry into the
bloodstream possibly by changing um the
rate of gastric emptying. It might slow
the rate of gastric emptying and thereby
also reduce the glycemic index of
particular foods. So I suppose if I were
going to eat a mango and I hadn't just
done a bunch of hard training, I might
sprinkle some cinnamon on it here. I
always enjoy kind of coming up with new
ideas of ways that I can eat foods
during these podcasts. I do want to
provide a cautionary note about
cinnamon. However, cinnamon contains
something called couadin which can be
toxic at high levels. So, you don't want
to ingest more than about a teaspoon,
maybe a teaspoon and a half of cinnamon
per day um because you'll start to
exceed the threshold at which cinnamon
could um start to be problematic. So,
we've talked about lemon juice and lime
juice and cinnamon. These are kind of
common place in many kitchens.
Then of course we can venture into the
more esoteric or I would say the more
advanced tools for adjusting sugar
intake. The one that comes to mind is of
course bourberine. Using bourberine is a
serious step. You should absolutely talk
to your doctor about it. It is true that
if you ingest bourberine your blood
glucose will plummet. And I point that
out because I've actually tried it
before. It gave me brutal headaches and
I felt really dizzy and I felt like I
couldn't see straight. And actually I
couldn't see straight. Why did it do
that? Well, it made me hypoglycemic. It
actually drove my blood glucose down too
far. And the reason it did that is that
I took bourberine on an empty stomach.
If I took bourberine along with a very
large meal that included a lot of
carbohydrates, then I felt perfectly
fine on even up to 750 milligrams or a
gram of bourberine. But again, talk to
your doctor. I would place bourberine
and of course metformin and globenomide
in the kind of the heavy hitting potent
tools for regula regulating blood
glucose. And there are some other
substances like um sodium caprate which
are known to augment the effects of
bourberine via thempk pathways. They
basically can increase the ability for
bourberine to have its glucose lowering
actions. But that of course is getting
into the really potent what I would call
sharp blade tools for controlling blood
glucose. And listen, anytime you're
dealing with blood glucose, you are
dealing with the brain's preferred
source of fuel. And anytime you're
dealing with the brain's preferred
source of fuel, you have to be
especially cautious about depriving the
brain of what it needs. These substances
like bourberine are very, very potent.
And you need to take them seriously.
There is yet another tool for
controlling sugar cravings and the
neural circuits that regulate sugar
craving and its downstream consequences.
And this tool is what I would call a
high performance tool, but it's one that
you probably didn't suspect, and that's
sleep. I've done extensive episodes
about sleep. We actually have an episode
called Master Your Sleep. Uh you can
find that episode easily at
hubermanlab.com. It's available in all
the various formats, YouTube, Apple,
Spotify, etc. And provides a lot of
tools. And on social media, I provide a
lot of tools. What is the role of sleep
in sugar metabolism, sugar hunger, and
the way that the brain regulates those
things? Well, there's a really exciting
study that came out just last year. This
study was published in the journal Cell
Report, Cellpress Journal, excellent
journal. The reason I love this study so
much is it involved having people, so
yes, this was done in humans, sleep in
the laboratory. That's not unusual.
There's a sleep lab at Stanford, there's
sleep labs elsewhere. What they did was
they actually measured from the breath
of these people and they extracted from
their breath the metabolites that would
allow them to understand what sorts of
metabolism was occurring in these
people's bodies at different phases of
sleep. They actually did this every 10
seconds throughout the entire night.
What they discovered was that each stage
of sleep was associated with a very
particular signature pattern of
metabolism and particular phases of
sleep are associated with sugar
metabolism or more with fat metabolism
or more with other aspects of
metabolism. And the reason why I think
this study is important to discuss in
the context of today's discussion about
sugar in the brain is that many people
have experienced
the effects of disrupted sleep on their
appetite. And in particular, it's been
reported that when people are sleepd
deprived or the quality of their sleep
is disrupted, their appetite for sugary
foods increases. Now, we don't want to
leap too far from this study to sugar
metabolism and the neural circuits
controlling sugar metabolism, but I will
say this. There is now a plethora of
data pointing to the fact that getting
quality sleep each night helps regulate
not only appetite, but also the specific
forms of metabolism that drive specific
appetites. So we can't overstate the
importance of getting regular sufficient
amount of high quality sleep at least
80% of the time not just for sake of
immune system function for clear
thinking etc but also for properly
regulating our metabolism including our
sugar metabolism. Thank you for joining
me for this discussion about sugar and
the nervous system and how they are
regulating each other in both the brain
and body. And last but certainly not
least, thank you for your interest in
science.