Intermittent Fasting & Hunger – What the Science says
Intermittent fasting has gotten a lot of attention
nowadays as there’s so many benefits to
be had from it.
It helps you lose weight, helps you gain muscle
and enhances your cognitive performance.
Specifically, it lowers insulin- the fat storing
hormone involved with diabetes, it raises
human growth hormone which does everything
from increasing muscle mass to making you
look better and live longer, and it promotes
the release of brain derived neurotrophic
factor, a key hormone for growing new brain
Intermittent fasting is simple, all you do
is not eat for 16 hours of the day.
But this is a radical idea for most people
considering we’re encouraged to be eating
all the time, being told things like “breakfast
is the most important meal of the day” and
“you need to eat six small meals to lose
Unsurprisingly hunger is the number 1 concern
when it comes to fasting.
So, let’s take a look at how hunger works
so we can better deal with it.
As Tim Russ said, “We often fear what we
do not understand; our best defense is knowledge.”
The first thing you should know is that hunger
is not a simple equation of no food plus time
equals more and more hunger.
In a very thorough series on fasting in his
blog called “Intensive Dietary Management,”
Dr. Jason Fung points to a study that shows
that while subjects went over 3 days without
any food, the hunger hormone ghrelin gradually
but continually decreased.
He says: “This means that patients were
far LESS hungry despite not having eaten for
the past 3 days.
This jives perfectly with our clinical experience
with patients undergoing extended fasting.
They all expect to be ravenously hungry, but
actually find that their hunger completely
This is because hunger is regulated by hormones
like ghrelin and it comes in waves.
You do not get progressively hungrier the
longer you go without food- your body adapts
to your REGULAR eating schedule.
It knows when you usually eat, so it comes
to expect food and make you hungry around
the same time each day.
Here’s a study from the Medical University
of Vienna done on subjects participating in
a 33 hour fast.
Despite the early morning being when people
have gone the longest without food, ghrelin
was the lowest.
And, their ghrelin didn’t rise more and
more the longer they went without food- ghrelin
rose at their normal eating times.
And, it fell back down even though they didn’t
So when you start fasting, it’s important
to know that hunger will arise and it is uncomfortable,
but it will come and go.
After a couple days, your hormones will adapt
to your new eating times and you will be less
and less hungry until it’s no longer an
This is why many people say that the first
4 days are the hardest.
Un-adapting and re-adapting your eating times
is uncomfortable, but your body is very good
I’ve been doing intermittent fasting with
an eating window of 2 to 4 hours for the past
year, and when I was first experimenting with
fasting, I actually tried eating once every
To my surprise, after only about 6 days of
this, I wasn’t that hungry on the no eating
But, like clockwork, I started to get pretty
hungry on the eating day right around the
usual time I would eat.
Another very interesting thing about this
hormone ghrelin, is that it may be making
you hungry in order to get you to take in
It would be very important to have a hunger
for salt as salt has many very critical functions:
It’s needed by the heart to pump blood properly,
and it’s a key component in cell-to-cell
communication and the optimal transmission
of nerve impulses to and from organs like
the heart and brain.
Low salt has been shown to stunt growth, increase
insulin resistance and increase uric acid
levels, stimulating oxidative stress in the
mitochondria and weight gain.
Despite all this, the standard low sodium
guidelines of only 2.3g of sodium per day
drastically underestimate how much salt the
body really requires for optimal functioning.
A study in the New England Journal of medicine
found that an estimated sodium intake between
at least 3g and 6g per day was associated
with a lower risk of death and cardiovascular
In fact, actually following the low salt guidelines
posed a particularly high risk to health.
And, Intermittent fasting may have you requiring
even more salt than usual.
Intermittent fasting lowers your insulin levels.
This is great, and is one of the main goals
of intermittent fasting, but it’s often
overlooked that insulin retains sodium, so
the lower your insulin goes, the more sodium
your kidneys secrete.
Losing a lot of sodium and other minerals
can leave you feeling sluggish and hungry.
Now, your body really doesn’t want to lose
too much sodium, so one thing it does is it
actually increases insulin levels in an attempt
to retain more sodium.
A 1991 study done on 147 people found that
a low salt diet significantly increased serum
insulin in these people.
Of course higher insulin is something you
don’t want, so let me explain more about
insulin and how it affects hunger.
So when you eat, insulin goes up and, insulin
helps you utilize carbohydrate for energy
or stores it as glycogen, which is basically
linked chains of glucose.
This glycogen is stored in the liver or muscle.
And when you have too much glycogen, the incoming
carbohydrate will be stored as fat.
Now, different foods will give you a different
insulin rise, fat stimulates insulin a minimal
amount while fiberless carbs provoke massive
rises in insulin.
But the point is, insulin’s job is to help
you absorb and store nutrients.
Next, after about four to six hours after
you eat, insulin levels will have gone down,
and the glucose level in your blood starts
to decrease, this prompts the pancreas to
Glucagon has almost the opposite function
Insulin stores food energy, but glucagon pulls
that energy out from your glycogen stores
and your fat stores.
So if you’re eating every two hours and
getting six small meals a day, you never let
glucagon do its job and your body never starts
burning into your glycogen or your fat stores.
Not a good strategy for losing weight.
Well what about hunger?
Well, let’s look at this study from 1986.
They found that if you inject an animal with
insulin, it will eat more.
Adding insulin increases appetite.
If you have insulin floating around with no
new glucose coming in, your blood glucose
will drop, but insulin needs to be low for
glucagon to use your fat or glycogen stores
With high insulin, you have no access to your
So it makes sense for extra insulin to promote
hunger as the only way you can get new energy
is to eat it and raise your blood glucose.
And as you might expect, giving an animal
glucagon, reduced food intake.
So glucagon is decreasing appetite.
Now they also infused glucose into the animal’s
small intestine and it reduced food intake,
so it’s of course not the case that carbs
don’t satiate you to some extent.
But if you’re eating every two hours, you’re
constantly pumping out insulin and the levels
never go low enough for glucagon to do its
This explains why you can eat a bowl of cereal
for breakfast or a huge plate of pasta for
lunch and be hungry in just an hour or two.
Refined carbohydrates like that will cause
the body to overshoot with its insulin secretion,
so even after all the carbohydrates are processed
after the meal, you still have a bunch of
insulin sitting around making you hungry.
This overshooting with insulin also explains
why if you binge eat at night, you’ll wake
up starving in the morning.
So after a meal, if you’re patient, glucagon
will allow you to start running on your stored
energy and you will be less hungry.
So while fasting, glucagon suppresses hunger
and is helping you power the body by burning
your stored glycogen and fat.
Then when you run out of glycogen, you’re
burning primarily fat for energy and your
body enters the state of ketosis.
This produces a lot of ketone bodies, which
your body and brain will use for fuel.
It’s a common misconception that the brain
runs only on glucose.
In 1967, George Cahill found that the brain
will derive 2/3rds of its energy from ketone
bodies made from fat.
Though, some processes in the body still require
glucose, but you don’t need to eat that
glucose – your body makes its own glucose
from things like glycerol and lactate.
This is called gluconeogenesis.
Earlier we saw that high glucagon and low
ghrelin during fasting will keep hunger in
But, we can add ketone bodies to the list.
There is data that show that ketone bodies
like beta hydroxybutyrate also reduce appetite.
Which isn’t a surprise considering they
act as an energy source for the body.
This is one of the reasons why a ketogenic
or low carbohydrate diet has such great synergy
with intermittent fasting.
These diets will keep insulin low, and allow
you to more use fat for energy, just like
So, if you’re able, I’d recommend trying
to shift your diet to one that is high in
good fat, high in fiber, high in micronutrients,
moderate on protein and low in carbohydrate.
You could even start eating like this before
you begin intermittent fasting and you’ll
already be adapted to be getting your energy
from fat so you’ll have less hunger and
adapt to intermittent fasting easier.
You might be wondering why I said moderate
Many people assume a low carb or keto diet
must have a lot of protein in it, but that’s
not quite the case.
Insulin is necessary for protein to be metabolized,
so you get some insulin rise when you eat
But not only that, when you eat more protein
than your body needs to make or repair cells,
the excess will be broken down into glucose
In fact, diabetics have been shown to have
a 30% decrease in daily blood glucose levels
just by restricting protein.
Now there’s one last interesting thing about
insulin and hunger – it’s how it interacts
with leptin, the satiety hormone.
Essentially when you eat food, depending on
the composition of the meal, your levels of
leptin go up and you feel more satiated.
But, when you have too much insulin, it reduces
leptin signalling- your brain can’t pick
up on the leptin signal, keeping you hungry.
In fact, obesity which is virtually always
accompanied by insulin resistance is also
accompanied by leptin resistance.
If you want to stay satiated and full of energy
rather than be hungry and lethargic, you need
to keep your insulin low.
Lucky for you, fasting seems to be the best
way to lower insulin.
So while intermittent fasting is hard for
the first couple of days, it’s helpful to
know that your body and hormones are working
in your favor, making it easier and easier.
And, if you get enough salt and minerals,
and eat high fiber, moderate protein and low
carbohydrate then you can make the whole process