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Where Does Body Fat Go? The Science Behind Weight Loss | Nutritionist Explains... | Myprotein
Jan 30, 2025
Expert nutritionist explains what happens to body fat when we lose weight.
Dr Richie Kirwan is here with Myprotein to break down the construct of losing weight, how fat loss works, and where it goes.
In this video, Richie tells us precisely what happens to that layer of insulation when we diet. So how do we lose weight? Fat loss occurs when the body is in a calorie deficit. That means we’re eating less energy from food than we’re burning as activity.
Follow Richie on IG: dr.richie.kirwan
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Chapters
00:00 - Introduction
01:12 - What is body fat?
02:54 - What is glycerol?
04:26 - Fatty acids
06:13 - Where does body fat go?
07:06 - Any more body fat questions?
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0:00
if you go on a diet and lose weight
0:02
where exactly does all that body fat go
0:04
let's talk about that
0:11
how's it going guys my name is Richie
0:12
Kerwin and today we're going to talk
0:14
about something that I'm surprised we've
0:15
never actually spoken about before we
0:17
obviously talk a lot about nutrition on
0:19
this channel and we've spoken a lot in
0:21
the past about how to lose weight but if
0:24
I follow my own advice and lose 10 kilos
0:26
of body fat where does it all actually
0:28
go 10 kilos of fat doesn't just
0:30
disappear into thin air right well
0:32
buckle up because we're going to find
0:33
out exactly what happens to that layer
0:35
of insulation when we diet
0:37
the first thing to understand is why
0:39
does fat loss actually happen if you
0:41
haven't watched any of my videos before
0:42
and if you haven't you're in for a treat
0:44
you'll know that fat loss occurs when
0:47
the body is in a calorie deficit what
0:49
does that actually mean that's when
0:52
we're eating less energy from food than
0:54
we're burning in activity energy is
0:56
measured using calories or in the case
0:58
of nutrition kilo calories but we still
1:00
say calories just because we're lazy one
1:02
kilocalorie is the amount of energy
1:04
needed to heat one kilogram of Water by
1:07
one degree Celsius that might sound
1:09
pretty obscure but think of it like this
1:11
everything your body does from staying
1:14
warm pumping your blood processing
1:16
information with your brain moving
1:18
yourself around reducing sweat all of
1:21
that needs energy and we get that energy
1:23
from the food we eat if we take in more
1:26
food energy than our body needs to fuel
1:28
right now our bodies are really good at
1:31
storing that extra energy for later and
1:33
the best way our body does that is by
1:35
building up stores of body fat in fact
1:37
in a 70 kilogram person with about 15
1:39
body fat they have around 10 kilograms
1:42
of body fat and that works out at about
1:44
90 000 calories of stored energy or
1:46
enough to theoretically last from 35 to
1:49
45 days or more fat makes up between 92
1:53
and 98 of the energy stores in our body
1:56
with the rest being stored as glycogen
1:58
which is a type of carbohydrate we store
2:00
in our muscles as lower having that
2:02
store of energy is really useful for
2:04
when we can't get enough food energy to
2:06
meet our daily needs when we haven't
2:08
eaten in a while and our body needs
2:10
energy it dips into our body fat stores
2:12
but before I explain how let's talk
2:14
about the form that fat is stored in our
2:17
body our body fat stores are made up of
2:19
cells called adipocytes adipose tissue
2:22
is the scientific term for body fat and
2:25
each adipocyte contains one large fat
2:28
droplet that is made up of millions of
2:30
molecules of something called
2:31
triglycerides a triglyceride is the
2:34
basic unit of fat and is made up of
2:36
three fatty acids which are long chains
2:38
of carbon atoms attached to a backbone
2:40
molecule called glycerol hence
2:43
triglycerides I go into a lot more
2:45
detail on the different types of fatty
2:46
acids in my video on fish oils so check
2:48
it out when we're low on energy our body
2:51
can detect this and needs to get energy
2:53
from our fat stores so it sends a signal
2:55
to our cells to produce enzymes called
2:57
lipases lipases break down triglycerides
3:01
into fatty acids and glycerol and these
3:03
can then pass out of our cells and into
3:05
the blood fat doesn't dissolve well in
3:07
water so those fatty acids get
3:09
transported by carrier molecules called
3:10
lipoproteins to the parts of the body
3:12
that need energy
3:15
glycerol is soluble in water and can
3:17
travel without a transporter protein the
3:19
glycerol is mostly used in the liver to
3:21
produce glucose and this process is
3:24
called gluconeogenesis
3:26
when glycerol gets converted into
3:27
glucose it loses some of the carbon it
3:30
contains as carbon dioxide let's
3:33
remember that carbon dioxide for later
3:34
that glucose that gets produced by
3:36
gluconeogensis can be used by special
3:38
tissues that prefer to use glucose like
3:41
the brain and red blood cells glucose
3:43
usually gets burned or oxidized in a
3:45
process called aerobic respiration
3:47
basically glucose passes through a
3:50
number of different steps in the cell
3:51
where it's broken down by glycolysis and
3:53
later inside our mitochondria in a
3:55
process called a Krebs cycle
3:57
as it gets broken down by enzymes it
3:59
releases energy along the way
4:01
that energy is used to make the famous
4:04
energy currency of the cell ATP or
4:07
adenosine triphosphate ATP is what our
4:10
body uses to fuel all the magic things
4:12
our bodies are capable of like running
4:14
marathons creating beautiful works of
4:16
art and staring at YouTube videos for
4:18
hours at a time as glucose is broken
4:20
down to make ATP carbon dioxide and
4:22
water molecules are produced as waste
4:24
products and we'll talk about those
4:26
later too so what happens to the fatty
4:28
acids well when the fatty acids get
4:30
released from our cells they can go to
4:32
two different places one of those places
4:34
is the liver just like the glycerol in
4:37
the liver fatty acids get converted to
4:39
something called ketones which are the
4:41
substance produced in high amounts when
4:43
someone doesn't eat carbohydrates like
4:45
on a keto diet those ketones can't be
4:48
used for energy in the liver but can be
4:51
used by other organs like the heart the
4:53
muscles and even the brain when glucose
4:55
is low just like glucose ketones can be
4:58
metabolized by the Krebs cycle in the
4:59
mitochondria of cells to produce ATP
5:03
and along the way more carbon dioxide is
5:05
produced as a byproduct fatty acids
5:07
themselves can also be used directly for
5:10
energy production in tissues like our
5:12
muscles and especially in our heart
5:13
which actually gets up to 90 percent of
5:16
its energy from burning fatty acids with
5:18
the rest coming from other fuels like
5:19
glucose and ketones fatty acids get
5:22
transported into our cells mitochondria
5:23
and are metabolized for energy in two
5:25
main steps
5:27
the first step is called beta oxidation
5:29
and this means that the long fatty acid
5:32
chain has little pieces broken off two
5:35
carbon atoms at a time like I said fatty
5:37
acids are chains of carbon atoms with
5:39
hydrogen atoms attached and it's easy to
5:41
imagine the enzyme in our mitochondria
5:43
working their way along the chain
5:45
cutting off two carbon atoms at a time
5:48
as that happens some water is also lost
5:51
as a byproduct the piece of the chain
5:52
that gets cut off then enters the Krebs
5:55
cycle which we mentioned earlier and is
5:56
used to make ATP and some more carbon
5:59
dioxide gets produced so we just
6:01
described how body fat gets metabolized
6:03
first by being broken down into glycerol
6:05
and fatty acids and then how the glucose
6:07
ketones or fatty acids are produced and
6:10
broken down to give us ATP
6:13
if you've gotten this far well done but
6:15
you're probably thinking this guy said
6:17
he was going to tell us where the body
6:19
fat actually goes well if you're paying
6:21
attention I've kind of already given a
6:22
hint you see I already said our body fat
6:25
was stored as triglycerides and that
6:27
these are made up of molecules of carbon
6:29
hydrogen and oxygen through all the
6:32
process of getting broken down to make
6:34
energy I've also mentioned that there
6:36
are two main byproducts carbon dioxide
6:38
and water that's what's happened to your
6:41
body fat it's been chemically converted
6:43
into molecules of carbon dioxide and
6:45
water then you just breathe out carbon
6:48
dioxide and some water vapor via your
6:50
lungs and some of that water gets
6:52
sweated out or peed out of your system
6:53
so when you're in a calorie deficit and
6:56
you need to dip into your body fat
6:57
stores for energy you're either
6:59
breathing out sweating out or peeing out
7:02
that carbon dioxide in water that used
7:04
to be your body fat so now you know and
7:07
have a story to bore people with at
7:09
parties you're very welcome as always if
7:12
you have any more questions let me know
7:14
in the comments below and remember to
7:15
like And subscribe to the my protein
7:17
YouTube channel for more great
7:18
evidence-based nutrition information
7:21
foreign
7:23
[Music]
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