The Science of Fat Loss

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The Science of Fat Metabolism - Part Two

How does food turn to fat?

What happens when you go to a buffet?  The amazing kind of buffet that has every food option you could imagine, enticingly laid out just waiting for you to gorge on.

You’ve got eggs, bacon, bread, pasta, salad, meat, cereals, cheese, fruit, scones, muffins, croissants… ugh now I’m hungry.

 Here’s the thing though, every single thing you eat during this indulging time needs to be processed by your body.

Once you are done digesting, some of the nutrients will go straight to the pile of stuff your body can burn right away, but other nutrients will need to be converted into something else.

For example the carbs and fats in your buttered toast can get directly oxidized into useable energy.  Awesome.

But the amino acids in your bacon have to be converted into molecules that then have to be broken down, if you want to get energy out of them.

 So, while you’re sitting there gorging on a giant meal, your body is busy working away, breaking down and converting all of this food into useable energy. 

 This process, as I’m sure you know, is called metabolism.

 Like I said last week, metabolism, is every single biological chemical reaction that is happening in your body.  

More importantly, it’s a never-ending series of reactions that are dedicated to doing two very contradictory things.

One of those chemical reactions is anabolism

Anabolic reactions build, construct and consume energy. In order to build muscle, you need to be in an anabolic state.

Catabolism breaks down bigger molecules (from either your body or food) to release the energy you need to stay warm, and move around. Cardio is an example of this as your body needs to break down fat stores to convert into energy.

Ok, but HOW do we derive energy from the food we eat?

 Two words….


This is the process of breaking down nutrient molecules to generate the all-amazing ATP, which means energy.

ATP is molecular currency and your muscles are constantly crying out for more!

 ATP contains chemical energy and our muscles are OBSESSED with turning energy into motion.  All we want to do is move.  We’re kind of addicted to it.   So our muscles crave ATP just as much as you crave actual money.

 Cellular respiration is the process of breaking sugar into a form that the cell can use as energy.  In other words, cellular respiration takes the food you ate from the buffet and uses it to create ATP (energy).

Now, how this energy is converted and where all depends on when you ate your last meal.

There are two nutritional states that we constantly switch between.

Absorptive (fed state), which is during or after eating.

Post-absorptive (fasted state), when the GI tract is empty and the body is running off stored supplies.


You’ve just eaten a massive meal, which puts your body in the absorptive state.

While you start to unbuckle your belt, your body gets to work breaking down the eggs, bacon, syrup, waffles, (salad?), muffins, etc. into a bunch of (mostly) glucose molecules that pass into your bloodstream.

The first bit of glucose gets delivered throughout the body and is tapped to generate ATP on the spot, through the process of cellular respiration.

But since you went a little crazy and had EVERYTHING IN SIGHT, there’s now more glucose floating around than your cells need at the moment.

 ATP is waaaay too unstable to be used for storage and your body will only need a certain amount.  This means all this extra glucose you’ve eaten will need to be stored as either glycogen, or… wait for it… fat.

How much of that energy gets stored depends on your basal metabolic rate (the number of calories your body needs to survive).  Or to put it into context, the more energy (ATP) you need.

The more ATP your body needs to make, the less will get stored as fat.

This number is influenced by your age, sex, body size, composition, or how much you move.

But there is an awesome trick here!

If you look up a BMR calculator you’ll notice that a higher weight needs more calories to survive, WOO! 

The lighter you are the fewer calories you need - not ideal if you love buffets.

If you’re like me - wanting to be slim but still get to gorge on massive buffets - then the trick is to weigh more by gaining MUSCLE and not fat. 

More muscle means you look better, can do more activities, you’re stronger annnnd you get to eat MORE.  It’s a win win win!

If you want to learn how to build muscle, find out here.

Ok back to fat science…


So your body likes to maintain a blood glucose level of 70-100 mg/dl.  But since you ate that massive meal, your blood glucose has now shot up to 140 mg/dl. 

This is not good.  High blood glucose levels are very dangerous so your body is forced to react!

Your body triggers special beta cells in the pancreas to start secreting more of the one hormone that regulates EVERYTHING that happens when you are in the absorptive state.

You may have heard of it… 


Insulin’s job is to move glucose out of the blood and into storage.  This triggers a shift from catabolic reactions (breaking down) to anabolic ones (building).

It puts a stop to glycogenolysis, which is the breaking down of glycogen in your liver and muscles to be released as energy.

And instead starts the process of glycogenesis, which links all this extra glucose together to make glycogen.

See, metabolism – the anabolic and catabolic reactions playing out.  You get it now yes?  I know I’m an amazing teacher.

Ok but seriously, this is the part you really care about –

Insulin also activates lipogenesis, where very cool chemical reactions in the liver convert glucose into triglycerides and then ship them off to adipose tissue for storage.  Aka, this is point your fat cells get more juice.


Now that your body has put everything where it needs to go (probably in places you didn’t want it to go, but hey, you had fun right?), you then enter into the postabsorptive state.

For several hours later, even though your small intestine is still working on what’s left of the buffet, your cells have been helping themselves to the remaining glucose in your blood and your blood sugar levels start to drop.

Yay… yes?  No, no.

Just as rising blood sugar levels is dangerous, low blood sugar is also very dangerous.

Your neurons are on an exclusive glucose meal plan and they always need sugar at the ready.

If your body senses a drop in blood sugar it SOUNDS THE ALARM.

This stimulates alpha cells in the pancreas to release the hormone glucagon - insulin’s arch nemesis. 



Glucagon raises blood sugar levels by triggering the liver and fat tissues to metabolize fat and glycogen stores.

It breaks down your fat stores, turning glycogen back into glucose so it can be released into the bloodstream.

Glucagon is crucial to the body’s response to a lack of food.  It encourages the use of stored fat as energy in order to preserve the limited supply of glucose.

No, don’t get any ideas about starving yourself to lose weight.


Ok, say for example you felt soooo guilty about the massive buffet you just ate so you decided to stop eating for two days in order to counteract the damage.  You thought, “hey I’ll just let glucagon burn my fat for me.”

Two days later and you’ve now burned through both your glucose and glycogen stores, and now you don’t have any sugar left to feed the brain!

In the spirit of keeping you alive, your body will react by launching into gluconeogenesis and convert fats and amino acids into glucose so ATP synthesis can continue in your brain cells.

This is your body’s last-ditch effort to protect the nervous system from the extremely damaging effects of low blood sugar.


Yes being skinny, weak and feeble can seem appealing, but you know, it’s also good to keep your BRAIN well fed so it doesn’t get ‘Hangry’… or dead.



The overall purpose of eating food is to replenish nutrients, because you are literally made up of the nutrients you eat.

If you are an average person (I’m sure you’re way better than average) 65% of your body is water.

16% protein of your body is protein, about 5-40% of you is fat (depending on how often you’ve been hitting up a unit 13 session, amiright’?) and about 4% goes towards minerals, like the calcium and phosphorus in your bones and the iron in your blood. 

Finally 1% of you is carbohydrates – most of which is being consumed as you are reading this, or is sitting around as glycogen waiting to be used. 

When you break it down, this really brings truth to the phrase “you are what you eat.”

Realistically though, it’s not like 70kg of food suddenly reconstructs to become the magnificent creation that you are.  Instead your body is constantly acquiring new stuff, extracting some of it to keep, burning some of it for energy and getting rid of the rest. 

And all the protein and fat and carbohydrates and nucleic acids that make up you, come from food.

No organism can survive without food.  So if you ever get that thought, “omg I ate so much last week I’ll just starve myself for a week to undo the damage!” – I’m sorry but it doesn’t work like that.

You will constantly need to take in and break down food in order to keep resupplying the body with the raw materials it needs to survive.

So starving should never ever be the answer to weight loss.

Maintaining a health body weight is largely about keeping a good balance between energy intake and energy output.  In other words, eat a little less at the buffet, or move way more that day.  Or, build more muscle so your body naturally needs more energy!

If you made it all the way to the end THANK YOU!  You are awesome and I really hope I was able to teach you something new(:

If you have any opinions, or if I got anything humiliatingly wrong, please feel free to leave a comment!


Happy Training,

Ciara Glover (@ciciglo)