Article Originally Submitted 2003
Your Liver: The one organ in the body that is actually metabolising the fats, carbs and proteins that you are putting into your body. It is your Liver that makes you burn fat, nothing else.
What’s that I hear you all cry? I thought that in a low-carb situation, the body gets its energy supplies from Fat, and that was all I had to worry about!
Well, yes, this is true, but the body has to make it usable first, and that is one of the Liver’s jobs. If your liver isn’t firing on all cylinders, you are not burning fat.
The liver is the most versatile organ in your body, and is certainly the busiest! Performing 500 or so functions, it can repair itself (within reason!), weighs between 2.5 and 4lbs, and has 3-4 pints of blood pumped through it every minute.
What I am going to talk about here is the role that the Liver plays in food metabolism.
(I warn you now, I am going to get a little technical.)
Your Liver is the main player in metabolising all three of the Macronutrient groups.
To get food into the body, it has to be transformed into water-soluble matter. To do this, a few things have to happen. Firstly, the Digestive tract contains Enzymes and Acids, produced by the Saliva glands and the Pancreas, which break down the Foods you eat.
Complex Carbohydrates are broken down into three simple sugars, Glucose, Fructose and Galactose by Carbohydrase. Proteins are broken down into Amino Acids by Protease. Fats are converted into Fatty acids and Glycerol by Lipase. Sugars and Amino Acids are then absorbed through the wall of the small intestine.
Fats, being of course not water-soluble then have to be emulsified. Bile from the Liver is injected into your Small intestine, and emulsifies the Fatty acids, and then a particular lipoprotein coat, called chylomicrons, transports them from the intestine to the liver.
Once in the blood, the Liver then further sorts out what you have just eaten.
Proteins: The Liver breaks down the complex Amino acids into “Amino (Nitrogen bearing) Groups” and L-Glutamic acid (or Glutamate ion) by a process called deamination, and then reforms these into a myriad of things! “Non-Polar” (i.e. fatty, non water-soluble) substance transport units, building blocks for your muscles, enzymes, and Nucleotides, (the building blocks that form your DNA and RNA, and ATP, the body’s “energy currency molecule”). It also forms serotonin, the brains neurotransmitter chemical, and porphyrins, which are the central oxygen-binding component of haemoglobin. The left over “carbon skeleton” after this little chemistry lesson has taken place (around 10%)is converted into either acetyl coenzyme A (Abbv. acetyl CoA), pyruvate or a “citric acid cycle intermediate”, and then processed down further into either ketone body for the former or glucose in the latter two, and used directly for energy.
Fats: The Liver breaks open the chylomicrons, and converts the Fatty acids inside into triglycerides and phospholipids, they are then reformed, and are then rushed off by Lipoprotein Lipase to dump their triglycerides into your fatty (adipose) tissue (i.e. that spare tyre!). However, the phospholipids are used for cell wall construction, and so are taken where they are needed.
Sugars: Fructose and Galactose are converted into Glucose, and then what Glucose is not needed instantly for energy is converted into Glycogen. The Liver can store around 2000 calories worth of Glycogen, which it can convert quickly back into glucose, when the body requires it. (You also store Glycogen in your Muscles) However, when your diet is high in carbohydrate, the Liver’s Glycogen store is usually full, so, the Glycogen is converted into pyruvate then to acetyl CoA. This is then converted into fatty acids, and again, off to the stores they go.
Right, take a breather… I know, this is a lot to take in, but I need to explain it all… If you eyes have glazed over, get up from the PC, and go get a Glass of water – I’ll be here when you get back 😉
Ok. That water nice and cool? Ready to concentrate again? Here we go then!
What changes under low carbohydrate conditions? From what I have just described, both Starches and Fats are “fattening”!
These processes are the basis of thought that forms the low-calorie diet theory. Protein is not deposited in fat stores, and so is not “fattening”. Carbohydrate is being metabolised into “Pure Energy” and we have trained our bodies to use it as “preferred” fuel, and so, it cannot be “fattening” either.
This has left heavily calorific Fat with the bum wrap. Under “normal” circumstances, the body never gets the chance to burn it, and so, it has to be “fattening”!
However, Glucose isn’t what actually provides the energy our cells burn, it has to be broken open, and a smaller unit, called ATP has to be created.
Glycolysis is the primary mechanism to convert glucose into ATP, and the process happens in the brain, muscles and the kidneys. Glycolysis is what drives our “Fast Twitch” muscle action, i.e. the Fight or Flight response.
However, there is also a penalty for this, pyruvate in the muscles is converted into Lactic acid. This is called Anaerobic Glycolysis, and is what gives you “The Burn” when exercising. This lactic acid is transported out of the muscle by the blood, and either converted back into pyruvate by other muscles, or into glucose by the liver (Gluconeogenesis). This recycling pair of Glycolysis/Gluconeogenesis is called the Cori-cycle.
Glycolysis can only occur with a steady supply of Glucose, and as I am sure you are aware, the body doesn’t store glucose as glucose, but rather as triglycerides. I note here, very importantly, that there is no mechanism in animals for the conversion of fatty acids to glucose. I.e. once the immediate supply of Glucose is exhausted, we have to stop sprinting whilst the body can convert either some glycogen, or the lactic acid it has just produced back into glucose.
So, how do people run marathons? They utilise ß-oxidation of Fatty acids. ß-oxidation occurs in the mitochondria of your liver and your muscles. In a body used to burning glucose, all that happens is that Fatty acids are broken down and two units of acetyl CoA are produced, which is then “wasted” through the citric acid cycle, to become CO2,. This process produces ATP, and can sustain the “Slow twitch” muscle fibres indefinitely. This is “breaking through The Wall”, when the body is forcibly switched from Glycosis to Lipolysis.
However, and this is the clincher for us, acetyl CoA doesn’t have to be wasted, but can be used to make Ketone Bodies.
It is converted in the liver into acetoacetate, which can be further reduced to form ß-hydroxybutyrate. These two compounds are referred to as ketone bodies. (A note here for Atkins’ dieters, only the former is measurable with Ketostix, which is why you can still be in BDK even when you are not turning them purple.)
They diffuse from the liver into the circulation and are used as fuels by several tissues, in the process of Ketosis (the formation of ATP from Ketone bodies, rather than glucose)
Heart muscle and renal cortex, in particular, use acetoacetate in preference to glucose.
In contrast, glucose is the major fuel for the brain and erythrocytes in a human on a “balanced” (i.e. high carb) diet. However, (and this is really important) the brain has the capacity to adapt to the use of acetoacetate during “starvation”, which is what low-carbing is to our glucose-trained metabolisms, and in “starvation” of long standing, acetoacetate meets more than 70% of the energy needs of the brain. The other 30% required from glucose, well, that can be synthesysed from protein.
This ability of the brain to adapt to the use of acetoacetate is important because, being water soluble like glucose, it is readily accepted across the blood/brain barrier as a valid source of ATP.
The amount of ATP produced by ß-oxidation or via Ketosis is approximately the same – there is no penalty to the body in converting acetyl CoA to this water soluble form. Also, the amount of ATP produced by Glycolysis is less than that produced by either ß-oxidation or ketosis. It is more efficient for the body to run on ketones that on glucose.
So, I come to the punch line: Without a healthy liver, you cannot have ß-oxidation, and you cannot produce either acetyl CoA or Ketone bodies, and therefore, are not burning your fats away.
How do I get and keep my Liver healthy? Keep drinking that water (which is very important, and a good first step!), and I will tell you next time…