Carbohydrates Part 2: What are carbs used for in the body?

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Simon Long

Simon Long

Simon is a highly experienced personal trainer and behavioural psychology expert
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The body loves to use carbs as its energy source. This is due to several factors. Firstly they break down easily into glucose, which in turn is easily transported in the blood, or plasma, as blood sugar. This allows it to easily be moved to wherever it is needed at the time. Compare this to fats, (Covered in Fats Part 1) which need to be broken down from triglycerides, attached to the lipoprotein albumin and then transported to the required area.

Carbs also create energy at a far faster rate than either fat or protein. This is again due to them being readily available in the blood stream. In turn this means that the metabolic pathway, or the path that the carb needs to follow to be turned into energy, is much shorter. Even before blood sugar gets to the muscles that need it they are likely to hold a store of carbohydrate as glycogen. Fat and protein on the other hand have a much longer journey. Fat we discussed above, and protein needs to be taken to the liver first, where it is transformed into glucose in a process called gluconeogenesis. Then this glucose has to be transported to where it is needed.

This far shorter metabolic pathway means that carbs create energy at a rate that is twice as fast as the other macronutrients. It is also more efficient, with 6% more energy being produced per reaction. It is this speed and efficiency that is the reason that carbs are the exclusive energy source for anaerobic (high intensity) exercise.

Use 2: An intramuscular store of energy

As mentioned in Carbs Part 1, we store carbs that are not used as glycogen in our muscles and liver. This glycogen can be converted back into glucose via glycogenolysis, when energy is required.

This readily available energy is key during the first 10 minutes of exercise as it can be metabolised without oxygen. This allows the body time to increase its intake of oxygen and meet the oxygen demand to metabolise the blood glucose and free fatty acids.

Humans on average hold about 6 grams of glycogen per kilogram of bodyweight. However with a structured increase in carbohydrate intake, known as carbo loading, these stores can go as high as 15 grams per kilogram of bodyweight. This means the average 70kg human will hold between 420 and 1050 grams of glycogen in their muscles. As each gram contains 4 calories of energy this equates to between 1680 and 4200 calories in total. This would only be enough, if no hard physical activity was taking place, in a day or twos worth of energy. If hard physical activity, such as exercise, was taking place then this energy could be burnt off in a matter of hours.

It is this reason that a reduced carbohydrate intake over a few days can quickly deplete these stores. It is worth noting that each gram of glycogen is stored with nearly 3 grams of water. So the glycogen stores of the individual above, when water was taken into account, would actually weigh between 1.6 and 4.2 kilograms. This is the reason that when people first start a diet they seem to lose a few kilograms in the first few days. Very little of it is fat, most of it will be their glycogen, and the water it is stored with, being used.

Use 3: The central nervous systems exclusive energy source

The brain and central nervous system (CNS) use carbohydrates in the blood as their almost exclusive energy source. If they are deprived of carbs then they will not function as efficiently. This is the reason that individuals who do low carb diets, such as the south beach, often complain of feeling tired and lacking energy and focus.

However the body is able to use ketone bodies to supply the brain with energy if carbohydrates are not sourced for prolonged periods of time (See Use 4: A fat metabolism primer below). This however takes about 8 days to kick in. It can be seen in action as quite often the fog which covers your brain when you first start to diet will dissipate after a week or so.

Use 4: A fat metabolism primer

When carbs are broken down they leave behind some waste products. Some of these are then used in the metabolic pathways that metabolise fatty acids.

If these fat burning primers are not available then fat mobilization (the release of fat from the adiposities) happens at a quicker rate then fat oxidization (the breakdown of this fat for energy). This results in partly broken down fat being present in the body. These are called ketone bodies.

These ketones will acuminate over time is carbohydrate continues to not be eaten and will result in the bodies tissues becoming more acidic. This is a condition known as acidosis, or in this specific instance ketosis.

Use 5: A protein sparer

As mentioned above if carbohydrates are not available and a reduced calorie diet is being eaten then the body needs to fill the energy deficit with something. A large amount of this will come from the stored fat in the adipose tissue. However the body will also start the process of gluconeogenesis in the liver. Gluconeogenesis breaks down molecules in the body that are not carbs or fat into glucose. This will include a lot of the bodies amino acids, the building blocks of protein.

If this protein is being converted into glucose for energy use then it cannot be used by the muscles tissues to maintain and repair themselves. This will mean that over time the bodies lean tissue, especially muscle, will be lost.


So that’s the uses of carbohydrates. Keep an eye out for more articles on carbs that are coming soon, including how the body uses carbs during exercise and what diseases and ailments can be contributed to carbs and why.

Also keep your ear to the ground to see future articles going more into depth about proteins and fats. A great way to be the first in the know is to follow me on social media. The links can be found below or at the top of the page.

As always if you have any questions give me a shout. If you have any comments them stick them in below. I always read them.

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Best wishes,

Si   =]