The body metabolism, on a sugar shortage

keto diet rabbit - comic


Wikipedia definition1 :

nutrient is a component in foods that an organism uses to survive and grow. Macronutrients provide the bulk energy an organism’s metabolic system needs to function while micronutrients provide the necessary cofactors for metabolism to be carried out. Both types of nutrients can be acquired from the environment.

You already know them: water, carbs, lipids, proteins, vitamins, electrolytes. They all have a role to play, but I’ll just focus here on those that can provide energy to the body.

digestion mechanisms


We all know that there are the slow sugars and the fast sugars. Excepted for the speed of action, which is an important factor, they basically play a similar role.

Carbohydrates are always consumed in priority to the two other sources of energy: lipids ans proteins.

Carbs are stored in the body as glycogen:

  • about 400 grams stored in the muscles (depends on your muscle mass)
  • 100 to 120 grams stored in the liver

In order to store that glycogen, the body uses water. It needs about 3 to 4 grams of water for each gram of stored glycogen. You therefore need 2 kilos of water (= 2 liters) in order to store those 500 grams of glycogen.

If you fast, your body will have to deplete its reserves of glycogen. You will consequently loose those 2 litters of water within 2 days. And you will get them back as soon as you start eating carbs again. When you ignore that mechanism, those important weight changes can seem disturbing or even alarming. You know now that it’s all fine. It’s just water.


Fibers from plants are cellulose. They belong to the carbs family, but they play nevertheless a much different role because the enzymes of our digestive tract are not able to cut those molecules appart. So, they travel through the small intestine and arrive to the colon. The colon is populated with bactérias : that’s what we call the microbiata, or intestinal flora. Once inside the colon, those bacterias will tear appart the cellulose using the fermentation process (that releases methan and CO2). Once broken down, they can be absorbed.


Lipids are the fats. We often run into the debate between bad fats: saturated fatty acids (from animal fats), and good fats: mono-unstarated and poly-unsaturated fats (olive oil, avocado and so on). That could be a subject for a whole article so I will not go into further details here.


Proteins, when in small amount, are responsible for the growth and repair of the muscles. There are found in the blood as amino acids.

In case of an excess of proteins and a shortage of carbs, amino acids can be converted into glucose by the liver (neoglucogenesis). This mechanism is triggerd by the glucagon hormone, which has an opposite effect of insuline. In such a case, the pancreas also release insulin, so that glucose can be absorbed by the muscles and the organs.

In case of an excess of proteins and a sufficient amount of glucose, amino acids can stay in the blood until they are used. Unlike glucose, the do not become toxic when raising. Neoglucogenesis is a mechanism that is triggered “on demand” (by the presence of glucagon). It therefore does not depend on the quantity of amino acids in the blood.

That means that proteins can help regulate the blood sugar level (glycemia). But they also triggers insulin release.

Nutrients in the blood

In order to be transported into the blood, nutrients take the following forms:

  • carbohydrates: glucose
  • lipids: fatty acids
  • proteins: amino acids

Our food system do the apology of carbs. It is however interesting to note that only glucose becomes toxic when is in excess in the blood. It is also the reason why glucose is the energy fuel that is used in priority by the body.


Alcohol is a different molecule. It is not a carbohydrate. Yet, strong alcohol like vodka do not contain any carbs. Alcohol is not considered as a nutrient because the organism does not need it.

When we drink alcohol, the ethanol molecule is absorbed and released into the blood. Still, it is even more toxic that glucose is for the body. So, when we intake alcohol, the liver uses its full capacity to transform ethanol into (acetaldehyde then) acetate, which is a less toxic form, and usable by the body.

When the liver is busy taking care of the alcohol, it becomes less efficient for the other tasks that are assigned to it.

Eating before or after alcohol consumption may diffuse its absorption over time and thus limit its effects.

What happens when we stop eating carbs

Our body posses very complex mechanisms that enables it a great flexibility. When a fuel source experiences a shortage, it is able to adapt and use another one. The downside is that sometimes, this mode switching can take a while before being fully optimal. So, how does the body react when you deprive it from carbohydrates?

Those mechanisms are split in order to simplify. They actually overlap.

1. After a meal with carbs

When? up to 3 hours after the meal

During and after the meal, the small intestine convert carbohydrates into glucose (thanks to enzymes). Then glucose goes through the intestinal wall and into the blood.

The blood glucose will be directly used by the muscles and the organs. Also, it will be stored into the liver and the organs for latter use, under the form of glycogen.

  • small intestine: carbs -> glucose
  • muscles:
    • glucose -> energy
    • glucose -> glycogen
  • brain: glucose -> energy
  • liver: glucose -> glycogen

glucose into glycogen - scheme

2. Once the blood glucose has been used

When? from 3 to 12 hours after the meal (can last up to 24h)

After about 3 hours, the small intestine does not provide anymore glucose and its blood level (the glycemia) lowers. Pancreas then release glucagon in order to indicate the liver and muscles that they may use up their glycogen reserves.

The liver will produce glucose and release it into the blood stream. Muscles directly use their own glycogen. This latter is only used locally: muscles cannot produce and release glucose into the blood like the liver does.

  • muscles: glycogen -(local use only)-> energy
  • brain: glucose -> energy
  • liver: glycogen -> glucose

glycogen into glucose - scheme

3. After glycogen consumption

When? from 12 hours to a week after the last meal (may even lasts several weeks)

From 12 hours after the meal, the glycogen reserves get nearly empty. This times approximately corresonds to the duration between dinner and breakfast. With a minimum of 3 meals a day, our body only experiences the first two operation modes.

The body is not able to transition directly into a glucose free mode. It will therefore use its amino acids and fatty acids from the blood and convert them into glucose. This happens in the liver.

At the same time, the transition into a fat running mode is starting: the liver starts producing ketone bodies from fatty acids, and releases them into the blood. Muscles will use most the those ketones. They will however begin to use fatty acids as a fuel source.

This transition mode is called ketogenic.

  • muscles:
    • fatty acids -> energy
    • ketones -> energy
  • brain: glucose / ketones -> energy
  • liver:
    • amino acids –neoglucogenesis–> glucose
    • fatty acids –neoglucogenesis–> glucose
    • fatty acids –> ketones

neoglucogenesis - scheme

4. Keto adaptation

When? more than two weeks after the last meal

From about 2 weeks after the meal (this duration may vary a lot from one person to another, the body is completely adapted to the glucose shortage. This is what we call keto adaptation.

The muscles have stopped using up ketone bodies as a fuel source. They mainly operate over fatty acids. The ketones are consequently found in a bigger amount, which enables a better supply for the brain (experienced as a sensation of feeling great, or even euphoria).

The livers keeps ensuring that the glycemia remains stable by converting fatty acids into glucose.

  • muscles: fatty acids -> energy
  • brain: ketones -> energy
  • liver:
    • fatty acids –neoglucogenesis–> glucose
    • fatty acids –> ketones

keto adaptation - scheme

Those mechanisms are those of fasting. However, the ketogenic diet, very low in carbs, low in proteins and high in fats, triggers similar energy operating modes (other mechanisms, like the immune system, may have a different response).


I think it is important to correctly understand those mechanisms before initiating an itermittent fast, a long fast, or a keto diet. Those changes may sometimes represent a big stress (but a benefic one) for the body. It therefore helps understanding the what’s happening when experiencing a hard time and prevent any panic behaviours.

  1. Wikipedia : Nutrient

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