The Fate of Macronutrients

A very simplified version of metabolism

Did you already read my first post about macros? If not, please do! In this post I will give you a very simplified insight what happens to the macronutrients in the body. I think this is necessary because there is still a lot of quackery around, demonizing macronutrients (like carbs) for some reason. Having some tools to understand what is behind metabolism may help get immune to it. If you do not want to read the whole article, here the spoiler: no matter if you are eating low carb, low fat or whatsoever, if you are in a calorie surplus, the body will start to store fat. If you want to know the why, read further.

We are now going to have a closer view what the fate of the macronutrients in the body is. Here is a simplified pic illustrating it and I’ll take you through step by step. To make it easier I’ve given each component a color, so that you can find it easier in my drawing.

Simplified metabolism of carbs, fats and aminoacids (proteins).


In a very simplified way, carbs are broken down into pyruvate in a process called glycolysis. Pyruvate‘s fate depends on the body’s needs, it is either:

  • turned into energy and CO2 or
  • converted into fats through acetyl-CoA or
  • converted into aminoacids through oxalacetate or
  • turned into glucose again through oxalacetate in case of low body sugar.

Do not forget, with carbs we mean everything which is digested and absorbed in the gut, no matter where it comes from, be it candy bars or potatoes.


To keep it simple, dietary fats are broken down into acetyl-CoA, which can be either oxidized into energy and CO2 or used for synthetis of (other) fats.

Do not forget, here we focus on dietary fats (triglycerydes), regardless if they are from an animal or plant source.


Simplified aminoacids can go two ways:

  • either through pyruvate into acetyl-CoA (energy and fat synthesis) or
  • some of the aminoacids can be used for glucose synthesis through oxalacetate.

Do not forget aminoacids are the building blocks of proteins. Proteins are broken down during the digestive process and aminoacids (or small aminoacid chains) are then absorbed.

Noticed who plays the central role in the metabolism? It is acetyl-CoA, the starting building block of fats!

THe main player of metabolism: Acetyl-CoA

If there is something that every macronutrient has in common, it is that it can be converted into Acetyl-CoA. This is the reason why everything you eat can theoretically be converted into fat and stored in the adipocytes (=fat cells).

In some case it may be easier than in others, for sure! A protein and fibre rich meal may not be the equivalent of the same calories of ice cream and biscuits. Still, at the end it is energy balance that matters and if you can reach it with ice cream and biscuits, well, you will lose fat.

Is it healthy on the long term? Probably not. Is it sustainable? Probably not. Nevertheless, the key for fat loss is not omitting one of the macros from your diet, but to be in an energy deficit (=eat less calories than you need). However, it is crucial to eat enough protein and useful to manipulate the carbs to fats ratio in your diet depending on your needs and preferences (it is NOT the same for everyone!).

I will cover every macro in my future posts. Hope you enjoyed my small biochemistry lesson. See you soon!

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