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If I understand things, and I most likely don't: mammals primarily use carbohydrates to produce metabolic energy when there are sufficient carbohydrates to do so. The use of fats for fuel requires the organism to break down the fat into constituent parts and from there produce either glucose (right?) or ketone bodies. Presuming that the organism is not in ketosis, it seems that its body would preferentially use carbohydrates for fuel.

This is where my confusion comes in: why would the mammal's body use fats for fuel if its diet contain sufficient carbohydrates? The real underlying layman's question is: why is that when I eat fat, I have to count those calories, if my body preferentially uses carbohydrates for fuel?

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  • $\begingroup$ The only way you wouldn't have to count calories from fat is if your digestive system wouldn't absorb them. Fat might not be used immediately as metabolic fuel but it will still be stored as adipose tissue in your body. $\endgroup$ – MikeKatz45 Nov 28 '18 at 15:45
  • $\begingroup$ So you're saying that the fats are presumably broken into their constituent parts and will be stored as fat tissue if I'm in a caloric surplus, yes? If so, then to change the question slightly, when I consume a deficit of carbohydrates compared to my base metabolic requirements, and consume the rest as fat (for simplicity), does the fat get turned into adipose tissue and then to fuel, or is it converted directly? $\endgroup$ – Michael Stachowsky Nov 28 '18 at 16:15
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    $\begingroup$ Yes, fat is not turned directly into energy and it is stored first before its usage. If you undergo a carbohydrate deficit you would first make use of your glycogen stored in the liver and muscles before proceeding to use fat for fuel via beta oxidation. I can try to provide a more detailed answer once I get off work later. $\endgroup$ – MikeKatz45 Nov 28 '18 at 16:32
  • $\begingroup$ @MikeKatz45, it seems that now the question is getting clear and you seem to be able to make an answer. If not an excessive effort for you, I'd love to read a simple explanation about how carbs and fats are consumed at the same time are used for current metabolic needs. Mainly how fats are converted into energy. $\endgroup$ – Jan Nov 28 '18 at 16:39
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To expand on my comments:

Just like you mentioned in your question, carbohydrates (mostly glucose) is used as the primary source of energy when available to satisfy metabolic needs in human cells, however lipids and even proteins will be used if required, in order to produce energy. Since all these biomolecules (carbs, lipids and proteins) can actually be used to sustain the metabolic activity of cells, that means all of them store energy to a greater or lesser extent (which we can quantify in a practical way as food calories). Other things that we eat or drink such as vitamins, cofactors, water, etc. are not accounted for in the calorie count because they can't be broken down to produce energy for metabolism.

Dietary carbohydrates are easily digested into smaller blocks (mostly glucose) and absorbed directly into the bloodstream, afterwards glucose is taken rapidly from the bloodstream by human cells and processed via glycolysis and lactic acid fermentation or the citric acid cycle (all these are metabolic pathways for glucose in humans). Each gram consumed of carbs yields approximately 4 calories of energy.

Lipids on the other hand undergo a more complex process of usage after their slower digestion compared to carbs since after their absorption they have to be transported in special molecules across the lymph, then into the bloodstream and then they can be taken by various kinds of cells. Considering just this, we can start seeing why lipids aren't a fast source of energy (they take long to be available as potential energy sources). Furthermore, the main destination of dietary lipids is the adipose tissue which constitutes a special tissue in which fatty acids can be stored for latter usage. Fatty acids not being used immediately to provide metabolic energy doesn't mean they lose their stored energy (doesn't mean they have no caloric intake value), its just "quiescent" for when its needed (when exercising or when fasting for example).

Each gram of consumed fat yields approximately 9 calories of energy, this quantity is higher compared to that of glucose because the metabolic pathway that breaks down fatty acids (beta-oxidation) in combination with the citric acid cycle produces more metabolic energy (ATP) compared to glycolysis in combination with the same citric acid cycle. However such amounts of energy are not usually required in rest.

It is important to mention that glucose can also be stored in the form of glycogen or even fat since it will be converted to these molecules if there is more glucose available compared to what the cell needs in a given moment. This glucose converted to glycogen or fat also has to be accounted for in the calorie count because its still a potential energy source (just not used right now).

References:

Dietary Carbohydrates - National Agricultural Library

Dietary Lipids - National Agricultural Library

Insights into digestion and absorption of major nutrients in humans - Goodman, B. E.

Food calories per gram of biomolecule - National Agricultural Library

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  • $\begingroup$ I'm tempted to suggest you to copy-paste the first two paragraphs from p. 430 from your 2nd link nap.edu/read/10490/chapter/10#430 , which clearly explain how fats are metabolized and why are metabolized after carbs. $\endgroup$ – Jan Nov 29 '18 at 14:21

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