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I have been reading the Wikipedia article about food energy, but as I'm not a biologist, I don't really understand it properly, so I came here to ask you guys.

My question is: If a human body needs X kcal of energy and it's taking it from the stored fat, how much fat (weight/kg) does it need to burn?

I know that 1 gram of fat represents 9 kcal of energy, but according to the food energy article, the efficiency of getting that energy isn't 100%, so the formula might be a lot more difficult.

I'm not sure what is the real efficiency value here. The article mentions more values, ranging from 18% to 40%.

P.S.: If this belongs to Chemistry.stack, please, tell me or move it there.

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I am not sure if I am able to answer your question, but I will give it a try and hope that someone tells us if I'm wrong.

Biologists talk about different kinds of energy depending on where you measure it. The first measurement is the gross energy (GE) which is measured through bomb calorimetry and is basically how much heat a substance gives off when completely burnt. I have recently read in a compendium that the GE value of fat is ~9,4 kcal/g.

The next stage is digestible energy (DE), which is how much of the forage you actually can digest, which would exclude for example fibres.

Next stage is the metabolizable energy (ME), the energy which can be used in actual metabolism (DE - energy losses from transporting nutrients etc). This are the value you reported. (I don't have a source, but I've heard the same values from multiple recognized sources.)

Now is the part where it gets a bit trickier, as the final measurement of energy is the net energy (NE). This is defined as the metabolic energy minus the heat increment, which could be simplified as the energy losses through the conversion between different types of energy (ie. from long carbohydrates into glucose etc.) within the body. Even more simplified it can be said to be the difference between the heat production in a fastened person and the heat production in a fed person, thus being the heat produced by eating, digesting, absorbing and transporting food.

I argue that because the fat already is incorporated in the body it has a very efficient value, and very little transportation within the body is needed. And because the energy (ie, the fat) is internal, it is not accounted for in the heat increment and therefore the NE of internal fat is the same as ME.

And according to the wikipedia-article you linked to United Nations recommendation of the average minimum energy consumption per person is 1800 kcal/day. Thus the amount of fat required to meet the minimum energy requirements of a person would be 1800/9=200g of fat per day.

If you want to read more about the different energy-measurements and the heat increment here is a good link. And don't forget that energy alone is not sufficient for a person as we need protein, vitamins, minerals etc...

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