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Tl;dr: What is the net joule value of fat if used for energy provision, compared to e.g. carbohydrates. So does 1 Joule Fat intake result in 1 - X Joule energy provided to the cell, or does it work in a completely different way?


My understanding is that if I take in one Joule of fat, my body takes in the energy to move ~100g of mass up by one meter (Wikipedia).

Now for my body to use that energy, it needs to convert it to some sort of Glucose, right?

From the Lipid Metabolism Wikipedia Article I get that my fat needs to become a Triglyceride to serve as energy source.

There are some steps involved, roughly the following:

  • gastric peristalsis (basically muscle contractions to mix it all up)
  • peristaltic contractions in the gut (again, mixing is key)
  • lipase, also working in the gut to finally create Triglyceride
  • then the fat gets "packaged" in Chylomicrons and put into the bloodstream

releasing energy to the cell involves the following

  • Lipase unpacking the Chylomicrons
  • Splitting of the Triglyceride in Glycerin (the stuff we want) and fatty acids

Now this seems like a really cumbersome process to me, and I wonder what the net Joule Value is that arrives at the cell. Is it significant, or is this process so efficient that it doesn't matter anyways?

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  • $\begingroup$ "the energy to move ~100g of mass up by one meter", so you are interested in mechanical energy output. For energy as such does not change. On the level of biochemistry, perhaps the interesting question is how much net ATP is gained, and how much mechanical energy can be gained from that. And, no, I'm not an expert but just like you waiting for one to come along. $\endgroup$ – Gyro Gearloose Jan 4 '18 at 17:11
  • $\begingroup$ @Gyro Gearloose: Okay, that makes perfect sense. As soon as I am home I will try to rephrase the question more specific. I have never had any deeper chemical / biological education, just starting to get an overview over the terminologies... $\endgroup$ – Haini Jan 4 '18 at 19:23
  • $\begingroup$ I'm not so sure about the balance, but you need some heat to function. Fat might generate a bit more "waste" heat, leaving less to use for mechanical purposes, but perhaps glucose is so efficient you actually just need to burn a bit of it to keep warm. Also keep in mind that also the digestion of glucose needs several of the steps you mention, and nutritional energy content is the chemical energy content with some correction factors, perhaps already including the factors you mention. $\endgroup$ – VonBeche Jan 17 '18 at 17:04
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So, what I was looking for, but was lacking the knowledge of, was the concept of "Thermic Effect of Food"

I really didn't know how to phrase my question well, due to the fact that I tackled the problem from the wrong angle.

Direct quote from the Wikipedia Article as it answers the question best:

The thermic effect of food is the energy required for digestion, absorption, and disposal of ingested nutrients. >Its magnitude depends on the composition of the food consumed:

Carbohydrates: 5 to 15% of the energy consumed [7] Protein: 20 to 35% [7] Fats: at most 5 to 15 %[8]

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