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A widely cited figure is a pound of fat contains 3500 calories of energy in fatty acids.

However, a pound of fat also contains adipose cells, arranged in adipose tissue, connective tissue etc.

When a human breaks down fat to use as energy, from what I understand it doesn't break down the fat cells, just liberates the triglycerides stored inside those cells.

Those fat cells, and connective tissue took energy to assemble as well.

How much energy is used to create a pound of fat tissue?

Let's assume that all the cells and the triglyceride stored inside them are created from scratch.

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A rough answer to a rough question

According to a study conducted in the 1960s, human adipose tissue, on average, contained (per 100g wet weight) 87g fat, 2g protein, 2g sodium and potassium and 10g water. (Nothing else was measured.) This suggests that approx. 3% of adipose tissue is solids other than triglyceride.

The figure quoted is presumably the energy yield for the complete oxidation of triglyceride. The energy yield for the complete oxidation of the non-triglyceride solids in adipose tissue — the appropriate, if hypothetical, comparison would be less than that for triglyceride, which is much more chemically reduced, so the yield for the oxidation of non-triglyceride might be approx. 1–2% that of triglyceride.

The energy needed to bio-synthesize any organic compound (triglyceride, carboyhdrate or protein) is more than is obtained from it by oxidation because a negative free energy change is needed to drive reactions. Hence, as regards the relatively constant (slowly turning-over) portion of the adipose tissue, the cost would be higher — perhaps 2–3% of that quoted produced by oxidizing the triglyceride.

The problem with this question

Some people say that there is no such thing as a stupid question. Nevertheless, there are questions that are formulated as if their answer will establish solve some problem, when in fact it will not because the premise on which it is based is false.

In this case a comparison is made between the triglyceride and non-triglyceride content of adipose tissue with the implication being that the latter should be assessed in the creation of the former:

“Those fat cells, and connective tissue took energy to assemble as well. How much energy is used to create a pound of fat tissue?”

As already mentioned, it is invalid to make a numerical comparison between the energy required to synthesize one compound with that released by the oxidation of another compound, as it costs more to synthesize a compound than is released by its oxidation.

However, quantitatively more fundamentally wrong is the implication that the one-time synthesis of the structural components of the cell should be costed into the repeated oxidation of triglyceride in that cell. How many grams of triglyceride are synthesised per gram of cell structural material during the lifetime of a cell? Very many, I should imagine if the cells turn over much more slowly than the lipid they contain.

It would be more pertinent (assuming that you are interested in such things) to ask what the cost is in terms of the energy used by whatever muscles are used by the lungs to breath in the oxygen needed to oxidize the fat, and the energy used by the heart muscle to pump the blood containing the oxygen to the fat cells so that it can be oxidized.

Finally, it is naïve to think of the energy cost of maintaining a fat cell in terms of the synthesis of its structural components. Other metabolic processes are continually occurring that require energy even in fat cells.

No doubt it would be possible to perform a calculation that took all these things into account (although what would you start with “from scratch” — ingested carbohydrate or carbon dioxide and water?). Personally, I am happy to accept that the system makes a profit and interest myself with the details of how it operates and adjusts to different circumstances.

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    $\begingroup$ Maybe more to the point regarding the second half of this answer: all of the maintenance of fat cells themselves (as well as all the activity of the lungs, heart, etc) is wrapped up in the concept of "basal metabolism", so it's sufficient to consider the calories stored in a given mass of fat when approaching the issue from a 'calorie economy' perspective. $\endgroup$
    – Bryan Krause
    May 21 '20 at 16:28
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    $\begingroup$ @BryanKrause — You’re right, the basic metabolic rate stuff would take all that into account. It’s the kind of thing physiologists do, and no doubt they have figures for different tissues. Perhaps one of them could enlighten the poster. $\endgroup$
    – David
    May 21 '20 at 17:57
  • $\begingroup$ Great answer thank you! $\endgroup$ May 26 '20 at 5:29

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