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As the Krebs cycle is involved with the conversion of food in to citric acid, why can't eating citric acid be used as a temporary primary source of energy - in place of fat/carbohydrate/protein?? It seems like a waste of energy to convert food into citric acid if there is freely available citric acid in your diet why wouldn't that preferentially be used for energy - as it would skip a lot of steps and therefore be more efficient. And what about drinking acetic acid for similar benefits, as "it is changed to citric acid by combining with oxaloacetic acid with the help of CoEnzyme A and ATP".

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  • $\begingroup$ how do you get citric acid in nature? $\endgroup$ – aaaaa says reinstate Monica Apr 2 '18 at 23:23
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    $\begingroup$ I order it online via amazon. Citric acid. ;-) Or I can find it in lemons too. $\endgroup$ – ol_matty_man Apr 2 '18 at 23:31
  • $\begingroup$ I might be wrong, but it seems that you would need 100kg of citric acid a day for 2400 kcal diet myfitnesspal.com/food/calories/495309621 $\endgroup$ – aaaaa says reinstate Monica Apr 2 '18 at 23:37
  • $\begingroup$ I think you mean 1kg. a day of citric acid, there are 2.5 calories per gram of citric acid. But the thing is I think that comparing equally to food calories is misleading, because if it were to be used for energy - it would be processed more efficiently than actual food which requires a lot of cellular energy to assimilate. $\endgroup$ – ol_matty_man Apr 2 '18 at 23:51
  • $\begingroup$ Please read what you have written: "If the krebs cycle is largely about converting food in to citric acid which then enters the krebs cycle." This is a conditional clause, rather than a sentence, but is obvious nonsense even if there were a main clause. How can a cycle do something to something if it is not already in the cycle in the first place? Please rewrite so it makes sense and in doing so spell Krebs with a capital K — it is the surname of Hans A Krebs who received the Nobel Prize for discovering the tricarboxylic acid cycle that bears his name. $\endgroup$ – David Apr 3 '18 at 11:58
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Why can't eating citric acid be used as a temporary primary source of energy - in place of fat/carbohydrate/protein?

No reason it can't. Citric Acid technically contains 2.5kcal/g -- which is almost half of protein/carbohydrates, almost a third of alcohol, and a fourth of fat.

So if someone needed 2500kcal/day, they'd need 1kg of straight Citric Acid. There's about 47g/L of Citric Acid in citrus fruit juice. So if you wanted it all from "natural" sources you'd need to drink 21.3L of your favorite citrus juice, though that's a bit comical.

In terms of being more efficient since Citric Acid bypasses the first two enzymatic reactions,the part where Citric Acid is processed into alpha-ketoglutarate via Isocitrate Dehydrogenase is the rate limiting step of the entire cycle. You wouldn't be saving any time by flooding your system just before the bottleneck.

Interesting question, though!

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  • $\begingroup$ Thank you MCM! So this raises the question, how is it possible to raise the rate of the citric acid in to alpha-ketoglutarate via isocitrate dehydrogenase step? $\endgroup$ – ol_matty_man Apr 3 '18 at 14:13
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    $\begingroup$ In vivo? No idea. That would require either a lot more enzymes floating around waiting for substrate (not sure how you'd do that or maintain it), or a much more efficient enzyme -- something that could be theoretically done if you had a few hundred hours of world-class supercomputer access and the right research team, but also suffers from the "I don't know how to get it in you and keep it around" problem. Chances are there's a better solution to whatever problem you're thinking this solves. $\endgroup$ – MCM Apr 3 '18 at 14:52
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    $\begingroup$ @ol_matty_man - I'm afraid I don't have some literature suggestions, as I don't fully understand what you wish to do. Basic metabolic processes are pretty well described via Wikipedia, and I'd suggest starting there if you're not familiar with the language. And, as I said, the problem is NOT the citrate -- it's the step going from citrate to alpha-ketoglutarate. That's the limiting step, and the bottleneck in the Krebs cycle. $\endgroup$ – MCM Apr 3 '18 at 21:53
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    $\begingroup$ @ol_matty_man - About 95% of the citric acid we ingest goes through a metabolic process in our bodies. Unless you ingest an unusually high amount, the majority of what you eat/drink is used and not directly urinated out. That said, I still think you fail to see the problem: Getting citric acid into our bloodstream is easy. Getting more energy after doing so isn't. It is not the amount of citrate that's the problem, but the rate at which it is processed. Like a road with a stop light, the thing that determines traffic speed isn't the number of cars (the citrate), it's the stop light (enzyme). $\endgroup$ – MCM Apr 4 '18 at 2:31
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    $\begingroup$ thank you @MCM - I had read conflicting information about citric acid here: livestrong.com/article/327212-what-products-contain-citric-acid where it said most is urinated out. $\endgroup$ – ol_matty_man Apr 4 '18 at 17:22

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