Why can't humans use dietary citric or acetic acid as a primary source of energy? 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".

> 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!