I asked BenaGene, a company that also sells Oxaloacetate, why, if citric acid directly metabolize to oxaloacetate, I should take its product rather than having a bit of lime juice every day. I received the following response:
Back when the "antioxidant" theory was big, there were several lifespan studies done with various antioxidants including Citric Acid (citrate) and ascorbic acid. There was no increase in lifespan.
While under certain conditions oxaloacetate can convert to citrate, and vice versa, it does not create the conditions we want. We want to flood the cellular system with oxaloacetate so that it converts to malate.
So why not just take a bunch of malate? (found in apples). It's because during the conversion of oxaloacetate to malate, NADH also converts to NAD+. This increase in the NAD+/NADH ratio is the key to activating AMPK which leads to similar genomic changes similar to a calorie restricted state. These changes then lead to increases in lifespan.
On the other hand, lime juice is very helpful for making margaritas.....
This is an interesting line of reasoning, which is not easily refuted. It seems quite plausible biochemically, and my strongest attack on it would probably be through the conjunction fallacy - while each of these steps seems reasonable, perhaps the entire chain is faulty.
However, there is one thing that seems blatantly out of place - and that's the scale of the process. The citric acid cycle as a whole operates at a catalytic concentration of 1-5 millimolar, in just about every cell of the body. Multiplied by 70 kg of weight per person, that would equal 70...
For those who are interested in this topic, I'm just wondering what longevity research today looks most promising to you and why. Whether that's SENS, cryonics, nanotech, brain uploading, etc is fine with me. Any links to actual research papers would also be greatly appreciated. I'm very interested in longevity, and am curious to see if anyone else would like to offer some thoughts on the current state of the art.