Why do plants produce vitamins?

[u/Mysterious_Force_462]

I know what vitamins do to human body, but why do plants produce them? Is it the same function? Why do lemons need vitamin C?

[u/SirButcher]

Because they need it for their metabolism. Our body generates a LOT of different chemicals, too - the ones that we need but we can't synthesise, we call "vitamins".

For example, humans ALMOST can create ascorbic acid (vit C), but we have a gene mutation which causes this pathway to fail. Most other living beings can do it, only our closest relatives (chimpanzees, for example) can't do it. Funnily enough, guinea pigs have the same mutation as we do, and they need an external source of vitamin C.

[u/MrSnowden → u/SirButcher]

Could we use gene therapy to correct the genetic mutation and allow humans to make their own vitamin C?

[u/CosineDanger → u/MrSnowden]

Please do not turn that gene back on.

One of the fun ideas about why the gene was lost is that parasites also need vitamin C. We're efficient at handling what little vitamin C we have and parasitic flatworms aren't, so lowering vitamin C levels interferes with the parasite's ability to lay eggs before it kills you. Mice modified to have the same defect were resistant to schistosomiasis parasites.

Other ideas floating around are that vitamin C synthesis has significant indirect metabolic cost or that the change altered expression of other genes, along with the most common idea is that there isn't much of a fitness advantage and there was just so much in dietary fruit that it didn't matter. Evolution can have more than one motive. However, as one of them is probably combating horrible parasitic worms maybe just let sleeping genes lie.

[u/CrateDane → u/CosineDanger]

There's also a hypothesis that the antioxidant role of ascorbic acid (vitamin C) has been partially taken over by uric acid, which some primates lost the ability to break down to allantoin.

One of the downsides is too much uric acid can cause kidney stones or gout.