Mice do it, cats do it, dogs do it, and even elephants do it. For some unknown reason humans and our primate cousins (and, yes, guinea pigs, too) are the only mammals that cannot make their own vitamin C. While the rest of the mammalian world happily and effortlessly takes glucose and turns it into vitamin C, we have been condemned to get ours from food alone.
We actually still have the same gene used by other animals to make vitamin C from glucose (in humans it's called GULOP)—it's just that our version looks genetically like someone cut out parts of it to make a paper napkin snowflake. This means that no matter how hard our DNA and body try, we're not going to be making vitamin C anytime soon. It's also one of the major limitations on our species' ability to travel long distances without a fresh supply from food.
We haven't figured out a way to fix the GULOP gene yet, and so until then you are completely dependent on consuming this key vitamin to shelter you from the damage caused by oxidative stress to your body. Thanks to advanced research studies, we now know that certain genes some people inherited also make them require more vitamin C because these genes don't work as well to prevent oxidative damage that can then harm their tissue and DNA.
So to get your necessary daily dosage of vitamin C daily—90 milligrams for men, 75 milligrams for women—reach for these surprising foods: