 Remember the arachidonic acid story? We make it, but we can also get it in our diet, eating other animals to make it. Same thing with cholesterol. Well, there are similar necessary components found exclusively, or almost exclusively, in the animal kingdom, not the plant kingdom, such as cornucine, cornutine, creatine, and taurine. But if something's made only by animals, what about those eating vegetarian? Thankfully, vegetarians are animals, too, so they make it themselves. Now true carnivores are the exception, right? Cats don't make taurine, for example, but that's because they're built to eat animals that do. But humans produce all these compounds on their own, unless they have some rare genetic inborn error of metabolism birth defect. There is actually a hereditary disease that may affect as many as 1 in 40,000 births. It's a mutation on chromosome 5 of a carnitine transport protein. They actually make enough carnitine, but because of the birth defect end up peeing too much out, and so develop a carnitine deficiency. And actually, there was a case report about 30 years ago of a 10-year-old boy in Israel in and out of the hospital every 4 or 5 months. No one knew what was going on. The clue only came when he decided to go meat-free, and that made things worse. Then he started having attacks every 3 weeks. Turns out he had that rare disorder. That's why he was sick. But it was being kept somewhat at bay by the exogenous external dietary source of carnitine in the meat that the other 39,999 kids out of the 40,000 don't need. He stayed vegetarian, but they gave him high-dose carnitine supplements, and at the time of the report he was 12, and completely cured.