 So we're going to discuss the fat-soluble vitamins first, starting with vitamin A. And incidentally, vitamin A got its name because it was the first vitamin discovered. And scientists thought, if we're going to use the alphabet, might as well start at the top. So it's nothing more than that and why it's called vitamin A. But you'll recall, I mentioned these concepts a few slides ago, that vitamins exist in nature in different forms. They're called vitamins and that they need activating by the body before they can start to function and do things for us. And in the vitamin A world, there are three vitamins that exist with vitamin A activity. We have retinol, retinol, and retinoic acid. And they have different functions in the body. Retinol is vital for the health of our retina, which is that kind of innermost light-sensitive tissue on the inside of our retina. I would kind of liken that to, let's say, film in a camera, in the days where we used to put film in cameras. But without film in a camera, the camera can't capture an image. But with film, it can capture that image. So it's that image capture medium. Retinol, on the other hand, enables color vision. It's vital for those visual pigments in those rod and cone-shaped cells of the retina. So I would kind of liken that to, let's say, a color print cartridge. Whereas retinoic acid, it kind of is all the non-vision-related function of vitamin A. It works at a DNA level to promote genetic expression and cell differentiation. Really crucial for the normal functioning and formation of tissues like the heart and the kidneys and the lungs, real high energy tissues. Now, retinol is the most active form or the most active vitamin. We can make the other two vitamins or metabolites, if you will, from retinol. But it's a one-way street. Once we convert or metabolize retinol into retinol and retinoic acid, we can't put it back as retinol. So the body is very good at kind of holding on to these really important bioactive forms and we will only convert what we need. Otherwise, we hold on to it in the liver. Now, the vitamin A in our food comes from two types. We get it from provitamin A and a provitamin is a substance that can be converted into a vitamin. So provitamin A can be converted into retinol, which is the active compound in the body. And provitamin A comes from a family of phytonutrients or phytochemicals called keratines or keratinnoids. The word keratin comes from the Latin word kerata, which means orange pigment. It's also where the carrot gets its name from. Now, the principal keratines in our diet, or I should say that have provitamin A activity, are the alpha keratin, the beta keratin, the gamma keratin, and a keratin called cryptozanthin. There are hundreds of keratines out there, but only about 10% actually have provitamin A functionality. Now, the most potent keratin is beta keratin. It's probably the most widely known because it has the most active vitamin A properties. Doesn't mean all of those other keratines are not valuable to human health. They very much are. Keratinnoids as a family provide a very strong antioxidant function for us, but they just don't all contribute to our vitamin A needs. Now, when we talk about preformed vitamin A, preformed vitamin A is exactly what it says. It's vitamin A that is ready-made. It doesn't take any conversion on our part for us to get at it. We only find preformed vitamin A or retinol in animal foods. That's because the animals eat the plants and they kind of do the conversion for us. And by eating those animals, we get kind of instant access to that preformed vitamin A. So, we could call that being more bioavailable to the human body. If you enjoyed this video, please give it a like and subscribe below so you don't miss any future content. To learn more about CNM or its courses, head to www.naturopathy-uk.com.