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Fat Digestion


Now, of all the MCAT topics on digestion, it is fat digestion and absorption that you need to understand the best. Okay, so make sure to pay attention here. Go back through this. This needs to be something that you have down very well, or you're gonna get burned on the MCAT.

So, for digestion, what you should think of is when we eat fats, there in this large fat globules, okay? And, I'll just write down here, its a big fat globule. And we have different what are called lipases, that will work on these globules. But they won't do that well until we can get this globule down into smaller components that the lipases can then really go to work on from many different angles.

And in order to get large fat globules into their smaller components, we need bile. And remember, bile is released from the gallbladder. And you should remember that colitis sicanin, when it senses fatty acids and proteins in the duodenum, it's released and it causes gallbladder contractions so that bile gets released.

And bile then will break it down, and the buzzword here is that bile emulsifies fats. You need to be familiar with that word, emulsify and that's what bile does. So it gets it down into these smaller components, and now these lipases can really go to work. And the lipases that we have that get to work on these fatty acid are the following.

We have actually a lingual lipase that starts working on larger globules in the mouth. We also have a gastric lipase that works in the stomach. And again though, that will be working on larger globules because when is our bile released? It's released from the gallbladder, and remember where the gallbladder connects up with the GI tract, in the duodenum, duodenum, whichever.

So lingual and gastric lipase are acting before bile's been able to emulsify the fat. But finally the last type and the major type of lipase is pancreatic lipase. And that's released, again, CCK stimulates gallbladder contraction which releases bile and it also stimulates the pancreas to release its enzymes, and one of those will be pancreatic lipase.

So then, pancreatic lipase is gonna be able to act on all these smaller globules and break them down. Now the goal here, we eat triglycerides, and triglycerides are made of this basic glycerol backbone, and they have three fatty acids. And just like you need to know, that carbohydrates are only absorbed as monosaccharides, you should know that we have to break down our triglycerides into a monoglyceride.

And two fatty acids. So you can see we cleave off these two fatty acids here, and that's what we break them down to. And that's how we absorb. So, thanks to the action of bile getting these smaller globules and pancreatic lipase working on these, this is what we end up with.

Monoglyceride and two fatty acids. And now for the discussion on absorption. To understand fat absorption, you should think of our GI tract as being this hallow tube. And then obviously there's a wall here, the walls aren't infinitely thin here.

So this is just the wall of our GI tract here, okay? And then outside of it here, so this is our GI, we'll call it the tube. And this is the wall of the GI tract. And then out here, obviously, we have blood vessel. And normally like for monosaccharides, the monosaccharides come down here and they get absorbed as monosaccharides and they cross.

They diffuse through some interstitial fluid and hop into the blood vessel. And then we have our nice glucose there that we can use for metabolism which we already talked about and converted into ATPs or whatever we wanna do with it. But for fats, you also need to be aware of the lymphatic system. And specifically, you'll have little projections of the lymphatic system called lacteals.

Okay, so with this in place now, we can understand fat absorption. So again we have the monoglyceride, we've eventually been broken down into monoglyceride and two fatty acids. You need to understand that, then in order to get absorbed into the wall of the GI tract, these get incorporated into something called a micelle. And it's just a carrier that packages this monoglyceride and two fatty acids, it's called the micelle.

And inside the micelle, the monoglyceride and two fatty acids can cross into the wall of the GI tract, okay? Once inside the wall now, the monoglycerides and two fatty acids will reform into the triglyceride, this guy. They will come back together and become a triglyceride again. And then, this triglyceride gets packaged into a new thing called a chylo, sorry, C-H-Y-L-O, chylomicron.

And this chylomicron now that contains the triglyceride, diffuses into the interstitial fluid, and it skips the blood vessel and jumps into the lymphatic system. It may jump into the lacteals. And then eventually it's in the lymphatic system. So that's the important thing about fat absorption.

The monoglyceride and two fatty acids hop into a micelle, and that's how they get into the GI wall. They recombine then into a triglyceride, and this triglyceride gets incorporated into a chylomicron. And this chylomicron then leaves the GI wall, and instead of hopping into the blood vessel, hops into the lymphatic system.

And now four a question that was just on the test last year. This lymphatic system eventually dumps into the blood system. And specifically this location, it dumps into the venous return, so it dumps into a vein, and you don't need to know exactly where that is. But the important thing is that the fats that jump into the lymphatic system eventually follow the lymphatic system all the way back to the point at which the lymphatic system dumps into our veins.

Also can be referred to as our venous return. And that is fat absorption. Now you should know there are some fat soluble vitamins and there are some water soluble vitamins. So the fats soluble vitamins are A, D, E, and K. Water soluble vitamins are the B vitamins and vitamin C.

And for the MCAT, how you could be ask to truly understand this would be for example, a situation where somebody is unable to absorb fats. For example, taking an instance where this is our duodenum coming around here. And we have the pancreas, and we have this duct here, and then we have bile secretions. If we have a plug here and we can't get bile or pancreatic lipase for that matter, into our GI tract, we won't be able to break down fats.

And if we can't, these fat soluble vitamins that we've eaten won't get absorbed as well, because they are not joining the fats as they get absorbed. And we'll end up lacking A, D, E, and K. Also additionally we won't have adequate fat stores, and so we won't have storage of A, D, E, or K. So a question that might ask, what vitamin would we be deficient in, if we were unable to secrete bile or pancreatic lipase, you would wanna select one of the fat soluble vitamins.

Additionally, fat soluble vitamins, because we store them in our fat in our body, it's easier to overdose on them because they can accumulate in the fat. Versus all the vitamin Bs and vitamin C, they are water soluble. So they will be soluble in our urine, and we'll be able to excrete them. So it's very difficult, on the other hand to overdose on a water soluble vitamins. So those are things you should understand about vitamins for the MCAT.

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