What is diabetes mellitus?

So, diabetes is a very common disease. It affects nearly 10 percent of the population and more than 25 percent, or one in four people over the age of 65 have diabetes. But what exactly is diabetes mellitus? Let me start by going through a couple of scenarios. Now the first scenario I wanna talk about is that of Joe here. So let me bring in Joe. Now Joe is a 15 year old boy. Over the past few months Joe just really hasn't been feeling well. He says, you know it's hard to kind of put his finger on it, but he's in general been a little bit more tired and fatigued than usual. In fact, it's caused him to lose a little bit of weight and he's already kind of a skinny guy to begin with, so that's definitely something that's abnormal for him. And you know his mother's with him and his mother says you know "Joe's been, it's kinda been odd, "Joe's been carrying around this water bottle "with him everywhere for the past few months. "He seems to just be drinking "liters upon liters of water a day." And when asked about it Joe acknowledges this, he says "Yeah, I have been a little bit "more thirsty than usual." and because of this, he says, "You know, I've been going to bathroom, "I've been needing to urinate all the time." Now, let's contrast Joe to Bruce here. Now, Bruce is very different than Joe. Bruce is a 45 year old gentleman. And he's come in to the doctor for his annual physical and when he goes to the doctor he says, "You know doc, I've been feeling pretty good. "I mean, yeah, maybe I've packed on "a few extra pounds around the waist, "but in general I'm feeling pretty healthy." Now, say both Joe over here and Bruce are seeing the same family practice doctor. You know for Joe, he says "Yeah, I am worried "about this thirst and urination and the losing weight. "I think we should probably check Joe's blood sugar." And then similarly with Bruce, even though he doesn't have any symptoms, the doctor recommends that Bruce has some routine screening done that would include checking his blood sugar. And it turns out that both Joe and Bruce have very high blood sugar. They have something called hyperglycemia, which stands for high blood sugar. And they're both diagnosed with diabetes. But how can such different situations with Joe and Bruce be caused by the same disease? Now, to answer this question, we first, really need to understand what exactly, diabetes is. Now the first thing I wanna mention is that I've been referring to diabetes mellitus and there are a couple different conditions that have the word diabetes in them. But generally when people refer to diabetes, they're referring to the disease diabetes mellitus, which is what we're gonna be talking about. So diabetes mellitus is a group of disorders that are characterized by an inability of the body to regulate its blood sugar levels. And you'll notice that I put glucose here in parenthesis after sugar, because glucose is the main type of sugar that the body uses. And this disregulation results in high blood sugar, or hyperglycemia. So, to get a better idea of this, let me just bring in a simple diagram here. So this right here in pink, imagine this is a blood vessel and in this blood vessel there is blood going through. And that blood has glucose. Now, glucose is a sugar and it's the body's preferred source of energy. And we get glucose from the foods that we eat, and then the glucose that we eat is stored throughout the body, but primarily in the liver here. And to regulate the amount of sugar that is in our blood we have two hormones. The first hormone I'll have over here in orange is insulin. And then the second hormone I'll have over here in blue is glucagon. And then down here in green is a cartoon of some cells in the body that are gonna need this glucose for energy. When the glucose level in the blood is getting too high, the pancreas releases this insulin. And what the insulin does is it stops the release of glucose from the liver into the blood and then it also has this glucose be taken up by the cells. And so this decrease in the amount of glucose being put into the blood from the liver and then the increase amount of glucose leaving the blood going into these cells causes that blood sugar level to go down. So insulin decreases blood sugar. But if insulin were to just act on it's own, the blood sugar would be always way too low, so when that blood sugar starts getting to a lower level, the pancreas then will release glucagon. And what glucagon does is it causes the liver to release that glucose into the blood, all that glucose that's stored there. And then it also causes the pancreas to stop producing the insulin so that less glucose is being taken out of the blood and put into cells. And therefore then, the effect of this by having the liver put glucose into the blood and then preventing the glucose from leaving the blood, glucagon results in an increased blood glucose level. So now, in diabetes, this insulin here is either not being produced or it's not functioning properly. So let's just cross it out. It's not doing its job. And what happens is that the effect of glucagon here is amplified. So you get this increase in blood glucose levels. And then as I mentioned, insulin causes glucose to exit the blood and go into the cells so that the cells can use the glucose for energy. But if it's not around, this doesn't happen. So despite the blood sugar, the blood glucose level is getting higher and higher, that glucose isn't able to get into the cells. And the body isn't able to use all of that energy. So in a sense, despite the presence of plenty of blood sugar and energy, the cells are actually kind of starving. And that's actually what can go on. You can have this really high blood sugar level, but the body in some senses thinks it's starving. And this idea of kind of starvation in the face of plenty of energy is a little bit confusing. So let me just make an analogy here to hopefully have it make more sense. So it's very similar here. Now imagine instead of this pink blood vessel, imagine this is a water main. Now, the purpose of that water main is to bring water to these houses. Now, imagine that one of these pipes that went from the water main to the house gets clogged. Now, despite plenty of water being travelling down the street and being right by the house, the house actually isn't able to get any water. And this is kind of like that same idea of, despite all of the glucose being in the blood, when you don't have insulin, you can't get the glucose into the cells. Just like if you have a clogged pipe that prevents water from coming from the water main into a house. So now let's go back to Joe and Bruce and understand how you can have the same disease that causes such different presentations in different people. Remember I said that diabetes mellitus is a group of diseases. They're not all the same disease. So, depending on the underlining mechanism that's going on, the disease can present in very ways. So Joe over here on the left, he is a very typical presentation of type 1 diabetes. And I'm gonna just abbreviate diabetes, DM, for diabetes mellitus. And in type 1 diabetes, Joe's body isn't producing any insulin. And that results in that high blood sugar. But over here with Bruce, he has a different underlying mechanism. His presentation is very characteristic for something called type 2 diabetes. And in type 2 diabetes, the body produces insulin, but the receptors on the cells are not as sensitive to insulin, so insulin just doesn't function like it's normally able to. And so it has a very different presentation. Even though for both of them, insulin is not working properly and it results in hyperglycemia, which is the characteristic finding in diabetes, depending on the underlying mechanism, whether or not you are not producing insulin or whether the insulin just isn't working properly, you still have hyperglycemia but the presentations can be very different.