Fruit Isn't the Enemy: Understanding Glucose, Fructose, and Carbohydrate Metabolism Part 1

In Part 1 of this episode, we begin a deep dive into one of nutrition's most misunderstood topics: carbohydrates. Join me as we explore what carbohydrates actually are, how plants create them through photosynthesis, and what happens the moment you take your first bite. By understanding the science behind glucose, fructose, and digestion, you'll gain the foundation needed to separate nutrition myths from physiology.

Transcript

SPEAKER_00 0:11

Howdy everyone, and welcome back to another episode of Metabolic Mindset. Um, in the next couple of episodes, gonna do kind of like a series to try to tackle one of the most misunderstood topics in nutrition and something that I come across a lot, which is sort of fear of fruit and carbohydrates. More specifically, in this episode, we're gonna look at fructose, glucose, and we're gonna start learning about the process of carbohydrate metabolism, which is very complicated. Um, it's gonna be a multi-parter, so look out for future episodes. And if it feels like this doesn't explain everything, it won't because I can't go on for three hours in one podcast. Again, the content in this podcast is intended for educational and informational purposes only. It's not personalized nutrition advice, it's not medical advice, it's not a substitute for working with a qualified healthcare professional. Opinions are my own and they don't represent the views of my employer or any professional organizations. So I'm Shara Perry. I'm a registered dietitian. I take a pro-metabolic approach to nutrition, and this is the Metabolic Mindset podcast. So let's get into it. Um, depending on who you ask, fruit is either one of the healthiest foods on the planet, or it's something that you should avoid entirely because it contains fructose or fruit sugars. And I hear this almost every week from new clients that start with me. And it usually goes something like, I don't eat bananas because they're high in sugar. Something like, I'll eat potatoes, but I'm not gonna eat grapes, or I love apples, but I heard that fructose turns directly into fat. So what's interesting to me is that those same people who are afraid of fruit will often have no problem eating rice, potatoes, pasta, bread. So today I kind of want to tackle a simple question of why are people comfortable eating starch and vegetables, but they're afraid of eating fruit? And to answer that question, we're gonna sort of follow a carbohydrate through the body, as boring as that sounds. Uh, to some people, I guess, not to me. But I think that understanding this process in detail and in even more detail, probably in the next episode, especially in regards to liver metabolism, will help explain a lot of these underlying principles and why a pro-metabolic diet is not afraid of carbohydrates. So, to start off, we're gonna cover where do carbohydrates come from, how do plants make them, uh, how your digestive system breaks carbohydrates apart, and then we'll briefly start touching on how the liver processes them. Um, that'll be more of the next episode for sure. Um, and by the end of this series on carbohydrates, my goal isn't to convince anyone to eat fruit. Obviously, I'm a huge, huge advocate of eating fruit and carbs, um, which, if you know me at all, that's that's a complete 180 from where I was five to ten years ago. And it's from a greater understanding of physiology. Uh, my goal really is to help you understand carbohydrate metabolism a little bit better, well enough to make your own decisions about whether or not you want to include carbohydrates in your diet. So let's start at the very beginning, right? Where do they come from? Whenever we talk about carbs, it's kind of nice to take a step back and think about okay, what is a carbohydrate? Where is it coming from? Um, so plants have been making carbohydrates for hundreds of millions of years, long before humans ever existed. In fact, carbohydrates is it's just a fancy of way of saying it's stored sunlight energy. Um, plants take sunlight, carbon dioxide from the atmosphere, water from the soil, and through photosynthesis, they manufacture glucose. And so they convert light energy into stored chemical energy. The glucose that's produced becomes sort of the currency of the plant, and sometimes the plant burns it immediately for its own energy. Sometimes it stores it or converts it, depending on what that plant needs. So if it's making a potato, it's gonna store the glucose as starch. If it's making fruit, it's gonna package glucose alongside fructose and sucrose. And if it's making seeds, it's gonna store large amounts of starch to feed the next generation of plant. All of those foods, potatoes, rice, wheat, corn, fruit, honey, even table sugar, they're different because plants have different biological purposes, but they are all still carbohydrates. So this is important to think about food for thought. A ripe peach isn't trying to poison you with sugar, it's trying to convince any animal to eat it, including humans. The fruit wants to be eaten, okay, because the seed inside will survive your digestion. And the animal walks a mile away, it's going to use the restroom, and now the seed has been planted somewhere else, right? So fruit, it gets kind of funny to think about it, but it's basically an evolutionary marketing strategy for furthering the next generation. Um, they're colorful, they're aromatic, they taste sweet, that's all for a reason. And the more appealing, the greater the chance that animals will spread the plant's offspring. And it's part of one of the oldest relationships on earth. Humans have been participating in that for hundreds of thousands of years. So not all carbohydrates look the same. When people hear the word carbohydrate, they often think it's one thing, but carbohydrates are actually an enormous family of molecules, and you can think of them like Lego bricks. The simplest bricks are called monosaccharides, mono, meaning one, and those are individual sugar molecules that your intestine can absorb directly. And there are three of those that are nutritionally important: the glucose, fructose, and galactose. Three monosaccharides. Everything else that you eat that's considered a carbohydrate has to be broken down into one of those three before your body can actually absorb it. So glucose is the one we hear about the most. It's the body's universal fuel. Okay, so almost every cell in your body can use glucose. Your brain uses a tremendous amount of glucose under normal, typical dietary conditions. We can use other molecules for brain fuel, but for now we're talking about standard typical dietary conditions. The brain is going to use a massive quantity of glucose. Uh, your red blood cells depend entirely upon glucose because they don't have mitochondria. And your muscles, of course, utilize it during exercise. So incredibly versatile in the body and used by nearly every cell type and every single organ. Fructose is a little bit different. It's still a carbohydrate, so it still contains four calories per gram, but it's going to enter metabolism through a different doorway, and we're going to talk all about that in the next episode. And then there's galactose. So you don't hear much about it. It's primarily found in dairy. And when you drink milk, you're consuming lactose. And lactose isn't just one sugar, it's actually two sugars that are stuck together. It's one molecule of glucose and one molecule of galactose. And the enzyme, lactase, is what helps to break them apart so that you can properly absorb them. And I'll probably do a whole episode on lactase and lactose and just talking about milk because it's complicated. Uh, but for now we're gonna move up one level. So disaccharides or dye, it means two. And these are simply pairs of sugars that are linked together. So sucrose or table sugar is glucose plus fructose. Lactose, like we just mentioned, is glucose plus galactose. And maltose, you never hear about maltose, is glucose plus glucose, double glucose. And from there we move up to complex carbohydrates. So those are long chains of glucose molecules that are linked together. Starch is simply thousands of glucose molecules that are connected, sort of like beads on a necklace. And plants are gonna use starch as an energy reserve, much like we use glycogen in our muscles. We store glucose in the form of glycogen in our muscles. Animals are gonna store glycogen, plants are gonna store um starch. Sorry. I blanked out because I've not had enough glucose today. Uh, so when you eat a baked potato, your digestive system isn't absorbing giant starch molecules. It's systematically cutting down that necklace into individual glucose beads until they're small enough to actually be absorbed by your intestines. And that's that's kind of an important mindset shift. So the body doesn't care whether glucose is starting off in oatmeal, if it's in rice, potatoes, or bread. Eventually, if it's digestible starch, which again could be a whole other podcast, is digestible versus indigestible sources of carbohydrate, if it's digestible, much of it is going to end up as glucose, individual glucose. Um, so now we know where carbohydrates are actually coming from, and we know some of the different forms that they take in food. And the next question is how does the body actually take a bite of food and turn it into usable energy? And this I don't telling you guys this is gonna be a dry cobbled podcast, but if you do want to understand how how carbohydrates work in the body, this is for you. Um, so the process of digestion, many of you are familiar with this. We're taught this in biology for whatever reason, it starts in the mouth before you even swallow. Um do do do. Don't know why my notes are out of order. Alright. So before before I jump into mouth digestion and the fun stuff there, you know, the the intestines, it's like they're receiving a shipment in the mail, especially if they get a complex carbohydrate. So your body can't use that package, it wants the individual molecules inside. That's exactly what digestion is, is the digestive system is just trying to open packages and get those into the smallest usable forms so that it can literally go into the bloodstream and be used in the body because your liver, or that's a terrible example, your spleen is not going to recognize a big chunk of sweet potato in the bloodstream. That's going to cause an autoimmune response, in fact, but it will recognize glucose. So that is the whole reason why we digest foods is to break down these larger packages into individual components. All right, so digestion beginning in the mouth. Um, that starts with the salivary glands excreting or secreting, actually, more accurately, an enzyme called salivary amylase. So enzymes like salivary amylase are just proteins and they act like little microscopic scissors. So each one is designed for a very specific job, and salivary amylase has one job and it's to cut long chains of glucose apart or to at least start that process for the digestive system. Um, amylase starts snipping at the little necklace and cutting it into shorter and shorter pieces, and it doesn't finish the job of digestion at all, but gets it kickstarted. If you remember, I was probably like middle school, they had you put a piece of cracker or bread in your mouth for a while, it eventually starts tasting sweet, and that's because amylase is creating smaller sugar molecules as the starch is being broken down, or the more complex glucose chains are being broken down. Uh, chewing is also happening in the mouth, and all that's doing is increasing the surface area of your food. So we can think of trying to melt a block of ice or trying to melt crushed ice. One is going to melt much quicker. And the smaller the pieces, the faster enzymes can actually work. It is important to chew your food. I use the word house with my clients a lot when we're talking about housing food because I do want them to get away from thinking about small meals that are calorie dense and get more into these volume foods, which can aid digestion. But if you're eating a lot of volume, you do have to properly chew it. So, stomach, moving on to the stomach. Once you swallow, that food's gonna travel down your esophagus and enter into the stomach. And very little carbohydrate metabolism and digestion actually happens there. So, stomach is an incredibly acidic environment. And the acidity is fantastic for beginning protein digestion, which we can talk about in another podcast probably. And it protects us, that acid actually protects us from harmful bacteria, but it inactivates salivary amylase. Any salivary amylase that we happen to swallow, it's deactivated by the acidic environment. So carbohydrates sort of have to wait their turn in the stomach. They get mechanically digested a little bit. So the stomach does actually churn food and it's gonna mix it with gastric juices and it's gonna release it eventually into the small intestine. So the stomach is more of like a blender for carbohydrates. It's gonna create that smooth mixture, it's called chyme, that can be digested much more efficiently downstream. But the small intestine is where most of the work actually happens. So if the mouth starts the job and the stomach sort of prepares the meal, the small intestine is where like the real magic is happening. Uh, so as partially digested food enters the first section of the small intestine, the duodenum, and um, you know, this word always gets me. You can say duodenum, but it's not technically correct. It's du it, do it with a duodenum or duodenum. It's whatever. It's the first part of your small intestine, it gets me every time. Uh the pancreas starts getting involved in in the first section of the small intestine. And the pancreas is gonna release pancreatic amylase. And pancreatic amylase is like salivary amylase's bigger, stronger brother. Um, it's gonna continue chopping those long starch molecules and complicated glucose chains into smaller carbohydrate fragments. Um, it's still not finished there, it's still too large to be absorbed, and that's where we get to these specialized enzymes that are sitting on the surface of your intestinal cells. And we call those brush border enzymes. Love that name because your intestine is literally a brush border and it's so aptly named, but they're called brush border enzymes. Um, and they're kind of like a final quality control station before the nutrients are actually allowed into your body, i.e., the bloodstream. So each enzyme has a very specific assignment. Uh, sucrase is going to break sucrose into glucose and fructose. Lactase, which a lot of us are deficient in, breaks lactose into glucose and galactose, those two simple sugars. And maltase is going to break maltose into those two glucose molecules that we talked about. And by the time all of those enzymes are done doing their job, every digestible carbohydrate has been reduced to its simplest form. And remember, those simplest forms are glucose, fructose, or galactose. And those are the only carbohydrate molecules that your intestine can actually absorb. Everything else has to become one of those first. All right, so crossing the intestinal wall, uh, getting nutrients into your bloodstream, at least I used to think of it this way, it's not passive. Um, your intestinal cells, they don't simply like leave the door open and hope that sugars are gonna wander aside. They use transport proteins. So, gosh, I hate the names of these, but each one is sort of each one of these transport proteins is designed for a particular passenger. Glucose is going to primarily enter your intestinal cells through a transporter called sodium glucose-linked transporter one, and you'll see it abbreviated SGLT1. And that transporter is kind of remarkable because it doesn't move glucose by itself, it's going to grab glucose and sodium at the same time. In other words, you know, we talk about the importance of electrolytes. This is a really big one. Sodium will literally help to pull glucose into the intestinal cell. And that's one reason why electrolytes are so important because during endurance exercise in particular, or just dehydration states, we need both sugar and sodium to properly hydrate. Um it's a scientific principle and it's it's rooted in physiology behind oral rehydration therapy that we use around the world to treat severe diarrhea. You don't want to drink plain water when you're having severe diarrhea and you're really dehydrated. You want a combination of glucose and sodium, and it will drastically improve water absorption. So fructose, that we were talking about glucose and we got a little fancy. Fructose takes a different route. Um, it uses a transporter called GLUT5. And unlike glucose, fructose doesn't hicharide with sodium. It enters through facilitated diffusion, moving down a concentration gradient. So when we say moving down a concentration gradient, we literally just mean moving from an area where it's concentrated to an area where it's not concentrated. And once it's inside those intestinal cells, both glucose and fructose are going to exit through another transporter called GLUT2. How many glute transporters do we have? It's confusing. But now the glucose and the fructose are together again. They're leaving the intestine and they're entering tiny little blood vessels on their way to your liver. And the portal vein, which you've probably heard about, is the highway, mega highway system that transports uh materials from the intestines to the liver. Um you hear this misconception a lot, where people will say, Oh, I don't want any fructose because it goes straight to my liver. God, you hope so. You hope that anything you eat is first going to the liver to be packaged appropriately and determined whether or not it needs to be moving into the bloodstream. So please, if someone tells you, I don't want to eat any fructose because it's gonna go to my liver, you want pretty much everything in your bloodstream to be going through your liver in ideal conditions, okay? Um let's clarify what's what's happening there. I'm so sorry. I think it's so funny. Um, most everything that you absorb from your digestive tract is going to the liver first. So glucose, fructose, galactose, amino acids, many vitamins, um, they're all collected in that special blood vessel called the hepatic portal vein, right? And you can think of the liver as the body's central shipping and distribution warehouse. Every single nutrient that's arriving from your intestine, first, it's gonna check in with the liver. And the liver, like I said before, it's gonna decide what to store, what to release, what to transform, and what to send elsewhere. So it's it's a shipping center, essentially. So when someone says fructose goes to the liver, yeah, they're technically correct. Um, but they're leaving out the fact that virtually every absorbed nutrient goes there too. Uh, the real difference isn't where fructose goes, it's what the liver chooses to do with it. And that's where glucose and fructose are finally going to begin taking sort of different metabolic paths, right? That's we're gonna pick up in the next section. I think once those sugars arrive at the liver, your body starts making decisions that are based on your current energy needs, uh, your glycogen stores, your activity level, and your overall metabolic state. So it is important to understand what the liver is gonna do with these different nutrients depending on what state you're in. And um understanding that is really key to understanding why fruit is not the metabolic villain that it's made out to be. But thank you for listening to this first very boring introduction to carbohydrates. If you do want personalized guidance and a pro metabolic approach that's tailored to your life, you can book a session with me. I work with Nourish. My link is going to be in the show notes. But thank you again for listening. Look out for part two, and I'll jump into a bit more detail with the liver and sort of your body deciding what it wants to do with these various nutrients.