Fruit Isn't the Enemy: Understanding Glucose, Fructose, and Carbohydrate Metabolism Part 2

In Part 2 of this series, we follow glucose and fructose to the liver, where their metabolic paths begin to diverge. Learn how the liver processes different carbohydrates, why fructose doesn't automatically turn into body fat, and how glycogen, ATP, fiber, and the food matrix influence your metabolism. We also explore why whole fruit is fundamentally different from sugar-sweetened beverages and why context—not fear—should guide the way we think about carbohydrates.

Transcript

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Welcome back to Metabolic Mindset. In part one of this episode, we talked about carbohydrate basics, kind of where they come from, initial digestion, how they're absorbed in the intestine, and then transport into the portal vein on their way to the liver. And so far, glucose and fructose have sort of been traveling together through that pathway. They were eaten together, digested together, absorbed through the small intestine, and then carried through the portal vein. But now they've arrived at one of those, one of the busiest organs in your body, which is the liver. So we're going to discuss glucose and fructose metabolism a bit further so that you can make your own educated decisions about carbohydrates and whether or not to include them. Spoiler alert, you should include them. Content in this podcast is intended for educational and informational purposes only. It's not personalized nutrition advice, medical advice, or a substitute for working with a qualified healthcare professional. And the opinions expressed are my own. They don't represent the views of my employer or any professional organizations. I'm Sara, a registered dietitian with a pro-metabolic approach to nutrition, and this is the Metabolic Mindset podcast. All right, so let's get into it. Every package that arrives in the liver gets sorted. Some nutrients are stored, some are burned immediately, and some are sent out to other tissues. And the liver isn't trying to detox your food, it's trying to decide how best to manage your body's energy. And this is where glucose and fructose finally begin taking different metabolic paths after those that we've discussed already in the first part of this episode, or rather the previous episode. So let's start with glucose and run down this pathway. So you can imagine you've just eaten a baked potato, and as that starch is digested, thousands of glucose molecules are going to enter your bloodstream and make their way to the liver. The liver doesn't keep all of that glucose. In fact, after a mixed meal, only about 20 to 30% of the incoming glucose is taken up by the liver on its first pass. The rest is going to continue into the bloodstream where it becomes available for the rest of your body. And if you're eating that, you're going, oh no, only 20 to 30% of it's taken up by the liver. No. Every tissue in your body can use glucose, nearly every tissue. Your brain needs it, your muscles need it, the kidneys use it, your immune cells use it, and even your red blood cells depend entirely on glucose because they don't have mitochondria. So the liver acts less like a sponge in this situation and more like an air traffic controller. Some glucose is going to stay, most is going to continue onto the tissues that need it. And as blood glucose begins to rise, your pancreas is going to have a perfectly normal response and it's going to respond by releasing insulin. And insulin is often portrayed as the villain in nutrition conversations. It's completely necessary. It's a messenger. And its job is to tell the body, hey, yo, fuel is here. Fuel's arrived. So that message allows your cells to increase glucose uptake. It's going to support glycogen synthesis, which remember is just the stored form of glucose. And it helps coordinate the body's transition from a fasting state into a fed state. And without insulin, those nutrients can't be handled properly, as we see in people with type 1 diabetes. So insulin isn't the problem here. It's when chronic metabolic dysfunction is present. All right, so the liver does store some glucose. One of the liver's most important jobs is maintaining blood sugar between meals. And in order to do that, it stores some glucose in the form of glycogen. You can kind of think of glycogen as a rechargeable battery. And when food is really abundant, the battery charges. When you haven't eaten for several hours, that battery is going to drain. And the liver can store roughly 80 to 120 grams of glycogen. And that's going to vary a little bit based on your body size, your diet, and your training status. But that's enough to help stabilize blood glucose overnight or between meals. But unlike body fat, glycogen isn't meant for long-term storage. It's designed to be readily available, sort of quick in, quick out, which is why we've talked about the importance of restoring liver and muscle glycogen in several of these podcasts. You can reference those if you're curious. And uh now we want to touch on fructose. So the fructose from an orange also reaches the liver through the portal vein, just like glucose. The key difference is that the liver is going to extract a much larger proportion of fructose during that first pass than it does glucose. Why does it do that? Um, your liver specifically, the liver cells express large amounts of an enzyme that we call fructokinase, also known as ketohexokinase. And as soon as fructose enters the liver cell, fructokinase kinase rapidly phosphorylates it and it converts it into fructose 1 phosphate. That phosphorylation step is gonna sort of trap fructose inside of the cell. And adding a phosphate group is a common strategy that the body is going to use to keep sugars from simply diffusing back out into the bloodstream. So that's where a lot of nutrition debates begin. You'll often hear people say things like, oh, fructose bypasses the body's normal regulation. That's true in a sense, but it's really incomplete. So the gatekeeper is called phosphofructokinase, like we just discussed. When glucose enters a cell, it doesn't immediately race towards energy production. Instead, it passes through a major regulatory checkpoint called phosphofructokinase or PFK. And you can think of PFK as kind of like a toll booth on the highway. If the cell already has plenty of energy, PFK is going to slow traffic. If the cell needs energy, PFK is going to open the gates. And it's one of the body's ways of matching fuel use to fuel demand. Fructose is going to enter metabolism downstream of that particular checkpoint. So that's why you'll hear people say, oh, you know, I don't want to have fruit or fructose because it bypasses PFK. But bypassing one checkpoint doesn't mean that fructose just runs wild or rampant in your body. The liver has many other ways of determining what to do with incoming carbon. So it considers your glycogen stores, your overall energy status, your hormonal signals, and your current metabolic needs. So while fructose enters through a different doorway than glucose, it still enters a highly regulated metabolic system. All right, so where does fructose go? People often believe that fructose turns directly into body fat because if they they've heard the term de novo lipogenesis, it's simply not what usually happens. Umce fructose is converted into its smaller metabolic intermediates, those carbon atoms have several possible destinations. So some of them are going to replenish liver glycogen, some are going to be converted into glucose, some become lactate, which can be used by other tissues as fuel, and then some are going to enter glycolysis and are burned to produce ATP. Only under conditions where glycogen stores are already full and overall energy intake consistently exceeds expenditure does a larger proportion of fructose contribute to de novolipogenesis, which is the creation of new fat. And even then, this process is metabolically expensive and it represents only one piece of the overall picture. Your body generally prefers to burn or store carbohydrate as glycogen before it converts it into meaningful amounts of fat. It's also just not something I see in practice. In general, the more a client eats fruit as a large proportion of their carbohydrates, which typically I prescribe carbohydrates at more than 50% of their total calories, you actually see better energy production, increased weights of rate, increased rates of weight loss, and better hydration status. And I genuinely have not seen once in practice a person start eating more fruits and root vegetables and gain weight, even if they were low carb or keto previously. I just don't see it. I think one of the biggest misconceptions in nutrition is that the liver is simply a filter. It's more like a traffic controller. It's really sophisticated. So every minute of every day, it's sitting there, it's asking questions. Do we need blood sugar right now? Should we refill glycogen? Are the muscles being activated through exercise? Has this person eaten recently? Are we fasting? Do we have enough available energy? And it's making thousands of metabolic decisions every second. So if a person's getting regular movement and they're not in a caloric surplus and they're eating whole foods, it's not been my experience that the liver categorizes fructose or glucose as either right or wrong. The body is sort of constantly adapting to your own unique needs. So now we understand something. Glucose and fructose are not exactly enemies that are competing against one another. They're simply handled a lot differently by the liver. Glucose is distributed broadly throughout the body, and fructose is processed primarily by the liver first. Both ultimately are feeding into the same overarching goal, which I think people don't talk about this enough. The goal is helping your body produce ATP, it's helping your body maintain energy balance and keep you alive. And the question becomes then if fructose isn't inherently harmful, why does fruit have such a different reputation than something like soda or candy? Let's talk about it. Fruit is more than just sugar. I can't tell you how many times I get the oversimplification that someone might not want to eat fruit because it's sugar. And I don't blame people for thinking this way, but let's let's break it down a bit. Um a big problem in nutrition in general is sort of reducing foods down to a single nutrient. And you'll hear people, myself included, if I'm giving a simple explanation. Fruit is fructose, potatoes are starch, butter is saturated fat, beef is protein. But fruits are foods are incredibly complex, and they're like biological packaging. So you can take an apple as an example. Yes, it's gonna contain fructose, it also contains glucose, it has water, fiber, potassium, vitamin C. Not to mention dozens of polyphenols, right, and plant compounds that we're still actively learning about. So thousands of different molecules interacting together in an apple. And when you eat an apple, your body isn't seeing fructose, right? It's seeing an entire food matrix. So what is that? What is a food matrix? Um, it refers to sort of the physical structure of a food and how the nutrients are are packaged and meshed together. So fiber, water, cell walls, and naturally occurring compounds are all going to influence how quickly nutrients are released and absorbed. You can compare that with a sugary soft drink. The sugars are dissolved in liquid. So there's essentially no fiber, there's very little chewing, there's minimal satiety, and the sugar is going to reach the intestines rapidly. So easy to drink hundreds of calories in just a few minutes without feeling particularly full. The amount of sugar can look really similar on a nutrition label, and I think this is what throws people. But the physiological experience is completely different, and your body is not just responding to individual nutrients, it's responding to the entire package that they arrive in. All right, so let's just talk a bit about these different packaging ingredients, if you will. So fiber. Fiber isn't digested into glucose. Instead, it's going to change the environment inside of your digestive tract. It's going to slow gastric emptying, meaning that food leaves the stomach more gradually. It can reduce the rate at which sugars are absorbed into the bloodstream. And it increases fullness by adding bulk to your meals. Some fibers are fermented by bacteria in your colon, and they're going to produce short-chain fatty acids like butyrate or propionate or acetate. And those actually support colon health and have beneficial effects on your metabolism and inflammation. It doesn't sort of magically cancel out sugar. We still want to encourage people to eat all of their carbohydrates with protein in an ideal world. But that fiber does change the context. And metabolism is kind of all about context. All right. The role of water. I don't think that we appreciate that a lot of fruits are mostly water and we don't talk about it. So the obvious example is watermelon. It's over 90% water. But there's others like strawberries, oranges, peaches. Those foods have a relatively low energy density. And in other words, you get a lot of volume for relatively few calories, which is why I love them for people trying to lose weight. Fruit is an excellent source of calories. I mean, and I mean that in sort of a paradoxical way, in that it doesn't provide a lot of calories, but it provides a lot of nutrients and fullness. It can help people feel really satisfied while it's still supporting an appropriate caloric intake. And that's that's part of the reason why you see clients lose weight after they start increasing fruit intake, which just absolutely blows people's mind. At first glance, that seems really backwards. Like they're eating more carbohydrates. Sometimes they're even eating more sugar in general from all the fruits and carbohydrates, but you shouldn't they gain weight? Not necessarily. And like I said, it's been very, very rare. If I can't honestly sit here and tell you of an experience that I've had where I've asked someone to eat maybe more fruits and root vegetables and seen them gain weight or have a negative metabolic impact. And you know, part of that has to do with the context. Um I feel like I can't even record a podcast without feeling like I need to talk for six hours. But in the context, they're usually replacing really highly processed caloric dense snack foods with foods that are sort of naturally filling, hydrating, and nutrient-rich. So quality of overall diet is improving, hunger decreases because someone's eating more throughout the day, their adherence improves, and they often find themselves eating more appropriately without even trying to. All right, another piece of this context is potassium. So sugar is going to get all the attention when we're talking about fruit, but potassium's great too. So many fruits, excellent sources of potassium. Bananas are going to get all the credit, but you know, oranges, kiwis, cantaloupe, apricots, potatoes, they're all going to provide meaningful amounts. And potassium plays important roles in nerve function, muscle contraction, fluid balance, and maintaining normal blood pressure, which is usually important for people that are trying to get healthier. If someone's physically active or they're training in the heat or they sweat frequently, potassium is extremely relevant. Ironically, some people avoid fruit because they're afraid of sugar, while they simultaneously miss out on nutrients that support their exercise performance. All right, so I know all of y'all are waiting for me to talk about. When does fructose become a problem? Because it can. Um, there are situations where high fructose intake can contribute to poor metabolic health. Spoiler alert: don't eat anything that has high fructose in the words, right? Don't eat high fructose corn syrup if you can avoid it. Uh notice the context. So the issue isn't usually fruit, the issue is chronic overconsumption of calories. And you can talk about this in any dietary framework. If you have overconsumption of calories, you're going to have a problem, especially from highly processed foods and things like sugar-sweetened beverages. You combine that with low physical activity and metabolic dysfunction, it's bad news bears. If liver glycogen is already consistently full, like let's say you're just absolutely housing carbohydrates, the amount that's appropriate for someone that's physically active, and your energy intake is consistently exceeding your energy expenditure. Yes, in this context, we cannot just house something like sourdough bread and apples. Good luck, by the way, eating too many apples. Honestly, good luck. But if, you know, we already have massive insulin resistance. Yes, too much fructose, too many calories in general can be a real problem. Um, when that fructose comes into this metabolic environment, the liver's gonna begin looking for somewhere to put the excess incoming energy. And under those circumstances, de novolipogenesis, which is again the process where the body is gonna convert excess carbohydrates like glucose or fructose or other non-fat precursors, it's gonna turn it into new fatty acids. That can increase, all right? We don't necessarily want that, and fat can begin accumulating within the liver. Really bad news bears. That's gonna contribute to metabolic dysfunction. That's very different from saying something like eating blueberries causes fatty liver disease. Context is gonna matter, dose matters, overall dietary pattern matters, physical activity matters. So, just a personal anecdote. Back when I switched from a higher fat diet and a highly carnivorous diet to a diet that included some carbohydrates, I did it entirely wrong. And I developed fatty liver disease and it was awful. Um, I was eating, yep, you can overeat any macronutrient to excess, right? But I was eating lots and lots of fat. So I was doing like tallow, ribey, avocados, butter. And then I thought I would play around with adding some carbohydrates. But I added too many carbohydrates in the context of a system that was already dealing with way too much fat. So I really take issue. Well, I took issue, I take issue with myself looking back, but I take issue with someone saying something like, Oh, I can't eat bananas, I can't eat blueberries, it's gonna give me fatty liver disease. Fatty liver disease is gonna develop because of a lot of different things, but one of them is caloric excess because your poor liver is trying to filter everything. And if it sees that your stores already topped off, it's gonna store as liver fat. So just think about that. I'm a normal healthy liver, if you're moving around, you're getting your 10,000 steps, you're weightlifting a couple times a week, you really put a lot of effort into those weightlifting sessions, or you do Pilates, whatever you do. If you're physically active and you look at yourself and you can genuinely say, I'm a physically active person, and I'm mostly eating whole foods and I'm controlling my calories. I'm not just, you know, housing 6,000 calories a day with no physical activity. You can have a nice healthy liver with low amounts of liver fat. And if you're curious about this, you can go get a DEXA scan and really look at the amount of fat that's stored up in and around your organs. It's called visceral fat. You want very low visceral fat, and you can achieve that easily with a reasonable amount of calories and a reasonable amount of. Physical activity. But sometimes we get looped into some sort of dogma where someone tells us, oh, you know, you can eat 100 grams of fat a day and be just fine as long as your calories are controlled. No, I disagree. I think, you know, there's been some recent evidence that overeating calories from carbohydrate results in less weight gain than overeating fat. So we kind of have to look at caloric surplus. And if you're feeling like I should not have all this liver fat, kind of assess the amount of physical activity that you're doing and your total caloric intake and see if potentially too many of your calories are coming from something like excess fat. I would just in my experience as a practitioner, I almost always see overconsumption of fat and underconsumption of carbohydrate. That is almost always the case. I rarely come across someone that is over-consuming carbohydrate. So just food for thought, honestly. What does the research say about fruit? One of the things I love about nutrition science is that sometimes we can step back from mechanisms and ask something like, what happens in real people? And when researchers look at large populations, whole fruit consumption, emphasis on whole fruit. We're not talking about fruit juicing or fruit concentrates, but whole fruit consumption is generally associated with better metabolic health. People who regularly eat fruit tend to have lower rates of type 2 diabetes, cardiovascular disease, and obesity. That doesn't automatically mean that fruit causes those outcomes. We really don't want to lump ourselves into the camp of people who use observational studies as evidence, but you can generally look across populations and see, okay, people who are eating more fruit, they may have other healthy habits, but they have lower rates of type 2 diabetes, cardiovascular disease, and obesity. Please maintain a curious mind about that. So I really don't like large observational studies, and I don't like to reference them, especially in something like a podcast, because you have poorly done research where someone might be evaluating red meat consumption, but they're evaluating people who are eating red meat from fast food and not physically active, and not necessarily looking at Jeff, who's training for a marathon and eats, you know, sirloin steaks a couple times a week in the context of a very healthy amount of calories and physical activity. So be careful about observational studies. Um what's striking about all of this is that we don't consistently see whole fruit behaving like sugar-sweetened beverages in these studies. If fructose itself were the primary problem, we'd expect fruit to produce similar outcomes as studies that use high fructose corn syrup, and it doesn't. And that's because foods are more than isolated nutrients, what we touched on earlier. Um, another thing I like to ask people that are afraid of fruit in particular is what are you afraid of? And usually the answer that I get back is sugar. And then, you know, if you just take a pause and ask someone, do you know of anyone who's become metabolically unhealthy because they've they're eating too many apples? And you know, you'd be hard pressed to find someone who answers yes to that question. People don't typically develop obesity or type 2 diabetes or fatty liver because they're occasionally eating peaches or oranges or berries. And in my experience, I I haven't I haven't seen someone develop obesity or type 2 diabetes or fatty liver, even when they're housing fruit. In fact, you can see improvements in diabetes and obesity when someone starts housing fruit. Um, conditions like those that we just mentioned usually develop over years of chronic energy imbalance or low physical activity because the machine doesn't work well when you don't move it. It's designed to move. Or if someone has um particularly high stress, you can often see that reflected in labs as well. If they're eating highly processed diets, etc., etc., you get the picture. I think that blaming fruit distracts us from addressing the things that matter a lot more in regards to nutritional context. So if you remember only a few things from today's episode, please let them be these. First, glucose and fructose are metabolized a little bit differently, but they're both trying to contribute to your body's energy economy, deciding whether or not you have readily available energy. Second, the liver isn't trying to punish you for eating fruit. It's doing what it's evolved to do, which is managing incoming fuel based on your body's needs. Third, whole fruit is far more than just sugar. It's water, it's fiber, it's vitamins, it's minerals, it's thousands of naturally occurring compounds that work together in your body. And fourth, metabolism is highly dependent on context. The same nutrient can have different effects depending on whether or not you're sedentary or active, if you're glycogen depleted or glycogen replete, if you're metabolically healthy or insulin resistant, and whether that nutrient comes from a whole food or an ultra-processed product. And finally, um, nutrition becomes much less confusing, okay, when we stop asking whether a food is good or bad, and we start asking a better question: what is the context? What is my body trying to do with this food? Once you understand all of that, it's a lot easier to make food choices based on physiology instead of fear, which is why my goal or aim with a lot of these podcasts, just try to understand physiology, right? All right, thanks for listening today's episode. Next episode, I think, is gonna be a lot of fun. We're gonna build a little bit on everything that we've discussed and explore one of my favorite topics in metabolism, which is energy availability. And we're gonna talk about why increasing carbohydrates often helps people lose fat, why some people naturally move more when they're well fueled, and how your body senses energy abundance versus energy scarcity. And finally, a really fun thing whether there might even be an evolutionary explanation for why fruit and carbohydrate availability changes with the seasons. Thanks again for listening. If you want personalized guidance and a pro metabolic approach that's tailored to your life, you can book a session with me through Nourish. My link is in the show notes. Take care.