Blood Sugar, Energy, and the Endurance Cyclist: Why Glucose Is Not the Enemy

Blood sugar has become one of those wellness terms that gets thrown around everywhere. One day, we hear that blood sugar spikes are dangerous. The next day, we see endurance athletes drinking 90 grams of carbohydrate per hour, eating gels, sipping glucose-fructose drink mixes, and using continuous glucose monitors to better understand performance. So what is the truth? Like most things in endurance sport, the answer depends on context.

For cyclists, blood sugar is not simply a health metric. It is part of the energy system that keeps us pedaling, climbing, thinking clearly, regulating effort, protecting the gut, and finishing long rides without completely falling apart. The problem is that blood sugar often gets discussed in a general wellness context, not an endurance-performance context. A sedentary person sitting at a desk all day is not using glucose the same way as a cyclist riding tempo, climbing for hours, racing gravel, or pushing through a long endurance block. As endurance athletes, we need to understand blood sugar differently. We need to respect it. We need to fuel it. And we need to stop fearing carbohydrates simply because they raise glucose.

For cyclists, glucose is not the enemy. Poor timing, under-fueling, over-fueling without gut training, low energy availability, and misunderstanding what the body is actually doing are usually where the trouble starts. Blood sugar refers to the amount of glucose circulating in the bloodstream. Glucose is a simple sugar, but that does not mean it is bad. It is one of the body’s most important fuels. When we eat carbohydrates, our digestive system breaks them down into smaller sugar molecules, including glucose. That glucose enters the bloodstream, where it can be used immediately for energy or stored for later. Some glucose is stored in the liver as glycogen, some is stored in the muscles as glycogen, and some stays in the blood to help maintain stable energy availability for the brain, nervous system, and working muscles.

This matters deeply for cyclists because muscle glycogen and blood glucose are two of the most important carbohydrate sources during training and racing. Your muscles can use stored glycogen directly. Your liver can release glucose into the bloodstream to help keep blood sugar stable. And when you eat or drink carbohydrates during a ride, that external carbohydrate can enter the bloodstream and become available as fuel. This is why blood sugar is not just a “health number” for endurance athletes. It is part of the performance engine.

The body does not want blood sugar to be too high or too low. It is constantly regulating glucose through hormones, especially insulin and glucagon. Insulin helps move glucose out of the blood and into cells, especially after meals. Glucagon helps raise blood glucose when it starts to drop, mainly by telling the liver to release stored glucose. During exercise, this regulation changes. When we start riding, our muscles begin pulling glucose from the blood more efficiently. Muscle contractions can increase glucose uptake even with lower insulin levels, which is one reason exercise is so powerful for metabolic health. At the same time, the liver works to keep blood glucose available. It breaks down liver glycogen and can also create glucose through gluconeogenesis, especially during longer efforts.

This is a beautifully coordinated system, but it is also a system under stress when we ride long or hard. The harder or longer we ride, the more the body has to manage the balance between glucose use, glucose release, glycogen storage, fat oxidation, hydration, stress hormones, and gut absorption. That is why endurance fueling is not just about eating sugar. It is about supporting a complex system while it is working hard.

When we talk about energy on the bike, we are really talking about ATP. ATP, or adenosine triphosphate, is the usable energy currency of the body. Your muscles do not directly use a banana, a gel, a bowl of oats, or a bottle of drink mix. They use ATP. Carbohydrates help create ATP through several pathways. First, glucose can go through glycolysis, a process that breaks glucose down into pyruvate and creates a small amount of ATP quickly. From there, pyruvate can enter the mitochondria, where it continues through the Krebs cycle and electron transport chain to produce much more ATP. When oxygen is available and intensity is manageable, carbohydrate metabolism can be very efficient. When intensity rises, carbohydrate becomes even more important because it can produce ATP faster than fat.

This is why hard efforts feel different from easy endurance rides. At lower intensities, the body can rely more heavily on fat oxidation. At higher intensities, especially tempo, sweet spot, threshold, VO2 max, climbs, surges, and race attacks, carbohydrate becomes the dominant fuel. Fat is incredibly useful for long endurance, but carbohydrate is the fuel that supports high power, repeated accelerations, mental sharpness, and the ability to keep pressure on the pedals when the ride gets hard. This is one of the biggest misunderstandings in endurance nutrition. Yes, cyclists should develop fat oxidation. Yes, metabolic flexibility matters. But when we ask the body to perform, especially at moderate to high intensity, carbohydrate availability matters a lot.

Muscle glycogen is stored carbohydrate inside the muscle. I like to think of it as the internal carbohydrate fuel tank. When we ride, especially at moderate to high intensity, our muscles break down glycogen to produce energy. The more intense the ride, the faster glycogen gets used. When glycogen gets low, power becomes harder to sustain. Perceived exertion rises. The legs feel heavy. The brain starts negotiating. You may still have body fat available, but you cannot access energy quickly enough to keep producing the same power. This is the classic bonk, or what people often call hitting the wall.

But bonking is not just feeling tired. It is often a blood glucose and glycogen problem. When liver glycogen drops and blood glucose falls, the brain starts to protect you. You may feel shaky, foggy, emotional, irritable, weak, or strangely disconnected from your effort. Anyone who has gone too deep into a ride without enough fuel knows that feeling. It is not a lack of toughness. It is physiology.

The brain relies heavily on glucose, which is why under-fueling can change more than your legs. During long rides, especially when carbohydrate availability is low, the brain becomes part of the performance equation. Low blood sugar can affect concentration, decision-making, motivation, reaction time, and emotional regulation. This matters in cycling because we are not just pedaling. We are descending, cornering, reading terrain, responding to wheels, making pacing decisions, navigating routes, managing group dynamics, riding in traffic, and staying calm when the body is under stress.

When athletes under-fuel, they often think the only consequence is tired legs, but sometimes the first signs are mental. You lose focus. You get negative. You start making poor decisions. You cannot hold your line as well. You feel overwhelmed by small problems. That is not a character flaw. That is your brain asking for fuel. For ultra-distance and bikepacking athletes, this becomes even more important. Long hours, sleep deprivation, limited resupply, cold conditions, altitude, and emotional fatigue all increase the importance of steady fueling. Blood sugar is not just about watts. It is also about staying human out there.

In general wellness conversations, blood sugar spikes are often framed as something to avoid at all costs. But context matters. A glucose rise after eating carbohydrate before or during exercise is not the same as repeated large glucose swings in a sedentary state. When you are riding, your muscles are actively taking up glucose. Exercise increases glucose demand. That means the same carbohydrate that might cause a bigger glucose rise at rest can become useful fuel during training.

This does not mean cyclists should eat unlimited sugar all day. It means we need to understand timing. Carbohydrates around training are different from mindless snacking without energy demand. A gel before a hard climb is different from eating candy while sitting at a desk. A carbohydrate drink during a four-hour ride is different from sipping soda all afternoon. For endurance athletes, the goal is not to flatten every glucose response. The goal is to match carbohydrate availability to the work being performed.

The liver is one of the most underrated organs in endurance performance. It helps regulate blood glucose by storing and releasing glycogen. When you are between meals, sleeping, or exercising, the liver helps keep blood sugar from dropping too low. During long rides, the liver releases glucose into the blood to support the brain and working muscles. But liver glycogen is limited. If you start a long ride under-fueled, ride hard, or go too long without carbohydrate, liver glycogen can become depleted. That is when blood sugar can drop. This is one reason breakfast before a long ride matters. It is also why starting a race fasted is usually not ideal for performance, especially if the ride includes intensity. A well-fueled liver gives the body a better chance of maintaining stable blood glucose over time.

When blood sugar drops, the body does not just politely ask for a snack. It responds with stress hormones. Adrenaline and cortisol can rise to help mobilize energy and protect the body. This can make an under-fueled athlete feel temporarily alert, wired, or even “fine.” But this is not true energy. It is stress compensation. Many endurance athletes get used to operating this way. They under-eat, ride hard, drink coffee, push through, and interpret the stress response as discipline. But over time, chronically training with low energy availability can affect recovery, hormones, mood, immune function, sleep, and adaptation.

This is especially important for cyclists doing big training blocks. You do not get stronger from the workout alone. You get stronger from absorbing the workout. Blood sugar stability, carbohydrate availability, and total energy intake all help create the internal environment where adaptation can actually happen.

Here is where things get even more interesting: blood sugar does not start in the blood. It starts in the gut. Before carbohydrate can become usable energy, it has to be digested, absorbed, transported, and tolerated. That means the gut is a performance organ. During exercise, blood flow shifts away from the digestive system and toward the working muscles, heart, lungs, and skin. This can make digestion harder, especially at higher intensities or in heat. At the same time, the athlete is asking the gut to process fluid, sodium, glucose, fructose, gels, bars, chews, real food, caffeine, and sometimes all of that while bouncing over gravel. No wonder GI distress is common in endurance athletes.

For cyclists, the gut is usually less mechanically stressed than it is in running, but it is still under pressure. Long duration, aero position, dehydration, heat, high carbohydrate concentration, low sodium, too much fructose, too little water, or foods that sit heavily can all create problems. This is why the gut has to be trained just like the legs, lungs, and mind.

Gut training is one of the most important concepts in endurance fueling. Athletes often say, “My stomach cannot handle that much carbohydrate.” Sometimes that is true. But sometimes the gut has simply never been trained to absorb that amount of carbohydrate during exercise. Gut training means practicing your race-day fueling strategy during training. It means gradually increasing carbohydrate intake. It means testing different forms of fuel. It means learning how much fluid you need. It means understanding whether your body tolerates gels, drink mix, bars, rice cakes, bananas, potatoes, or liquid calories better.

It also means practicing under realistic conditions. A fueling plan that works on an easy one-hour spin may not work during a hot gravel race with repeated climbs. This is why race nutrition should never be a surprise. Your gut needs reps too.

One of the biggest advancements in endurance fueling is the use of multiple transportable carbohydrates. Glucose and fructose use different transporters in the intestine. Glucose primarily uses the SGLT1 transporter, while fructose uses the GLUT5 transporter. When athletes consume only glucose or maltodextrin, there is a ceiling to how much carbohydrate can be absorbed and oxidized effectively. But combining glucose and fructose can allow higher carbohydrate delivery because the body is using more than one intestinal transport pathway.

This is why many modern endurance products use glucose-fructose combinations. Older guidelines often focused on 30 to 60 grams of carbohydrate per hour. Now, many trained endurance athletes can consume 90 grams per hour or even higher in certain situations, but only when the gut is trained and the intensity, duration, and conditions justify it. This does not mean every cyclist needs 100 grams per hour. It means the ceiling is higher than we used to think. For many recreational riders, 40 to 70 grams per hour may be plenty. For long races, hard gravel events, ultra rides, or high-intensity endurance efforts, 70 to 100 grams per hour may be useful if trained. The key is not copying the pros. The key is matching the fuel to the work and building tolerance over time.

Carbohydrate is powerful, but more is not always better. If carbohydrate intake exceeds what the gut can absorb, the extra carbohydrate can sit in the intestine and pull water into the gut. That can lead to bloating, nausea, cramping, diarrhea, and that sloshy stomach feeling. This is where osmolality matters. A highly concentrated drink mix, especially without enough water, can slow gastric emptying and irritate the gut. That is why some athletes do better with a bottle of concentrated fuel plus a bottle of plain water. Others do better spreading fuel across drink mix, gels, and real food. Some need more sodium. Some need lower concentration. Some need less fiber and fat before hard rides. Some need to avoid taking in a massive amount of carbohydrate all at once.

The goal is not just to get carbohydrate into your mouth. The goal is to get it through the gut, into the bloodstream, and to the working muscles. That is the real fueling skill.

Blood sugar and hydration are also deeply connected. Fluid helps move nutrients. Sodium helps maintain blood volume and supports fluid balance. Carbohydrate and sodium together can improve fluid absorption in the small intestine. But if an athlete takes in a lot of carbohydrate without enough fluid, the gut may struggle. If an athlete drinks too much plain water without sodium during long hot rides, they may dilute blood sodium and still feel terrible. If an athlete gets dehydrated, gastric emptying can slow and GI symptoms may worsen. This is why fueling plans need to include carbohydrate, fluid, and sodium together.

For long rides, especially in heat, the question is not just, “How many grams of carbs am I taking?” It is also, “How much fluid am I drinking? How much sodium am I losing? How concentrated is my bottle? Can my gut absorb this at race intensity? Do I have plain water available if my stomach gets too concentrated?” That last question is huge. Sometimes the fix for GI distress is not less fuel. Sometimes it is better dilution.

There is also a lot of discussion around low-carb training, metabolic flexibility, and fat adaptation. Some lower-carbohydrate sessions may have a place in advanced programming, especially for specific endurance adaptations. But this has to be used carefully. Training low is not the same as under-fueling, and it is not ideal for every athlete, every session, or every goal. High-quality intensity requires carbohydrate. Repeated hard sessions require recovery. Female athletes, masters athletes, athletes with high life stress, and athletes already struggling with energy availability need to be especially cautious with chronic carbohydrate restriction.

Low-carb approaches can reduce the ability to hit high power outputs, increase perceived exertion, impair mood, and make it harder to recover from demanding training. For cyclists, the goal should not be to avoid carbohydrate. The goal should be to use carbohydrate strategically. Fuel the work required. Eat enough to adapt. Practice race fueling. Understand that becoming a strong endurance athlete does not mean becoming afraid of glucose.

Continuous glucose monitors, or CGMs, have also become popular in endurance sports. They can show glucose trends across the day and during training. For some athletes, they may offer insight into fueling timing, under-fueling patterns, recovery meals, sleep-related glucose changes, and how the body responds to different foods. But CGMs are not magic. They measure interstitial glucose, not direct blood glucose, and there can be a lag between what is happening in the blood and what appears on the sensor. They can also create anxiety or over-analysis, especially when athletes start judging every normal glucose rise as a problem.

For cyclists without diabetes, CGMs may be interesting. They may even be helpful. But they should not replace common sense, performance feedback, hunger cues, recovery markers, power data, or a well-practiced fueling plan. A flat glucose line is not the goal of endurance performance. A strong, fueled, adaptable athlete is the goal.

The bonk is one of the clearest examples of blood sugar and glycogen meeting reality. A bonk usually happens when carbohydrate availability drops too low. Muscle glycogen is depleted. Liver glycogen is low. Blood glucose may fall. The brain senses the threat. Power drops. Mood changes. Coordination fades. Everything feels harder than it should. You may still have plenty of fat stored on your body, but fat cannot always provide energy quickly enough for the intensity you are asking for. This is why “I have enough fat to fuel the ride” is technically true but practically incomplete. Yes, the body stores a lot of energy as fat. No, that does not mean you can perform all endurance efforts well without carbohydrate.

Under-fueling does not always look dramatic. Sometimes it is subtle. You may notice that your power fades late in rides. You may feel unusually cold. You may get irritable or emotional. You may crave sugar intensely after training. You may wake up at night hungry. You may struggle with recovery. You may feel flat during intervals. You may get dizzy after hard efforts. You may feel like your legs are disconnected from your brain. You may have GI problems because your gut is being asked to perform under stress without enough consistent fuel practice.

Many cyclists blame fitness when the real issue is fueling. They think they need more discipline, more intervals, or more toughness. Sometimes they need breakfast. Sometimes they need 60 grams of carbs per hour instead of 20. Sometimes they need to stop saving calories during the workout and then overeating later because the body is desperate. Fueling well is not weakness. It is performance maturity.

After a hard ride, blood sugar and glycogen restoration matter. Your muscles are more sensitive to carbohydrate after exercise, which makes this a great time to replenish glycogen, especially if you have another session within 24 hours. Carbohydrate after training helps refill glycogen. Protein helps repair muscle tissue. Fluid and sodium help restore hydration. This is where a carb-plus-protein recovery approach can be useful. For cyclists training frequently, recovery nutrition is not just about today. It is about tomorrow’s session. If you finish a hard ride depleted and do not refuel well, the next workout may suffer. Your mood may dip. Your cravings may increase. Your sleep may be affected. Your immune system may take a hit. The body does not adapt well when it is constantly trying to survive.

Gut health is not only about what happens during exercise. Daily nutrition matters too. A healthy gut generally benefits from enough total food, a variety of plants, fiber, colorful fruits and vegetables, polyphenols, fermented foods if tolerated, adequate hydration, and consistent meal timing. But there is a difference between everyday gut-health nutrition and race-day gut strategy. A high-fiber, colorful, whole-food meal may be perfect on a normal day. That same meal may be a disaster two hours before a hard race.

This is where athletes need nuance. Whole foods matter. Fiber matters. Micronutrients matter. But during hard training and racing, the gut often needs simplicity. Lower fiber. Lower fat. Lower protein. Familiar carbohydrate. Adequate fluid. Enough sodium. Easy absorption. You can care deeply about health and still use gels during a race. You can eat vegetables and still drink carbohydrate mix on the bike. You can value stable blood sugar and still intentionally raise glucose during exercise because your working muscles need fuel. This is not contradiction. This is context.

For practical fueling, shorter easy rides under 60 minutes may not require much carbohydrate during the ride, especially if you are well-fed beforehand. For rides lasting one to two hours, 30 to 60 grams of carbohydrate per hour can be a helpful range, especially if intensity is moderate to high. For rides over two to three hours, many cyclists benefit from 60 to 90 grams per hour, depending on intensity, body size, experience, and gut tolerance. For long races or ultra-endurance events, some trained athletes may push higher, but this should be practiced carefully over time.

The best fueling plan is not the most aggressive one. It is the one you can absorb, tolerate, and repeat under real conditions. A strong plan usually includes a carbohydrate-rich meal before longer or harder rides, fueling early instead of waiting until you are already fading, sipping regularly instead of taking everything at once, using glucose-fructose blends for higher carbohydrate targets, matching fluids and sodium to sweat rate and heat, training the gut before race day, and keeping emergency carbs available. The biggest mistake most cyclists make is fueling too late. By the time you feel the bonk coming, you are already behind.

Blood sugar is not something cyclists need to fear. It is something we need to understand. It rises and falls for reasons. It responds to food, stress, sleep, hormones, exercise intensity, hydration, heat, recovery, and training load. It is not a moral score. It is information. For endurance athletes, the goal is not to suppress blood sugar at all costs. The goal is to create a body that can use glucose well, store glycogen, absorb fuel, ride hard when needed, and recover afterward.

As cyclists, we ask a lot from our bodies. We ask them to climb mountains, hold tempo, chase wheels, ride through heat, handle stress, think clearly, and keep going when the road gets long. Blood sugar is part of that story. Not the whole story, but a very important one.

In the end, blood sugar is not just about glucose. It is about energy availability. It is about the brain. It is about the gut. It is about the liver. It is about hormones. It is about performance and recovery. It is about learning how to fuel the work you are asking your body to do.

For endurance cyclists, carbohydrates are not something to fear. They are a tool. A powerful one. When used well, they help protect power, mood, gut function, hydration, recovery, and long-term adaptation. The strongest cyclists are not the ones who ignore hunger, fear glucose, or brag about how little they can eat. The strongest cyclists are the ones who understand their bodies. They know when to fuel. They know how to practice. They know that the gut is trainable. They know that blood sugar is not the enemy. And they know that endurance is built not by starving the system, but by supporting it.