The Physiology of Fuel Management: What I Learned from Cycling 377 km

Beyond hyper-modern bikes and advanced training techniques, the single biggest revolution in modern professional cycling is without a doubt nutrition strategy. Fully loaded tanks at the start and strict fuel management along the way allow pros to sustain unprecedented power outputs. But how does that actually work, and can everyday cyclists simply copy this strategy?

The advice sounds simple: depending on the intensity, you consume 80, 100, or even up to 140 grams of carbs per hour. But does the same strategy apply to a short pro race versus an ultra-endurance effort, or is there a world of difference between fuel management for 3 hours versus 10+ hours in the saddle?

To test how my own body responds, I rode 377 kilometers last weekend — good for roughly 15 hours of cycling. I consciously chose a flat, smooth route along waterways with barely 500 meters of elevation, allowing me to ride continuously without traffic lights or city congestion.

My plan was straightforward: cruise disciplined between 160 and 180 Watts for hours. At more than 60 Watts below my aerobic threshold (LT1), I was riding deep in Zone 2. At that intensity, you burn almost exclusively fat… right?

As for supplies, I was well prepared: my rear pockets were stuffed with gels, cola gummies, and gingerbread with honey. My bottles were filled with carbohydrates and electrolytes. With this stash, I should be able to last a very long time.

The route was completely flat, the power meter read accurate numbers, and the fueling strategy seemed solid. Yet, a few times during the ride, I felt my strength fading away. Was something going wrong physiologically and biochemically, or is it simply inevitable that you hit a wall on distances like this?

To be clear: I am not an experienced ultra-endurance athlete. Those who are typically have additional gear that I didn’t use, such as handlebar, frame, and saddle bags. Their routes are likely planned tightly around water refill points. The goal of this experiment is primarily to see what happens biologically and what issues you encounter when fuel management isn’t taken 100% seriously.

The 21% Efficiency Factor

Perhaps surprisingly, the human body is actually a fairly inefficient combustion engine: our muscles have an average mechanical efficiency of around 21%. The remaining energy is lost as heat.

That means for every €5 worth of fuel (food) you put into your body, only €1 goes directly into turning the pedals. The other €4 is lost as body heat — which is precisely why you get hot and start sweating!

Handy rule of thumb vs. reality

In cycling, a popular rule of thumb states that the mechanical work on your bike computer in kilojoules (kJ) equals your total energy expenditure in kilocalories (kcal). That rule assumes a mechanical efficiency of 23.9% (since 1 kcal = 4.184 kJ).

However, if you calculate with a more realistic efficiency of 21%, your actual metabolic burn is slightly higher:

  • At 155 Watts: 155 W * 3.6 = 558 kJ of mechanical energy at the pedals. At 21% efficiency, your internal engine actually burns around 635 kcal per hour (558 / 0.21 / 4.184) to make that happen.
  • At 180 Watts: 180 W * 3.6 = 648 kJ of mechanical energy, which translates to over 738 kcal per hour.

This math makes it much easier to estimate how much energy your body is actually demanding while on the bike.

The Dream Start: Flat, Tailwind, and 120 BPM

For the first 180 kilometers, I was able to pace by the book. With a tailwind, I cruised at a relaxed 150 to 160 Watts. My heart rate stayed extremely low: between 120 and 130 beats per minute (well below my LT1 and deep in Zone 2).

With values that low, you might expect your carbohydrate reserves to remain untouched while you coast purely on fat. Biochemically, it’s not that black and white.

Muscles never burn exclusively fat or exclusively carbohydrates. It is always a blend — a hybrid mix. Even right now, as I write this article, my body is burning a small amount of sugars. When pushing 150 to 160 Watts, carbohydrate combustion is definitely part of the equation.

The exact ratio between fat and carbohydrates depends on several factors: training status, nutritional state, duration of effort, and genetics. According to lactate tests taken three months earlier, my LT1 sits around 240 Watts. LT1 is the threshold where the body starts burning noticeably more carbohydrates. Around 155 Watts, my primary fuel source is fortunately fat. Based on my testing and literature, I estimate my ratio at that intensity to be 70% fat and 30% carbohydrates.

As established, the body burns about 635 kcal per hour at 155 Watts. At a 70/30 ratio, that breaks down to:

  • 30% of 635 kcal = 190 kcal from carbohydrates.
  • Divided by 4 kcal per gram = ~48 grams of carbohydrates per hour.

After 6 hours of riding at this “easy” pace with a tailwind, total consumption quickly reaches 280 to 300 grams of carbohydrates.

Because I started fully loaded and ate systematically along the way, my glycogen levels stayed balanced. However, my reserve tank was no longer completely full. (By “fully loaded,” I mean maxed-out muscle and liver glycogen stores, a process that starts days before the event and ends 1 to 2 hours before the start).

The Forgotten Reality: The Size of Your Tank

Let’s pause to consider how large your glycogen tank actually is. A well-trained 73 kg athlete stores approximately:

  • Muscle glycogen: ~400 grams (1600 kcal)
  • Liver glycogen: ~100 grams (400 kcal)
  • Blood glucose: ~5 grams (20 kcal) — literally one teaspoon of sugar.
  • Total: ~500 grams of carbohydrates, worth about 2000 kcal of directly accessible sugar.

At an output of 155 Watts (635 kcal/hour) and a 70/30 ratio, you burn ~48 grams of carbohydrates per hour. On paper, without fueling, you could ride 500 / 48 = over 10 hours before the tank runs completely dry.

Here is where the problem lies: you can never tap into 100% of your glycogen. Your body always preserves a baseline reserve for your brain and vital organs. Furthermore, fat oxidation stalls once glycogen reserves drop too low (“fats burn in the flame of carbohydrates”). Experience shows that after 5 to 6 hours of continuous cycling, you reach a critical tipping point, even if you keep eating.

This is why eating early and consistently is so critical: you don’t just prevent bonking, you slow down the depletion of your internal battery.

The Tipping Point: Headwind, 180 Watts, and Accumulated Fatigue

At kilometer 180, the wind shifted. Where I was previously freewheeling with a tailwind at 155 Watts, I now had to keep constant pressure on the pedals to maintain speed. My power output shifted from 150–160W to 170–180W.

Those 20 extra Watts immediately push energy expenditure up to 738 kcal per hour. More importantly, the physiological demand shifts. Although 180 Watts is still well below my LT1 (240 Watts), the combination of a slightly higher intensity and hours of accumulated fatigue triggers a noticeable shift in fuel selection.

Because slower fat oxidation can no longer meet the rising demand for fast energy (ATP) after 6 hours on the bike, the fuel mix tilts from 70/30 to 55/45 or even 50/50.

At 45% carbohydrate reliance, you are suddenly burning:

  • 45% of 738 kcal = 332 kcal from carbohydrates.
  • Divided by 4 kcal/gram = ~83 grams of sugar per hour.

An extra 20 Watts sounds negligible on a bike computer, but combined with progressive muscle fatigue, it causes a significant spike in carbohydrate burn.

On top of that, physiological strain plays a major role. In the early hours, muscle fibers are fresh. After 6 hours, micro-muscle damage and cardiovascular drift set in (your heart rate rises at the exact same power output). To keep pushing 180 Watts after 8 or 10 hours, your body requires more oxygen and energy. Your mechanical efficiency drops, meaning you consume more calories per hour at the same power output, forcing your muscles to rely on the easiest available fuel: your remaining muscle glycogen.

The Silent Saboteur: Fluid and Salt Loss

What many fueling plans overlook is the critical role of fluids and electrolytes. On a 15-hour ride, you easily lose 5 to 8 liters of fluid, along with grams of sodium.

A sodium deficit leads to two major physiological complications:

  1. Impaired Gut Absorption: Your intestines rely on sodium-dependent glucose transporters (SGLT1) to absorb sugars and water. As sodium depletion builds, your ability to absorb carbohydrates from gels and mix decreases, even if you keep eating regularly.
  2. Muscle Function & Fatigue: Sodium deficiency causes muscle cramping, an elevated rate of perceived exertion (RPE), and general lethargy.

That feeling of heavy, dead legs is often not just a lack of sugar, but a lethal combination of glycogen depletion and electrolyte deficiency.

Flavor Fatigue and the Supermarket Lifesaver

Now that we are burning close to 80 grams of carbohydrates per hour, we simply need to consume more sugar. Sounds easy on paper, right? But after 6 to 8 hours in the saddle, you inevitably run into Flavor Fatigue.

Your brain and stomach revolt against yet another sweet gel or energy bar. Even sweet drink mix starts to taste nauseating. No matter what food you have in your pockets, you can barely force it down. Subconsciously, you start delaying fueling. Gummy candies are taken 15 minutes late, and your water bottle goes untouched. Deep down, you can feel yourself fading.

Physiologically, gastrointestinal blood flow decreases as blood is diverted to working muscles. Digestion slows down. Solid food sits longer in the stomach, creating a bloated, nauseous feeling.

What can you do at that point? In my case: stop, pull out the phone, find the nearest bakery or supermarket, and buy “real” food. Instead of technical sports nutrition, I stopped three times for 20 minutes to eat high-calorie comfort foods: a rice tart, an éclair, a bag of salty chips, and an ice-cold Coke.

Scientifically, this was far from “optimal” (fats delay gastric emptying), but practically, it was a lifesaver. The change in flavor, texture, and the added salt broke through the flavor fatigue. It provided a mental reset and brought back the appetite to eat.

The Data Analyzed: My Carbohydrate Balance over 377 km

Time (Hours) Distance Avg. HR Intake (g Carbs/h) Burn (g Carbs/h) Net Effect on Glycogen Estimated Glycogen Buffer Status / Physiology
0h (Start) 0 km - - - - 450 g Tanks loaded 100% full
Hours 1 - 5 0 - 138 km 125 bpm +50 g -48 g Fully covered by intake ~450 g Dream start: Exogenous sugars cover burn
Hour 6 ~165 km 130 bpm +50 g -52 g -2 g / h ~448 g First mild fatigue setting in
Hours 7 - 8 165 - 215 km 138 bpm +35 g -85 g -50 g / h ~348 g Tipping point: Wind shifts, 180W → rapid drop
Stop 1 (Hour 9) ~240 km 139 bpm +120 g (incl. stop) -88 g +32 g (liver buffer) ~380 g Stop 1: Éclair, Cola & Chips (Liver recovery)
Hour 10 ~265 km 141 bpm +30 g -90 g -60 g / h ~320 g Flavor fatigue on the bike hits
Stop 2 (Hour 11) ~290 km 142 bpm +110 g (incl. stop) -90 g +20 g (liver buffer) ~340 g Stop 2: Bakery & savory snacks
Hour 12 ~315 km 143 bpm +25 g -92 g -67 g / u ~273 g Muscle fatigue demands higher sugar burn
Stop 3 (Hour 13) ~340 km 141 bpm +100 g (incl. stop) -88 g +12 g (liver buffer) ~285 g Stop 3: Final mental & physical recharge
Hours 14 - 15.8 377 km 138 bpm +10 g -80 g -70 g / h ~160 g Finish: Intake drops; FatMax saves the ride!

Visualizing Glycogen Buffer Evolution

What the data shows: The first 6 hours were a physiological plateau: intake and expenditure were balanced. From hour 7, the wind shifted, carbohydrate burn jumped from 48g to over 85g per hour, and glycogen reserves plummeted.

Hours 9, 11, and 13 show the immediate impact of the three store stops. Without those real-food breaks, my sugar tank would have emptied completely around hour 12. Thanks to a well-trained FatMax, my engine kept turning over smoothly at the end, even when on-bike sugar intake almost completely stopped due to flavor fatigue.

The Lifesaver: 160–170 Watts on FatMax

When your glycogen tank is nearly depleted due to wind, elapsed time, and an accumulated intake deficit, you become entirely dependent on your FatMax. This is the maximum power output you can sustain primarily on fat oxidation.

You rarely hit a complete wall on a bike because you have 0.0 grams of sugar left; rather, the rate of fat oxidation is simply too slow to supply ATP fast enough for higher speeds. Your FatMax is the intensity where fat oxidation operates at its absolute peak. At that point, tiny amounts of carbohydrates are still being burned as an igniter flame for the Krebs cycle, but those reserves are strictly guarded by the body for vital functions and the central nervous system.

A cyclist who never trains fat oxidation specifically will collapse when glycogen runs dry (the classic bonk, barely able to push 100 to 120 Watts).

By consistently focusing on my aerobic base (Zone 2) in training, I was able to comfortably hold 160 to 170 Watts with minimal sugar intake, operating almost exclusively on fat oxidation. It didn’t feel fast or punchy, but the engine kept running to finish all 377 km successfully.

Preparation: More Than Just Nutrition

Anyone attempting ultra-endurance rides should prepare mechanically with bags for extra clothes, food, and spare parts. But beyond gear, there is one core physiological insight every cyclist needs to remember:

The rate at which you burn carbohydrates is vastly underestimated.

Many riders consume little to nothing on rides under 2 to 3 hours. That works if you start with maxed-out glycogen stores, but once those reserves drain, recovery takes time. Fully restoring depleted glycogen stores after an exhaustive ride requires 24 to 48 hours of structured carbohydrate loading (8–10 grams of carbs per kg of body weight per day).

Why not start fueling 15 to 20 minutes into every ride — even 2 to 3 hour rides — and repeat three times per hour?

  1. Blood Glucose Stability: You provide working muscles with a steady supply of circulating glucose.
  2. Glycogen Sparing: By eating from minute one, you delay the depletion of your internal battery, keeping you fresher late in the ride.

Conclusions & Practical Tips

  • Carbohydrate burn accumulates quietly: Even on a flat course with a tailwind, you burn tens of grams of sugar per hour at 160–180 Watts. If you push near tempo or FTP without fueling, you will burn through your sugar reserves rapidly.
  • Prevent flavor fatigue on long rides: Pack variety from the start. Alternate sweet sports nutrition with savory snacks (wraps, salted nuts, small rolls) so you can keep eating.
  • Train your fat oxidation: Your Zone 2 volume is your insurance policy. When sugars run low, your FatMax dictates whether you drop to a crawl or ride smoothly to the finish line.

My 4 Golden Rules for Every Ride:

  1. Know your numbers: Get a physiological test to determine your LT1 and estimated FatMax. This tells you the exact wattage you can sustain without over-taxing your sugar reserves.
  2. Plan your intake in advance: Determine ride intensity beforehand, calculate your hourly target (e.g., 60–90g/hour), and set a repeating timer on your bike computer for every 15–20 minutes. Tip: Practice this high intake in training to train your gut to process these amounts without distress.
  3. Vary taste and texture: Pack at least three different textures: sweet (gels, chews), salty (nuts, crackers), and real solid food (small wraps, rice cakes).
  4. Don’t forget sodium: On rides over 4 hours, plain water is not enough. Add electrolytes to your bottles to ensure proper fluid and sugar absorption in the gut and prevent cramping.