Table of Contents
French biochemist reveals the surprising science of what your body actually burns during different workouts - and why that glucose gel might be pointless for your 30-minute gym session.
Key Takeaways
- Your body has three fuel storage systems: liver glycogen (100g), muscle glycogen (400g), and unlimited fat reserves
- At 50% max heart rate, you burn mostly fat and can exercise for hours without eating anything
- One pound of body fat contains enough energy for 12 hours of moderate-intensity walking
- High-intensity exercise (85% max heart rate) burns glucose almost exclusively and depletes reserves quickly
- For workouts under 2 hours, you don't need mid-exercise fuel - just swish sweet liquid and spit it out for 3-7% performance boost
- Your intestines can only absorb 60g glucose plus 30g fructose per hour, creating a fuel bottleneck for ultra-endurance events
- Post-workout refueling doesn't require sugar bombs - normal meals with veggie starters work better
- Metabolic flexibility (ability to burn both fat and glucose efficiently) is a key health marker you can improve
- Working out fasted is fine for moderate intensity but can hurt performance for intense sessions
Your Body's Fuel Storage System: The Foundation You Need to Understand
Before we dive into what to eat and when, let's talk about how your body actually stores and uses energy. Think of your body as having three different fuel tanks, each with its own characteristics and purposes.
Your liver is like a small but easily accessible fuel tank. It can store about 100 grams of glycogen - that's roughly the amount of glucose in two large orders of fries, and about half of what your body needs per day just to function. When you eat carbs, they break down into glucose molecules, and insulin helps shuttle the excess into your liver where it gets converted into glycogen (basically glucose molecules holding hands in long chains).
Your muscles are the main storage depot. They can hold about 400 grams of glycogen - equivalent to seven large orders of fries. This is where things get interesting for exercise, because when you're moving around, your muscles can tap directly into their own fuel reserves without having to wait for delivery from somewhere else.
Then you have your fat cells, which are basically unlimited storage. When your liver and muscles are topped off and there's still excess glucose floating around, your body converts it to fat and stores it in fat cells. And here's the mind-blowing part: every single pound of fat on your body contains enough energy to fuel 12 hours of moderate-intensity walking.
Jesse Inchauspe tested this herself with a VO2 max test, where she ran on a treadmill with a mask measuring exactly what fuel her body burned at different intensities. The results showed a clear pattern: at lower intensities, her body burned a mix of fat (shown in white on the graph) and glucose (shown in green). But as the exercise got more intense and her heart rate climbed toward maximum, the white bars disappeared and her body switched almost entirely to burning glucose.
Understanding this system is crucial because it explains why the same snack advice doesn't work for every type of workout. A leisurely hike taps into completely different fuel sources than a high-intensity interval class.
The Chill Zone: Why Your Easy Workouts Don't Need Fuel Stations
Let's start with what Inchauspe calls "chill exercise" - anything at about 50% of your maximum heart rate. This is the intensity where you're hiking with a friend and can carry on a conversation, doing easy cycling along a river, or taking a brisk walk. You don't feel like you're working particularly hard, and you could sustain this pace for anywhere from 5 minutes to 4 hours.
At this intensity, your body becomes a fat-burning machine. Studies show that up to 60% of the fuel comes from your fat reserves, with the rest coming from glucose in your bloodstream (either from food you've recently eaten or released from your liver's glycogen stores).
There's a fascinating study where scientists had people cycle at 55% of their max heart rate for four straight hours. In the first two hours, participants burned a mix of fat from their bloodstream, glucose from their muscles, and glucose from their blood. But in the last two hours, fat burning massively took over as the primary fuel source.
This has huge implications for how you should think about fueling these workouts. "If you're going for a long walk, a hike, if you're dancing at chill intensity, if you're doing moderate exercise, the answer here is you don't need snacks," Inchauspe explains. "Your body has everything it needs in its fat reserves."
This is actually liberating information for most of us. When you go on a hiking trip, those snacks you pack are for enjoyment and pleasure, not because your body actually needs them for performance. Your fat reserves contain so much energy that they could power hours and hours of this type of exercise.
If your goal is fat burning specifically, this moderate intensity is also your sweet spot. Higher intensity workouts are fantastic and have their own benefits, but they won't burn nearly as much fat for fuel because your body switches to preferring glucose at higher intensities.
This brings up the common question about fasted workouts. For moderate-intensity exercise, working out on an empty stomach is totally fine because your body is mostly burning fat anyway. Even though your blood glucose might drop slightly as your muscles use some of it, the majority of your fuel is coming from fat stores that don't depend on what you ate recently.
Metabolic Flexibility: The Health Marker You've Never Heard Of
Here's where things get really interesting from a health perspective. Some people can't burn fat for fuel even during low-intensity exercise. Their bodies just burn glucose all the time, regardless of intensity, and this condition is called being "metabolically inflexible."
Being metabolically flexible means your body can efficiently switch between burning glucose and fat depending on what it needs. This is actually a crucial marker of overall health that we should all be working toward. When you're metabolically flexible, your body can tap into its fat reserves during longer, easier activities and save the glucose for when it really needs that quick-burning fuel.
Signs that you might not be metabolically flexible include being hungry all the time, wanting to snack constantly, or finding that even moderate exercise like an hour-long walk feels exhausting. If you can't go more than a few hours without eating, or if gentle exercise wipes you out, your body might be overly dependent on glucose and struggling to access its fat stores.
The good news is that metabolic flexibility isn't fixed - you can train your body to get better at burning fat. The key is reducing the number of glucose spikes you experience throughout the day. When your glucose levels are constantly spiking and crashing, your body becomes dependent on those regular hits of glucose and loses the ability to efficiently burn fat.
"The way you train your body to become metabolically flexible is by reducing the amount of glucose spikes that we experience," Inchauspe explains. "As we steady our glucose levels, our body no longer depends on these big spikes of incoming glucose in our bloodstream."
This is where her famous glucose hacks come in handy - eating vegetables first, taking walks after meals, having vinegar before carb-heavy dishes. These strategies help keep your glucose levels more stable, which over time trains your body to be more flexible about what fuel it burns.
The High-Intensity Switch: When Your Body Demands Glucose
Everything changes when you crank up the intensity to about 85% of your maximum heart rate. We're talking about sprinting, fast-paced running like a 5K or 10K, high-intensity interval training classes, vigorous cycling with hills, swimming at race pace, or CrossFit-style workouts. At this intensity, your body makes a dramatic shift in fuel preference.
Studies show that at 85% max heart rate, your muscles are burning almost exclusively glucose. The fat-burning capability that sustained you beautifully during moderate exercise basically shuts down because converting fat to usable energy is too slow for the demands of high-intensity work.
This creates a problem that didn't exist during easy exercise: you can run out of fuel. Remember, you can only store about 500 grams of glycogen total between your liver and muscles. At high intensity, you're burning through about 150 grams of glycogen per hour. Do the math, and you've got maybe 3 hours before your tanks are empty.
But it's not like you can perform at maximum intensity for 3 hours and then suddenly drop. As your glycogen reserves decrease, your body starts rationing the fuel and reducing how hard you can push. Performance drops gradually as your fuel levels fall.
This is why proper fueling becomes critical for longer, high-intensity efforts. If you're doing intense exercise for more than about 2 hours, you need to start thinking strategically about carb intake both before and during the workout.
For pre-workout fueling, the recommendations are pretty straightforward: make sure your glycogen reserves are topped off. The night before a big effort, eat plenty of good carbs like rice, pasta, bread, and potatoes (not talking about cookies and ice cream here). The morning of, have more carbs to replace whatever glycogen got used overnight. Don't do long, intense workouts fasted unless you're okay with compromised performance.
The Mouth Rinse Trick: Performance Hacking for Short Intense Workouts
For intense exercise lasting less than 2 hours - think boot camp classes, hour-long HIIT sessions, or short races - you don't actually need to eat during the workout. Your glycogen stores should be sufficient to fuel that duration. But scientists have discovered a fascinating performance hack that sounds almost too weird to be true.
It's called carbohydrate mouth rinsing, and it can boost performance by 3-7% without consuming any calories. Here's how it works: take some sweet liquid (glucose solution or fruit juice), swish it around in your mouth for 5-10 seconds so your body gets the message "oh, there's something sweet coming," then spit it out.
That's it. You don't even have to swallow the liquid. Just the act of having sweet taste receptors in your mouth triggers your brain to think fuel is on the way, so it doesn't dial down your performance capacity as aggressively.
Studies have shown this works specifically for muscular endurance (though not for maximum strength). The theory is that your brain is constantly monitoring fuel availability and adjusting performance accordingly. When it senses sweetness in your mouth, it assumes glucose is incoming and allows you to maintain higher intensity for longer.
"You don't actually have to swallow the glucose or the sugar," Inchauspe explains. "Just by swishing it around, your body and your brain think 'oh cool, there's some sugar coming, there's some glucose coming, so I don't have to decrease how much we can push during this one or two hours.'"
This is perfect for those intense gym classes where you want every possible advantage but don't want to deal with actually eating or drinking sugary stuff during the workout. You can test this yourself - bring a small amount of fruit juice to your next HIIT class, take a sip and swish it around for several seconds during rest periods, then spit it out.
Ultra-Endurance Fuel Strategy: The Glucose-Fructose Science
Once you get into exercise lasting 2-3 hours or more at high intensity, you enter the realm where external fueling becomes essential. This is when athletes often "hit the wall" - that horrible feeling where your body just says "nope, I can't maintain this pace anymore, I'm out of fuel."
The challenge is that your digestive system becomes a major bottleneck. At 85% max heart rate, you're burning about 120-180 grams of glucose per hour. But your intestines can only absorb about 60 grams of glucose per hour, no matter how much you consume. That creates a deficit that gets worse the longer you exercise.
Scientists discovered something clever though: while you can only absorb 60 grams of glucose per hour, you can simultaneously absorb about 30 grams of fructose per hour through a different pathway. Your liver then converts that fructose into glucose, giving you access to roughly 100 grams of usable glucose per hour if you consume the right mix.
Studies show that athletes using this glucose-fructose combination during long events get 75% more glucose burning, higher power output, and faster race times compared to glucose alone. The key is finding that sweet spot of about 60 grams glucose plus 30 grams fructose per hour.
You can get this from sports drinks, gels, or solid foods - whatever your stomach tolerates best. If you go with solid bars or food, make sure to drink plenty of water since they can be dehydrating. The exact delivery method matters less than hitting those ratios.
But here's the important caveat: this glucose-fructose strategy is specifically for ultra-endurance events lasting 3-4+ hours at high intensity. If you're not doing that kind of exercise, you don't need to be consuming fructose supplements. Outside of this very specific context, having too much fructose in your diet is actually detrimental to health.
Post-Workout Refueling: Why You Don't Need the Sugar Bomb
After intense exercise, your body becomes incredibly efficient at absorbing glucose to replenish those depleted glycogen stores. This is both good and bad news. Good because refueling happens quickly. Bad because it also means you'll absorb glucose really rapidly from whatever you eat, potentially creating big spikes and crashes.
The conventional wisdom of immediately eating sugary foods after workouts is actually counterproductive for most people. Yes, you want to replenish your glycogen, but you don't need to create massive glucose spikes to do it. Those spikes cause inflammation and lead to energy crashes that make you feel terrible after you've already stressed your body with exercise.
Instead, treat post-workout refueling like any other meal: use the glucose hack of eating vegetables first. Have a salad or some other veggie starter, then follow it with protein and plenty of good carbs like rice, pasta, or potatoes. The vegetables help slow the absorption of glucose from the starches, allowing you to refuel steadily without the spike-and-crash cycle.
"You do not need to replenish after intense prolonged exercise with cookies, with fast food, with ice cream, with chocolate, with sugar," Inchauspe emphasizes. "You can refuel with a normal healthy meal that doesn't create a glucose spike."
This approach gives you all the glycogen replenishment you need while avoiding the negative consequences of rapid glucose absorption. Your body will still quickly shuttle that glucose into your muscles to rebuild glycogen stores, but it'll happen more gradually and without the inflammatory response.
Adapting Glucose Hacks for Athletes
For people doing regular intense training, some modifications to typical glucose management strategies make sense. The biggest change is around breakfast and pre-workout meals. While Inchauspe usually recommends savory breakfasts where starches and fruit are optional, athletes need to make sure those glycogen tanks stay topped off.
If you're doing 4 hours of intense exercise at 10 AM, your breakfast absolutely needs to include good amounts of starch and fruit to replenish what got used overnight. The savory breakfast approach still works - you're just adding more carbs to it rather than making carbs the entire meal.
Similarly, post-workout meals should include generous amounts of starches for glycogen replenishment. The key is still using the veggie-first approach to prevent spikes, but the total carb content will be higher than what sedentary people need.
Apart from those modifications around workout timing, all the same glucose hacks apply to athletes. Reducing glucose spikes is beneficial for everyone, whether you're training for the Olympics or just trying to feel better day-to-day. The difference is that athletes need to be more strategic about when they include larger amounts of carbs to fuel their training demands.
The beautiful thing about understanding these fuel systems is that it takes the guesswork out of workout nutrition. No more wondering whether you need that pre-workout snack for your evening walk, or feeling guilty about not eating during your weekend bike ride. Your body's got plenty of fuel stored up for most activities - you just need to know which fuel system you're tapping into.
