Why Not All Carbohydrates Fuel You the Same Way

What Happens to Your Gut During Exercise

At rest, your digestive system is well-supplied with blood and running efficiently. The moment you start exercising hard, however, that changes.

During exercise, blood flow is redirected away from the GI tract toward your muscles, heart, and lungs. During intense or prolonged exercise, GI blood flow can drop by 50–80%. 

Then, gastric emptying slows down and absorption becomes less efficient. Anything your gut can't clear quickly starts drawing water in, fermenting, and causing the cramping, bloating, and urgency that affects somewhere between 30 and 90% of distance runners.

The harder and longer you push, the more this matters. A banana at an aid station in the first hour of a ride is very different from a gel at kilometer 100 of an Ironman bike leg. Fueling strategy and fuel type both need to account for that shift.

 

Not All Carbohydrates Use the Same Pathways

Different sugars use different transport proteins, meaning that when carbohydrates are broken down into simple sugars, they don't all take the same route into your bloodstream. 

Glucose uses a transporter called SGLT1, an active transport system that requires sodium and is known to absorb at a maximum rate of roughly 60 grams of glucose per hour. If you take in more glucose than your body can absorb through SGLT1, it pools in your gut rather than becoming fuel for your muscles.

Fructose uses an entirely separate transporter, GLUT5, which works independently of sodium and doesn't compete with glucose for the same pathway. Its absorption rate is slower on its own but it's a different entry point into the bloodstream entirely.

Using carbohydrates that use different transport proteins with different capacities and different locations means more total carbohydrates can reach your bloodstream simultaneously.

 

Combining glucose-based carbohydrates with fructose, means athletes have higher carbohydrate oxidation rates and can absorb up to 90g/hr or more compared to the ~60g/hr ceiling when using glucose alone. 

There’s also a GI tolerance benefit as, when one transporter saturates and carbohydrates accumulate in the intestine, water gets pulled in osmotically and fermentation increases. This is where bloating and cramping tend to originate. 

Using both glucose and fructose pathways reduces buildup, which is why some athletes who struggle with GI issues on single-source fuels actually find better tolerance with a combined source product, even if they assume the opposite.

This is also the reason a 2:1 glucose-to-fructose ratio has become a standard design principle in endurance sports nutrition, because that's roughly where both transporters can work at close to full capacity without either one being overwhelmed. 

Whole Foods vs. Sports Nutrition Products

Whole foods play an important part in any athlete's fueling plan, especially before and after training. Bananas, dates, honey, and maple syrup all contain mixed carbohydrate profiles. Honey and maple syrup sit at roughly a 1:1 glucose-to-fructose ratio, which makes them useful as mid-exercise fuel. 

Research comparing honey to commercial sports nutrition products during steady-state cycling found no significant differences in carbohydrate oxidation rates or endurance capacity, which is meaningful for athletes who prefer a more natural option.

A few things to keep in mind before stuffing your jersey with bananas, though:

Portability matters when you're on a bike for four hours or running a marathon. A banana works at a relaxed café stop; it's harder to manage at race pace.

Fiber content in foods like dates slows gastric emptying, which can be useful before training but could turn problematic during exercise.

Osmolality affects how quickly a product clears your stomach. Commercial energy gels vary enormously in osmolality, ranging from 303 to over 10,000 mmol/kg. High osmolality draws water into the gut rather than promoting emptying, which is why even within sports nutrition products, not all gels are the same from a GI standpoint.

Precision dosing matters when you're targeting 90g+ per hour. Whole foods make consistent, measurable intake harder to achieve.

Well-designed sports nutrition products are engineered around absorption pathways, carbohydrate ratios, osmolality, and gastric emptying rates, because all of those things determine how much of what you consume actually reaches your muscles. That matters most in long events, high heat, or any strategy targeting the upper range of hourly carbohydrate intake.

 

Most GI problems during racing aren't caused by carbohydrates themselves. They're caused by exceeding what a stressed gut can absorb. FODMAPs, which are fermentable carbohydrates found in many common foods, can draw water into the intestine and ferment in the gut. Exercise reduces GI blood flow, making the gut more sensitive to the same carbohydrates it handles fine at rest.

The same mechanism applies when a single transporter saturates. Carbohydrates accumulate in the intestine, water follows osmotically, absorption slows, and a fueling plan becomes a real GI problem. 

More carbohydrates consumed is not always more energy delivered.

How to Actually Approach Your Fueling Strategy

Train your gut: Repeatedly exposing your gut to carbohydrates during exercise improves its capacity to absorb and tolerate higher intakes. Studies have shown meaningful adaptation in as little as two weeks of consistent carbohydrate feeding during training. Race day is not the time to find your ceiling.

Build gradually: If you're currently at 40–50g/hr, don't jump to 90g. Work up across training sessions in 15–20g increments and track how your gut responds. The ceiling is trainable, but not overnight.

Match intake to duration: Sessions under 60–75 minutes generally don't require much beyond water. Beyond 90 minutes, and certainly past two hours, the case for higher hourly carbohydrate intake and multiple carbohydrate sources strengthens considerably.

Combine carbohydrate sources: If you're targeting higher intakes, you need both transporters working. Look for products combining glucose or maltodextrin with fructose in roughly a 2:1 ratio.

Start early, stay ahead: Glycogen depletion is much harder to reverse mid-race than it is to prevent. Begin fueling earlier than feels necessary, at lower doses, and maintain it.

Hydration supports absorption: Concentrated gels in particular require adequate fluid to clear the stomach and minimize osmolality-related GI stress. Carbs and fluids work together, not independently.

Products like Beet It Sport Top Up 100 are built around these principles, combining glucose and fructose in a 2:1 ratio to support efficient absorption and deliver usable energy during hard training and racing.