Glucose, Fructose, and Maltodextrin: Why the Type of Carb in Your Gel Matters More Than You Think

You have heard the number. Sixty grams of carbs per hour used to be the ceiling. Now elite marathoners are pushing 90, even 120. You have done the maths on your gel schedule, trained your gut on long runs, and you are still hitting a wall somewhere past mile 18.

The problem might not be how much you are taking. It might be what is in the packet.

Most runners obsess over total grams per hour but never flip the gel wrapper to read which carbohydrates are actually inside. That ingredient list is not decoration. It determines the biological ceiling of how much fuel your gut can absorb — and if you are using the wrong type, you are leaving performance on the course no matter how diligently you follow a plan.

Your gut has two doors, not one

Carbohydrate absorption in the small intestine happens through specific transport proteins embedded in the intestinal wall. Think of them as doors. Each type of door only opens for certain molecules.

SGLT1 is the primary transporter for glucose and maltodextrin (which is just a chain of glucose molecules that breaks apart quickly). It can move roughly 60 grams of glucose into your bloodstream per hour. That is a hard biological limit — you cannot force more through by taking more gels. Once SGLT1 is saturated, the excess glucose sits in your gut, pulls water in by osmosis, and creates the bloating, cramping, and nausea that every runner dreads.

GLUT5 is a separate transporter that handles fructose. It operates independently of SGLT1 and can move an additional 30 to 40 grams per hour.

Here is the key: these two systems run in parallel. When you take a gel that contains both glucose and fructose, you are using both doors at once. Instead of 60g per hour through one transporter, you can absorb 90 to 100g per hour through two.

This is called dual-transport carbohydrate delivery, and it is the mechanism behind every headline you have read about elite athletes fuelling at 90g or more per hour. It is not willpower or gut toughness. It is biochemistry.

The ratio that unlocks the ceiling

Not every combination of glucose and fructose works equally well. Research consistently shows that the optimal ratio sits between 2:1 and 1:0.8 glucose-to-fructose. That means for every 2 grams of glucose (or maltodextrin), you want roughly 1 gram of fructose.

At this ratio, both transporters run near their maximum capacity without overwhelming either one. Go too heavy on fructose and you risk osmotic diarrhea — GLUT5 has a lower throughput than SGLT1, so excess fructose ferments in the gut. Go too heavy on glucose and you are back to the single-transporter ceiling.

Here is what the ratios look like in practice for a 90g per hour target:

| Ratio | Glucose/Maltodextrin | Fructose | Total | Notes | |-------|---------------------|----------|-------|-------| | 2:1 | 60g | 30g | 90g | Well-studied, widely available | | 1:0.8 | 50g | 40g | 90g | Newer research, used by some elite teams | | 1:1 | 45g | 45g | 90g | Higher fructose load — some athletes tolerate it, some do not |

The 2:1 ratio has the deepest evidence base and is the safest starting point. If you tolerate it well and want to experiment, some recent studies suggest that shifting slightly toward 1:0.8 may improve oxidation rates further — but the marginal gain is small, and the risk of GI distress from higher fructose rises.

Where maltodextrin fits in

You will see maltodextrin on more gel labels than glucose. That is not a problem — maltodextrin is a polysaccharide made of glucose chains that break apart rapidly in your gut. For the purpose of intestinal absorption, maltodextrin behaves almost identically to glucose. It uses the same SGLT1 transporter.

The advantage of maltodextrin over pure glucose is osmolality. Maltodextrin is a larger molecule, so it exerts less osmotic pressure in solution. Translation: a gel made with maltodextrin is gentler on your stomach than one made with the same weight of pure glucose. This is why most well-formulated endurance gels use maltodextrin as the primary carb source rather than dextrose or glucose syrup.

When you read a gel label, treat maltodextrin and glucose as interchangeable for the purpose of calculating your glucose-to-fructose ratio.

How to read your gel label in 30 seconds

Flip your gel packet over. Look at the ingredients list and the nutrition panel.

Step 1: Identify the carb sources. Look for: maltodextrin, glucose, dextrose, glucose syrup (these all count as "glucose-side"). Then look for: fructose, fruit juice concentrate, crystalline fructose (these count as "fructose-side"). Some gels also use sucrose, which is a 50/50 split of glucose and fructose — count half toward each side.

Step 2: Check total carbs per serving. Most gels deliver 20 to 30 grams per packet.

Step 3: Estimate the ratio. If the ingredients list starts with maltodextrin and fructose is second, you likely have a dual-transport gel with roughly 2:1 ratio. If the list is maltodextrin only (or maltodextrin plus glucose), you have a single-transport gel.

Step 4: Do the hourly maths. If your target is 90g per hour and your gel delivers 25g, you need roughly 3.5 gels per hour — about one every 17 minutes. But this only works if those gels contain both glucose and fructose. Three single-transport gels give you 75g of glucose — and your gut can only absorb 60g of it. The remaining 15g becomes the reason you feel awful at mile 20.

The gel audit: what is actually in the popular options

Here is a simplified breakdown of common endurance gels by carb composition:

Dual-transport (glucose + fructose):

• Maurten Gel 100: 25g carbs, maltodextrin + fructose, roughly 2:1 ratio
• SiS Beta Fuel: 40g carbs, maltodextrin + fructose, 1:0.8 ratio
• Precision Fuel & Hydration PF 30: 30g carbs, maltodextrin + fructose

Single-transport (glucose/maltodextrin only):

• GU Energy Gel (original): 22g carbs, primarily maltodextrin
• Hammer Gel: 21g carbs, primarily maltodextrin
• Clif Shot: 24g carbs, primarily maltodextrin + brown rice syrup

Mixed or variable:

• Science in Sport GO Isotonic: 22g carbs, maltodextrin + small amount of fructose
• Spring Energy: varies by flavour, some use whole food carb sources

If you are currently using a single-transport gel and trying to hit 90g per hour, you have found your problem. Switching to a dual-transport option — or combining a glucose-based gel with a fructose-containing drink mix — opens the second transporter and removes the absorption bottleneck.

Why gut training still matters

Understanding carb types does not replace gut training — it complements it. Even with the right dual-transport ratio, your gut needs practice handling high carb loads under the stress of exercise.

A landmark study from Monash University found that just two weeks of structured gut training — consuming 90g of carbohydrate during daily one-hour runs — reduced gastrointestinal symptoms by 60 to 63 percent. The mechanism: repeated carbohydrate exposure upregulates the production of both SGLT1 and GLUT5 transporters. More doors open.

The practical takeaway: start gut training with dual-transport gels during your long runs at least four to six weeks before race day. Begin at a tolerable dose — maybe 60g per hour — and increase by 10g per week until you reach your target. If you start with the right carb composition from day one, you are training both transporters simultaneously.

Sweatr calculates your personal carb ceiling based on your body weight, training load, and wearable data, then matches it to products in the Sweatr library with the right carb composition. No spreadsheet. No label reading. Just a plan that fits your body.

Common mistakes with gel carb types

Mistake 1: Drinking a glucose sports drink with a glucose gel. If your gel is maltodextrin-only and your drink mix is also glucose-based, you are double-loading one transporter. Switch one to a fructose-containing option or use plain water with a dual-transport gel.

Mistake 2: Assuming "natural" gels are better for absorption. Whole-food gels made with dates, honey, or rice syrup can be gentler on the stomach for some athletes, but their carb composition is almost always glucose-dominant. You still need a fructose source alongside them.

Mistake 3: Chasing 120g per hour without the ratio. The athletes hitting 120g per hour in elite fields are using precisely calibrated dual-transport formulas, have months of gut training, and are running at paces that maintain adequate gut blood flow. Copying their gram target without their carb composition is a recipe for GI disaster.

Mistake 4: Ignoring concentration. Even with the right carb type, taking a gel without water creates a hypertonic bolus in your stomach. Always chase a gel with at least a few mouthfuls of water to dilute it and speed gastric emptying.

Putting it together: your carb type checklist

Before your next long run, answer these four questions:

1. What is your hourly carb target? If you do not know, start with 60g per hour for a half marathon and 60 to 90g per hour for a full marathon. Adjust based on your body weight and pace.

2. Are you using dual-transport gels? Check the label for both maltodextrin (or glucose) AND fructose. If fructose is not listed, you are capped at 60g per hour of absorbed carbs.

3. What are you drinking alongside them? If your gel has fructose, use plain water or a low-carb electrolyte drink. If your gel is glucose-only, consider a fructose-containing drink mix to open the second transporter.

4. Have you practised at race intensity? Gut training only counts if you do it at the pace and conditions you will race in. A gel that sits fine at easy pace may cause problems at threshold effort because blood flow to the gut drops as intensity rises.

Sweatr builds your race-day fueling plan from your Apple Watch and Garmin data — including the carb type and product recommendations that match your individual target. No guesswork, no label-reading, no surprises at mile 20. Download Sweatr and let your data decide.