The Sugar-Fueled Illusion of Performance
Picture this: You're 60 miles into a 100-mile ultramarathon. You've been dutifully sipping your sports drink every 15 minutes, just like the conventional wisdom suggests. Yet your stomach is churning, your energy is crashing, and you're questioning every life decision that led you to this moment on a remote mountain trail.
Ultra runners work so hard to avoid GI distress during races but something hiding in that brightly colored bottle you've been sipping from for hours is working against you.
There is a hard ceiling on how much carbohydrate you can absorb per hour, and in the last few years it moved. Upward, and further than most fuelling advice has caught up with. That story, and the gut training that makes the higher numbers possible, is the subject of the carbohydrate revolution in ultramarathon nutrition. What follows here assumes you have picked a number and want to know what to put in the bottle to hit it.
The Gastric Emptying Bottleneck
The first barrier ultra runners encounter with simple sugars is gastric emptying. Research on ingested carbohydrate regulation demonstrates that high-osmolality solutions (characteristic of sugar-heavy sports drinks) empty from the stomach more slowly than lower-osmolality alternatives.
This delayed gastric emptying creates several cascading problems:
- Gastrointestinal distress: Liquid sloshing in your stomach for hours.
- Delayed energy availability: Fuel sitting in your stomach instead of reaching your muscles.
- Nausea and vomiting: The infamous "ultra runner's stomach" that can end your race.
- Reduced hydration: When your stomach won't empty, you can't hydrate effectively.
This is the practical reason the source of your carbohydrate matters as much as the amount. You can be eating the right number of grams per hour and still be getting almost none of it, because it is sitting in your stomach.
Simple vs. Complex Carbohydrates: Understanding the Difference
Not all carbohydrates are created equal, especially when you're asking your body to function for 20-40 hours straight.
Simple Carbohydrates (The Problem)
What they are: Monosaccharides and disaccharides: glucose, fructose, sucrose, found in sports drinks and candy.
How they behave:
- Rapidly absorbed (when tolerated).
- Cause sharp blood sugar spikes followed by crashes.
- High osmolality leads to GI distress.
- Provide quick but unsustainable energy.
- Can overwhelm intestinal glucose transporters.
Complex Carbohydrates (The Solution)
What they are: Polysaccharides found in whole foods like potatoes, sweet potatoes, rice, and maltodextrins.
How they behave:
- More gradual digestion and absorption.
- Stable, sustained energy release.
- Lower osmolality reduces GI distress.
- Better tolerated during ultra-distance events.
- Provide accompanying nutrients (electrolytes, vitamins).
The Potato Revolution in Ultra Running
Ask any experienced ultra runner about their fueling strategy, and many will enthusiastically tell you about boiled potatoes. This isn't just ultra-culture folklore, there's solid science behind it!
Potatoes offer several advantages over sports drinks:
- Complex Carbohydrate Structure: The starch in potatoes (amylopectin) is oxidized at high rates similar to glucose, according to research on carbohydrate feeding strategies, but with better GI tolerance.
- Natural Electrolyte Content: Potatoes are rich in potassium (421mg per medium potato), magnesium, and sodium when salted, exactly what you're losing through sweat.
- Psychological Satisfaction: Real food provides a mental boost that sweet sports drinks cannot match at 3 AM on a mountain trail.
- Lower Osmolality: Easier on the stomach, reducing the risk of the GI issues that force many runners to DNF.
The Blood Sugar Rollercoaster
One of the most insidious effects of relying on simple sugars is the blood glucose volatility they create. When you consume high amounts of glucose or sucrose:
- Blood sugar spikes rapidly.
- Insulin surges to manage the spike.
- Blood sugar crashes below baseline.
- You experience bonking, fatigue, and cognitive impairment.
- You reach for more simple sugars to compensate.
This cycle becomes increasingly problematic as races extend beyond the first couple of hours. The metabolic stress compounds, your insulin sensitivity decreases, and your body becomes less efficient at utilizing the very fuel you're consuming.
How Much, and From What
Start with the target, then work out what delivers it. Current race-day guidance runs roughly like this:
- Pre-race (1-4 hours before): 1-4g of carbohydrate per kg of body weight
- Races under 81km: 30-50g per hour
- Races over 81km: 60-90g per hour
- Key stages of a race, for well-trained athletes: up to 120g per hour
Those upper numbers are only reachable if your gut has been trained for them, and only if you are combining glucose and fructose sources at roughly a 2:1 ratio so you are not queueing everything through a single set of intestinal transporters. Both of those are covered properly in the research piece. Do not read 120g/h here and try it on Saturday.
The practical question is what those grams arrive in. Four broad options, and most runners end up using three of them in the same race:
- Engineered products: gels at 20-30g a serving, chews, bars. Precise, portable, and monotonous after eight hours.
- Drink mixes: convenient, but this is where the sports-drink problem lives. See below.
- Real food: lower osmolality, better tolerated late, and the only category that still appeals at 3am.
- Broth and savoury: not much carbohydrate, but it keeps you eating when nothing sweet will go down.
Practical Fueling Recommendations for Ultra Runners
Replace sports drinks with electrolyte solutions!
Modern electrolyte supplements provide the minerals you need without the sugar overload: Hammer Nutrition Fizz, or Endurolytes are well balanced electrolyte-only formulations without the high sugar content.
Separating your hydration from your calories is the single highest-value change most runners can make. It lets you drink to thirst and eat to a schedule, instead of coupling the two and being forced to over-drink to hit a carbohydrate target.
On sodium, aim for roughly 500-700mg per litre of fluid, or 300-600mg per hour, and increase both fluid and sodium in the heat.
Want to control your carbohydrate/electrolyte mix even more? Try these homemade solutions.
Mix 1/4 tsp salt, 1/4 tsp lite salt (potassium), and a squeeze of lemon in 20oz water.
Bone broth: Warm, salty, and comforting during night sections
Embrace Real Food Fueling
Build your nutrition plan around whole food sources:
Boiled Potatoes:
- Prepare small cubes, lightly salted
- Easy to carry in ziplock bags
- Consume 1-2 cubes every 30-45 minutes
Sweet Potatoes:
- Higher in vitamins A and C
- Naturally sweet without added sugar
- Excellent for cooler weather races
Rice Balls (Onigiri):
- Sticky rice formed into palm-sized portions
- Can wrap in nori for additional minerals
Bananas:
- Natural package, easy to carry
- Potassium-rich
- Fruits are a mixture of simple and complex carbs
Strategic Use of Maltodextrin-Based Products
If you prefer engineered nutrition, choose products based on maltodextrins rather than simple sugars. Research shows maltodextrins are oxidized at rates similar to glucose but with better gastrointestinal tolerance.
Whatever you choose, choose it in training. The rule that matters more than any product recommendation on this page: nothing goes in your vest on race day that has not already been through a four-hour training run.
The Bottom Line: Ditch the Gatorade!
Research reveals that relying on simple sugars from sports drinks is a suboptimal fueling strategy for ultramarathons. The science of carbohydrate oxidation points toward a superior approach of separating your hydration from your calories. Use electrolyte-only solutions for hydration and consume real food or complex carbohydrates for energy. This will provide you with more stable energy while reducing GI distress.
Your next 100-miler doesn't have to involve a stomach full of sloshing Gatorade and the inevitable bonk at mile 70. Try boiled potatoes, electrolyte water, and the occasional banana instead. Your gut, and your finish time, will thank you. Go get that buckle!
Research Cited
- Longer-Term Effects of the Glycaemic Index on Substrate Metabolism and Performance in Endurance Athletes.
- Effect of Sugar- and Polyphenol-Rich, Diluted Cloudy Apple Juice on the Intestinal Barrier after Moderate Endurance Exercise and in Ultra-Marathon Runners
- Glycaemic Impact of Low- and High-Glycaemic Index Carbohydrate Diets in Ultra-Endurance Athletes: Insights From Continuous Glucose Monitoring
- Effect of the glycaemic index of pre-exercise carbohydrate meals on running performance
