Endurance athletes should consume 60-90g of carbohydrate per hour during races over 90 minutes, scaling to 90-120g/hr for gut-trained athletes in events over 4 hours. The 2:1 glucose-to-fructose ratio maximizes absorption by using two separate gut transporters (SGLT1 and GLUT5) in parallel. The plan also covers hourly sodium, fluid, and a per-hour food menu blending gels, drink mix, and real food.
Inputs map to published research on glucose-fructose co-ingestion (Jeukendrup 2010, 2014) and sport-specific GI tolerance (Burke et al. 2019). Results are a calibrated starting point, not a medical prescription. Practice the plan on 4 to 6 long training sessions before race day to confirm GI tolerance.
See your glycogen tank, your bonk time, and your exact hourly carb, sodium, and fluid targets. Three concrete food menus calibrated to your weight, gut training, and the weather.
Not Medical Or Nutrition Advice
This calculator and the resulting plan are educational only. Endurance sports carry inherent risks and individual nutrition needs vary. Athletes should consult a qualified healthcare professional or a registered sports dietitian before applying any nutrition, hydration, or supplementation strategy, especially if you have a medical condition, are pregnant, take medications, or are training through injury. Use this information at your own risk.
From The Founder
Story: The Seasonal Reset
Every winter, my routine is a steady grind. I spend my days clocking 1 to 2 hours on the indoor trainer, supplemented by weight training and the occasional run. But there is a significant difference between staying active and staying “road ready.”
As the daylight hours stretch and the weather warms, I finally move my rides back outside. Because my deep endurance has usually suffered over the cold months, those first 3 or 4 rides are a brutal awakening. My endurance capacity is not there yet.
I find myself suffering more than usual. My fitness economy has not reached optimal levels, meaning my body is less efficient and requires more calories than it does during my peak summer or fall fitness. Sometimes, I even bonk.
I have learned to be patient with my body during this phase. Rebuilding that aerobic base is a process that cannot be rushed. To help you avoid the same fueling traps I fall into every spring, use this calculator to better determine your specific carb needs.
Cameron Hoffman
Founder · Beetroot Pro® / Endurance360®
The Science
Why Fueling Math Matters
The glycogen tank is smaller than you think
Trained, carb-loaded athletes carry roughly 500 to 600 grams of glycogen across muscle and liver. That is about 2,000 to 2,400 kilocalories of stored fuel. At marathon race pace a 70 kg runner burns 700 to 900 kcal per hour, with carbs supplying 60 to 70 percent of that energy. The math is unforgiving: without intra-race carbs you run out of stored fuel somewhere between hour 2 and hour 3. That is the wall.
This is also why marathoners famously hit the wall around mile 20. It is not a mental thing. It is a fuel-tank-empty thing.
The 60 g/hour absorption ceiling and how to break it
Your gut absorbs glucose through one transporter (SGLT1) that maxes out at roughly 60 grams per hour. Past that point, anything more sits in your stomach causing nausea, bloating, and cramps. This is why many athletes get sick when they switch from one gel an hour to two.
The way around the ceiling is to use a second transporter, GLUT5, which absorbs fructose. With a 2:1 mix of glucose and fructose, the gut can move 90 to 120 grams per hour cleanly. Most quality sports gels and drink mixes use this ratio. Honey, maple syrup, and many fruit-based foods are naturally close to it. Pure glucose tablets and pure white sugar are not.
If your fueling plan calls for more than 60 g/hr, verify that your sources are dual-source. This single check prevents most race-day GI disasters.
Train your gut like you train your legs
Gut absorption is trainable. Athletes who practice fueling at 90 g/hr during long sessions can hit those numbers on race day with no GI issues. Athletes who never practice past 30 g/hr cannot suddenly run 90 g/hr in a marathon, regardless of what their calculator says.
Practical protocol: take your race-day target into your two longest training sessions per week for at least 6 to 8 weeks before the event. Train at the rate. Use the exact products you will race with. The phrase "never try anything new on race day" applies as much to fueling as it does to shoes.
Heat and the absorption paradox
In hot weather, blood that would normally service your gut gets redirected to your skin for cooling. This reduces gut absorption capacity by roughly 10 to 20 percent. Counterintuitively, this means in extreme heat you should slightly lower your carb target while raising sodium and fluid.
Most calculators get this wrong because they assume "more sweating = need more of everything." More sodium and more fluid: yes. More carbs: no. Push the carbs and you stack the gut, lose blood flow, and start a downward spiral that ends with you walking the last 6 miles.
Where Beetroot Pro® fits
Dietary nitrate raises plasma nitrite and nitric oxide, which improves the efficiency of mitochondrial respiration. Translated: at the same pace, you use slightly less oxygen per unit of work. The AIS Sports Supplement Framework classifies dietary nitrate as Group A, the highest evidence tier.
For race day, plasma nitrate peaks 2 to 3 hours after intake. Take your final Beetroot Pro® dose with breakfast, 2.5 hours before the gun. For best results, load daily for 3 to 6 days before the event so your tissue stores are saturated.
Fueling and nitrate work on different problems. Fueling supplies the kilocalories you burn. Nitrate makes those kilocalories go further. The two are additive, not redundant.
Where Endurance360® fits
Carb fueling is acute, race-day math. Endurance360® works on the chronic side: it builds the aerobic capacity and recovery durability that determine how fast you can hold a pace in the first place. The blend includes clinically dosed creatine for high-end power, beta-alanine for buffering capacity, and adaptogens (rhodiola, cordyceps) for stress tolerance during heavy training blocks.
Beta-alanine and creatine both require multi-week loading to take effect, so daily intake during your training block is what unlocks race-day benefits. Beetroot Pro® then handles the acute oxygen-efficiency layer in the final week.
A trained athlete fueling correctly without a built aerobic base is still going to bonk. A built aerobic base without race-day fueling still hits the wall. Use the calculator above for the fueling math, and stack Endurance360® across your training block so the engine is actually capable of finishing the race you are fueling for.
Food Reference
No brand names. Use these carb values to mix and match a menu that hits your hourly target. Test any new food in training before race day.
Format
Food
Carbs
Notes
Gel/Liquid
Energy gel (generic)
~25g
Dual-source if quality brand
Gel/Liquid
High-carb drink mix
~40g/serving
Best dual-source delivery
Gel/Liquid
Maple syrup (1 tbsp)
~20g
Vegan, ~1.5:1 ratio, natural
Gel/Liquid
Honey (1 tbsp)
~17g
~1:1 glucose/fructose
Gel/Liquid
Coca-Cola (12 oz)
~39g
Late-race rescue, caffeine bonus
Real Food
PB and J quarter (white bread)
~15g
Fat slows absorption, save for hour 3+
Real Food
Banana (medium)
~25g
Potassium, gut friendly
Real Food
Banana (half)
~13g
Easier to handle mid-stride
Real Food
Medjool date
~15g
Vegan, dense, natural
Real Food
Fig bar
~20g
Slower release, real food feel
Real Food
Gummy candy (handful)
~30g
Cheap, often dual-source
Savory
Salted boiled baby potato
~15g each
Ultra running staple, 200+ mg sodium
Savory
White rice ball with salt
~30g
Cycling and ultra favorite
Savory
Pretzels (small handful)
~25g
~400 mg sodium per handful
Savory
Salty bouillon broth (1 cup)
~2g
Stomach reset, sodium boost
Savory
Pickle juice (1 oz)
~0g
Cramp protocol, sodium
The Complete Stack
Build The Engine. Then Fuel It.
Endurance360® builds the chronic capacity through your training block. Beetroot Pro® delivers the race-day oxygen efficiency. Stack both, fuel right, and the wall stops being a thing that happens to you.
This calculator synthesizes peer-reviewed sports nutrition research. Endurance sports carry inherent risks. Consult your physician or a registered sports dietitian before significantly altering your diet, training, or supplement intake.
Jeukendrup, A. E. (2014) · A step toward personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine 44, S25 to S33.
Stellingwerff, T., Cox, G. R. (2014) · Systematic review: carbohydrate supplementation on exercise performance or capacity of varying durations. Applied Physiology, Nutrition, and Metabolism 39(9), 998 to 1011.
Jones, A. M. (Exeter) · Plasma nitrate and nitrite kinetics, beetroot loading protocols, oxygen efficiency mechanisms.
Race Day Fueling: Common Questions
The carbohydrate sweet spot for endurance racing is 60 to 90g per hour for most athletes, with gut-trained athletes pushing to 120g per hour. Combine glucose (maltodextrin) and fructose at a 2:1 or 1:0.8 ratio to clear the single-transporter ceiling. Train your gut over 4 to 6 long sessions before race day, and dial the target back 10 to 15% in hot conditions.
How many carbs do I need per hour during an endurance race?
The published range is 60 to 120 grams of carbohydrate per hour, scaling with event duration and gut training. Most athletes target 60g per hour for events under 2.5 hours, 80 to 90g per hour for marathons and half-Ironman, and 90 to 120g per hour for full Ironman, ultra, and gran fondo events. The calculator returns your number based on body weight, sport, and gut training status.
What is the 2:1 glucose-to-fructose ratio and why does it matter?
Glucose absorbs through the SGLT1 gut transporter, which saturates at roughly 60g per hour. Fructose absorbs through GLUT5, a completely separate transporter. Running both in parallel raises the total absorption ceiling to ~90g per hour at 2:1 glucose:fructose. The 1:0.8 ratio (used by SiS Beta Fuel and Maurten 320) pushes the ceiling to ~120g per hour for athletes with trained guts.
How do I avoid GI distress at high carb intakes?
GI tolerance is trainable. Start at 60g per hour and add 10g per hour every 2 to 3 weeks of consistent practice. Stay isotonic (6 to 8% carb concentration in your bottle). Use multiple transportable carbs (glucose plus fructose, not glucose alone). Practice race-day fueling on at least 4 to 6 long training sessions before the event. Avoid high-fiber foods in the 24 hours before racing.
Should I carb load before a marathon or long race?
Yes for events over 90 minutes. The protocol is 8 to 12 grams of carbohydrate per kg body weight in the 36 to 48 hours before the race. For a 70kg athlete, that is 560 to 840g of carbs daily. This saturates muscle and liver glycogen stores. The calculator returns your specific carb-loading target alongside the intra-race fueling plan.
Can I use real food instead of gels for fueling?
Yes, for events where you have time to chew and digest. Bananas (~25g carbs), rice cakes with jam (~30g), Honey Stinger waffles (~21g), and dates (~17g each) all work well. Most athletes blend real food and gels: real food early when GI capacity is high, gels late when chewing becomes difficult. The tool includes a real-food menu alongside the gel and drink-mix plan.
How does carb intake change in hot weather?
Hot conditions reduce GI tolerance, so the practical upper limit drops. Most athletes can sustain 60 to 75g per hour in 80F+ heat versus 80 to 90g per hour in temperate conditions. Dial back the carb target by 10 to 15% in hot races and prioritize sodium and fluid replacement. The calculator scales the recommendation by your input temperature.
