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Leading research generally shows that about 90 grams of carbs per hour can be consumed during endurance sports, and a 2:1 ratio of glucose to fructose is optimal. But, it turns out, that might not always be the case. We know now that, at times, a high-carb fueling plan of up to 90-140 grams of carbs per hour might be beneficial—increasing performance and delaying fatigue—but it must be done following a few guidelines or you risk GI distress and the inability to absorb those carbs. When does a high-carb strategy make sense, though, and how do you implement it?
When To Use High-Carb Fueling
When you should consume more than 100g of carbs per hour comes down to a few questions:
- Do you burn enough carbs and calories per hour to merit it?
- Is your race in a hot/humid environment where hydration may become a stronger nutritional bottleneck than carb intake rates?
- How long is your race?
When it comes to calorie burn, the answer is probably yes. If you average more than about 123 watts you can likely benefit from more than 100g of carbs per hour during training or racing. Holding 123 watts for one hour burns about 440 kilocalories, or roughly the equivalent energy of 100 grams of carbohydrate. If you average over 160 watts, then you may be a candidate for considering as much as 140 or 150 grams of carbs per hour during training and racing.
The issue underlying the climate considerations is the likelihood of dehydration. Dehydration causes decreased performance, reduced cardiovascular capacity, and hinders gut absorption. Unfortunately, very-high carb consumption requires slightly more concentrated (hypertonic) beverages, and hypertonic beverages slightly slow fluid absorption, only furthering dehydration.
That means that when racing in a hot or humid environment, sweat loss may exceed fluid and electrolyte replacement ability. When racing or training in anything over 80 degrees or 75% humidity, you should stick to carb consumption of 90-100g per hour rather than higher.
The last factor is race length. If the race is more than seven hours long, you will probably be unable to sufficiently replace lost fluids by the end of the event. Dehydration greater than 2% of body weight is then likely and with it impaired cardiovascular performance and gut absorption, exacerbating the dehydration and fueling problems outlined above.
For events over 12 hours, hydration status becomes a strong performance-limiting factor. Fueling in these events is a precarious balance between fluid and sodium intake and getting sufficient carbohydrates. Sticking with 60-90g per hour is probably the safe bet. Push carb consumption as high as possible without limiting fluid absorption due to beverage hypertonicity.
On the other end of the spectrum, for events under two hours you can likely stick with even lower carb consumption, 30-90g per hour is sufficient to meet energy needs. Increasing carb consumption is unlikely to make you faster.
All of that means that targeting more than 100 grams of carbs per hour is best reserved for events two to six hours long.
How Do You Use High-Carb Fueling?
Before you target a very-high-carb fueling strategy, you want to be sure to manage these issues:
- Be hydrated. If you have a 2% or greater reduction in body weight due to water loss, you are asking for gut issues and reduced performance. Using a rehearsed pre-race fluid and sodium consumption protocol to ensure optimized hydration allows gut absorption rates to begin high, and so then hydration does not become a higher priority than carb consumption rate.
- Practice high-carb fueling in training before using it in racing. Preferably try very-high-carb over shorter durations as training for your gut, both for logistical purposes and to enhance gut carb absorption ability.
- Consume a high-carb diet. Without sufficient carb consumption, carb absorption in the gut is limited, and ability to burn carbs effectively in the muscle becomes impaired.
When using high-carb fueling, follow these instructions on the day of:
- Start immediately. Your warm-up should include the high-carb strategy.
- Keep your stomach as full as comfortable. The fuller your stomach is, the faster the gastric emptying rate. Consume frequently; recency of consumption also speeds gastric emptying.
- Maintain hydration status at all costs. Sodium is needed for hydration maintenance, otherwise you might end up with hyponatremia (diluted sodium concentration in blood, decreased performance, and health risks!). Consuming one liter of fluid per hour is usually tolerated well and is necessary to allow for maximum carb consumption. Packing in 140 grams of carbs, even in one liter of fluid, makes for a concentration of 14%! A concentration of 8-12% is probably more optimal when well-hydrated; 6-8% is better when lesser hydrated.
- Include sodium with your fluids and carbs to maximize gut absorption. Sugar is best absorbed in the presence of sodium: 500-1000mg of sodium per hour and per liter of fluid. Reducing beverage osmolarity by using sodium citrate is good for gut comfort.
- The more you sweat, or the less you drink (the more dehydration progresses), the more you need to back off a high hourly carb rate.
One key to keep in mind: A 2:1 glucose to fructose ratio is the oft-cited gold standard approach for maximizing carb utilization in training. But closer to a 1:1 ratio is probably better. Fructose consumed in isolation causes gut discomfort above 30 grams per hour. Glucose or glucose polymers like maltodextrin consumed in isolation tends to cause discomfort above 60 grams per hr.
In combination with fructose, glucose-related problems persist—70 grams of glucose per hour might be tolerated in a well-trained gut while using at least a 1:1 ratio of fructose to glucose. In combination with glucose, fructose-related problems vanish, and more than 70 grams per hour of fructose is commonly well-tolerated.
In summary, if you’re looking to test a high-carb fueling strategy and see any potential benefits from additional carbs: be hydrated, consume lots of carbs in and out of training, start fueling immediately, stay full, target upper limits of fluid and electrolyte consumption, and use a 1:1 ratio of glucose to fructose.
Dr. Alex Harrison, a certified USA Triathlon coach, holds a PhD in Sport Physiology and Performance. He is the author of The RP Diet for Endurance and more than a dozen articles. When he isn’t pumping out training and nutrition plans in his RV-garage-turned-mobile-office, he can be found on his bike, clinging for dear life to his wife’s wheel.