This week our question goes to Namrita O'Dea. A licensed dietitian and member of the Topeak/Ergon racing team, O'Dea helps riders fine tune their bodies for maximum performance through her business 55 Nine Performance Nutrition.

Question: I did a couple of 100-milers last year and had trouble with bonking. Someone told me that in a race that long you need to change the kind of sugars you are ingesting because your body slows processing it if you stick to only one. Basically, you could still be eating but your stomach has shut off. Is that true? And what do you recommend for feeding during a long endurance MTB race.

Carbohydrates, electrolytes (mainly sodium), and fluid are the three most important things to replace during an endurance race. The amount and the timing of intake are important to ensure that you are digesting, absorbing, and delivering nutrients in a timely manner, without any distress on your gastrointestinal (GI) tract.

If you are using a sports drink, it should be around 6-8% carbohydrate (approximately 30-60 grams per hour, or 120-240 calories per hour). Going above 8% carbohydrate may slow the emptying of fluids from your stomach (gastric emptying). Drinks of >10% carbohydrate can cause symptoms of dehydration and GI distress. Any of these can cause the feeling of your stomach “shutting down.”

Glucose, sucrose, and complex carbohydrates (glucose polymers) are all effective, but a blend of glucose and fructose may be better utilized during exercise. Studies have shown 20-50% higher carbohydrate oxidation rates when a mixture of glucose-fructose is used, compared to glucose alone. You will find that most sports nutrition products are formulated with a blend of carbohydrates. Fructose should not be the predominant carbohydrate, though, as it can also cause GI distress.

Generally, the goal is to replace as much fluid as you can tolerate without any GI discomfort, up to the rate of your sweat loss. Rate of sweat loss can be determined by checking pre- and post- exercise body weight. One pound of weight loss = 16 oz of fluid.

There still isn’t any specific recommendation on sodium replacement as needs likely vary between individuals. So, it’s a good idea to experiment with this during training. Some people lose more salt in their sweat than others do, but it is not yet clear if these individuals need to replace more sodium to compensate for their increased losses.

During a long race, intake should be steady, every 15 to 20 minutes, to maintain nutrition without overloading your digestive system. Most people do well with 200-300 calories per hour. And, when possible, your race-specific nutrition strategy should be developed while training in similar conditions. Just as you train with specificity, you want to develop your nutrition plan with specificity.

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