The bike-run bonk is a simple case of over-nourishment with a twist. The twist is that the stomach is able to tolerate a greater volume and concentration of nutrition, and is also able to empty more quickly, when an athlete is bicycling than when that same athlete is running. So what qualifies as optimal nourishment during the bike leg of a triathlon suddenly becomes over-nourishment on the run.
In addition to that unpleasant sloshy feeling (or even full-blown nausea), a secondary problem results: inadequate supply of fluid and energy to your blood and muscles, which can quickly result in a classic energy bonk. Isn't that ironic?
You crammed all that nutrition down your throat on the bike to prevent dehydration and glycogen depletion and it winds up causing these very things—in addition to gastrointestinal distress.
The way to avoid the bike-run bonk is to fuel yourself during the final 30 minutes of the bike leg in a way that anticipates the reduced capacities of your stomach on the run. Here are five specific tips to help you avoid the bike-run bonk.
Go Light
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Throughout the majority of the bike leg, take full advantage of the opportunity to take in fluid and energy at a high rate. A typical cyclist can absorb 1.2 to 1.5 liters of fluid and 80 to 100 grams of carbohydrate per hour at race intensity. You can also tolerate a fairly full stomach on the bike, and it's a good idea to keep your stomach as full as you comfortably can by taking in nutrition frequently, because the fuller your stomach is, the faster it empties.
But with around 30 minutes remaining in the bike leg you must sharply reduce your rate of nutrition intake and allow your stomach volume to come down to a level that is manageable for the run. I recommend taking an energy gel with water or a few swigs of a sports drink with 30 minutes to go and another drink with 15 minutes to go, and that's all. If it's hot, drink at 30 minutes, 20 minutes and 10 minutes.
Stay Liquid
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Fluids are absorbed into the bloodstream more quickly than solid foods. Therefore I recommend you get as much of your nutrition as possible from fluids (where energy gels taken with water count as fluids) throughout the bike leg. This will not only minimize your chances of getting blocked up after the bike-run transition, but it will also maximize the rate of nutrient delivery to your blood and muscles throughout the bike leg itself.
You may swallow more calories if you chow down on a lot of energy bars during a triathlon, but you will absorb more calories if you avoid solids and stick to liquids, because they are absorbed more quickly.
Choose Fat-Absorbing Nutrition
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Two nutrients, sodium and protein, help you get more hydration per ounce of fluid consumed, while caffeine helps you absorb carbohydrate faster. Ounce for ounce, sports drinks with higher sodium concentrations provide better hydration, because they accelerate gastric emptying and improve fluid balance in the body. For this reason, use a sports drink that contains at least 15 mg of sodium per ounce.
Protein appears to enhance both fluid absorption and fluid retention. In a recent Spanish study, a carb-protein sports drink was found to empty from the stomach significantly faster than a carb-only sports drink in cyclists pedaling at 70 percent of VO2 max.
Finally, the results of a new study published in the Journal of Applied Physiology suggest that caffeine may enhance the effectiveness of sports drinks consumed during exercise by accelerating the absorption of carbohydrate in the intestine.
Practice
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There's an easy way and a hard way to discover your personal fueling limitations. The hard way to find them is by experiencing the bike-run bonk in a long-distance race. The easy way is to do some long, race-pace brick workouts in training. In preparing for a half-Ironman, build up to at least a two-hour ride followed by a one-hour run. In preparing for a full Ironman, build up to at least a four-hour ride followed by a one-hour run.
During these workouts, fuel yourself at the maximum comfortable rate until 30 minutes remain in your ride, then go light and observe your body's response during the run. If you experience gastrointestinal distress, you know you need to go even lighter.
Don't Overdo It
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Triathletes are often panicked about getting in enough nutrition in these events, but it's actually quite easy to consume fluid and calories at the maximum rate your body can absorb them. And on the run, it's all too easy to exceed your limits, because they are so much lower than on the bike.
Make every effort to stay on the safe side of your limits, and don't fret about not getting enough nutrition. Although it may seem paradoxical, by focusing more on emptying your stomach than on filling it, you will have a better chance of avoiding both Paul's fate and the classic energy bonk in your next long-distance race.
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