Avoiding the Bike-Run Bonk
In 2004 I flew to Penticton, B.C., to see a couple of my athletes compete in Ironman Canada. One of them had a day to remember; the other had a day to forget.
The latter, Paul, was doing fine through the end of the bike leg. He arrived at T2 right on his goal pace. But things fell apart quickly on the run. Almost immediately his stomach began to feel bloated and sloshy. His legs grew heavy and seemed starved for energy and soon he was even experiencing some lightheadedness. By the 5K mark of the marathon he was walking.
This scenario – which I call the bike-run bonk – is common in long-distance triathlons. The athlete feels good or at least okay on the bike, only to suffer a gastrointestinal meltdown in the early going of the run. Fortunately, as common as the bike-run bonk is, it is completely avoidable.
The key to avoiding the bike-run bonk is understanding exactly what it is. So what is it? The bike-run bonk is a simple case of overnourishment – 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 faster, while an athlete is bicycling than it is while the same athlete is running. So what qualifies as optimal nourishment during the bike leg of a triathlon suddenly becomes overnourishment on the run.
The essential difference between cycling and running with respect to nutrition is the far greater amount of stomach jostling that occurs on the run. This jostling is the likely cause of the unpleasant sloshy feeling that often becomes full-blown nausea if the stomach volume is too great. Stomach jostling probably also contributes to a reduced gastric emptying rate (i.e. slower absorption of nutrition through the stomach and intestine) during running as compared to cycling. The result is a nutrition backlog in the stomach, small intestine, and possibly the colon that’s not unlike the damming of a river and subsequent flooding of riverfront properties. Such a backlog and the resulting accumulation of fluid in places it should not be (e.g. the colon) is also a cause of that terrible bloated feeling.
If that wasn’t bad enough, when your pipes get stopped up in this manner 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.
A few ounces of prevention
The cause of the bike-run bonk, then, is taking in too much nutrition (and perhaps too high a concentration of nutrition) during the latter portion of the bike leg. It’s not too much with respect to the latter portion of the bike leg itself, but it becomes too much in the early portion of the run leg. 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 four specific tips to avoid the bike-run bonk.
1. Go light
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 especially hot, drink at 30 minutes, 20 minutes, and 10 minutes.
This advice is precisely the opposite of what I hear many coaches and triathletes preaching. They encourage long-distance triathletes to “stock up” on nutrition towards the end of the bike leg for the same reason I’m telling you to cut back --because it’s impossible to consume nutrition at as high a rate on the run. What these coaches and triathletes are missing is that not only can you not consume as much nutrition on the run, but you also cannot tolerate as much in your stomach or absorb it as quickly, so “stocking up” on nutrition before the run is a recipe for disaster. In fact, one of the reasons the bike-run bonk is so common is that this advice is so frequently given, and followed.
2. Stay liquid
Fluids are absorbed into the bloodstream faster than solid foods. Therefore I recommend that 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 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 faster. I never consume any solids during long-distance triathlons, and I have never suffered the bike-run bonk or the more classic glycogen depletion bonk in any of them.
3. Choose fast-absorbing nutrition
Not all fluids are equal when it comes to absorption and retention. By consuming fluids that are absorbed faster and retained better, you can actually get better hydration and faster energy delivery from less fluid. This will help you “go light” during the final 30 minutes of the bike leg, and throughout the run, with less risk of experiencing severe dehydration or glycogen depletion.
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% of VO2max. And in a new study from St. Cloud State University, a carb-protein sports drink was retained in athletes 15% better than a carb-only sports drink (meaning 15% less of it wound up in the bladder).
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. So it’s a good idea to use an energy gel with caffeine or to supplement your sports drink with caffeine from another source, especially in light of the fact that caffeine is also proven to enhance endurance performance and reduce perceived effort.
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. If you experience no GI symptoms but suffer an energy bonk, try taking in a little more nutrition next time, but don’t count on being able to get away with it. You may actually have to reduce your pace to avoid both the bike-run bonk and the energy bonk.
Too much is no better than too little
While I don’t have scientific proof of it, in my experience, triathletes are far more likely to bonk due to overnourishment than undernourishment in long-distance triathlons. 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.