Brick Workouts: How to Train Your Gut for the Bike-to-Run Transition

The transition from the bike to the run in a triathlon is notorious for "jelly legs"—the sensation that your quads have completely forgotten how to function. But an equally dangerous, less talked about phenomenon is the "jelly stomach."

"You can bike beautifully on 90 grams of carbohydrate an hour, but the second you transition to running, the vertical oscillation turns your stomach into a cocktail shaker."

Brick workouts (stacking two disciplines back-to-back, usually a bike followed immediately by a run) are essential not just for muscular adaptation, but for gastrointestinal adaptation. If you ignore nutrition during your bricks, you are ignoring race-specific reality.

The Physiology of the Transition

When you are cycling, your body is in a seated position. Your core is relatively stable, and blood flow, while directed to your legs, still allows for reasonable stomach function. Because of this, it’s standard practice (as outlined in our Ironman 70.3 Nutrition Plan) to eat aggressively on the bike.

However, when you start running, two things happen instantly:

1. Mechanical Stress: Every footstrike sends a shockwave up your legs and into your core, aggressively jostling the contents of your stomach.
2. Vascular Shift: The mechanical demand of running requires even more blood flow than cycling, further starving the GI tract of the blood it needs to digest fuel.

If you have 500ml of undigested sports drink sitting in your stomach when you start the run, you are highly likely to suffer from severe Runner's Stomach.

Fueling the Final Miles of the Bike

The key to a successful transition begins 30 to 45 minutes before you get off the bike.

• Hour -1:00 to -0:30: This is your final window for aggressive fueling. Take your last solid food or heavy gel dose here.
• Hour -0:30 to T2: Dial back carbohydrate intake. Switch to sipping purely water to help flush existing carbohydrates out of your stomach and into the small intestine where they are absorbed.
• Avoid the Chug: Do not realize you are dehydrated at mile 50 of the bike and attempt to chug a whole bottle of water before entering T2. It will not absorb in time.

Fueling the First Miles of the Run

Once you are in your running shoes, you enter what coaches call the "settling period."

For the first 10 to 15 minutes of the run (roughly miles 1 and 2 of a 70.3), consume nothing but small sips of water. Your body is undergoing a massive physiological shift. Allow your heart rate to settle into its running zone and let your stomach adapt to the bouncing motion.

Once you pass this window and feel your rhythm returning, you can begin introducing highly concentrated sources of carbohydrates (like gels or chews) in small doses, washed down with water. Avoid returning to the massive liquid-calorie strategies you used on the bike; they are too voluminous for the stomach to handle while running.