Why you bonk on a GLP-1

Bonking is running out of stored carbohydrate — glycogen — in the middle of work. Your brain and muscles depend on it. When it drops, power drops, focus drops, and everything feels harder than it should. It is not weakness. It is fuel math.

Semaglutide, tirzepatide, and other GLP-1s suppress appetite. That is the point. But appetite is the signal that tells you to eat, and on a GLP-1 the signal is quiet. You eat less overall, which means fewer carbs, which means less glycogen stored. Then you train, and your tank runs dry faster than it used to.

The medication did not reduce your fitness. It reduced your fuel intake, and your fitness cannot show up without fuel.

You cannot rely on hunger to time your fueling. You have to schedule it.

• The night before: a normal dinner with carbs, not low-carb.
• The morning of: 60–90g carbs 2–3 hours before a hard session, even if you are not hungry.
• During: for anything over 60 minutes, start fueling at minute 20, not when you feel empty.
• After: 40–60g carbs plus 20–30g protein within the first hour.

If your heart rate is higher than usual for the same pace, if your mood crashes mid-workout, or if you suddenly cannot hold a power you held last week, you are likely glycogen-depleted, not detrained. Eat first, then judge the workout.

Match carbs to the work. Easy 45-minute session: 30–40g before is enough. Long hard ride or run: 60–90g per hour. Race: up to 90g per hour. The bigger the work, the more aggressive the fueling has to be — because the medication will not remind you.