Did I overheat, dehydrate or bonk?

On Sunday I rode a solo century on Cape Cod. It was pure sun and through the day warmed up to about 85º F and 75-80% humidity. I was going fine for first 75 miles or so, but the last 25 mi. were hell. That last 25 mi. my heart rate was about 15-20 bpm below where it was earlier in the day, and my power dropped about 20-30W with it. I wasn’t totally delirious, but was worried a bit about making it home. It was hard to tell, but it also seemed like I wasn’t sweating as much toward the end.

I’m at 240 ftp and was holding around .64 IF for most of the ride (until it started declining with HR and power). So I don’t think it was a pacing issue. I’ve done this distance before, even this route before.

Fuel wise, over 6.5 hours moving time I had:

  • (pre-ride oatmeal)
  • The carbs from 2 bottles of Skratch
  • 2 Clif bars
  • 1 Skratch bar
  • 2 packages Skratch chews
  • 1 bacon and egg on ciabatta sandwich


  • 2x22oz. bottles Skratch
  • 4x22oz. bottles SIS electrolyte mix
  • 1x12oz. Coke

Bingo. You rode in 85% humidity and you dehydrated. Not to say you wouldn’t have lost some watts near the end of the ride, but it sounds like you felt yourself hitting a wall. Your body started regulating your effort some to start to conserve the resources it had available for the work you were asking it to do.

240 ftp and .64 IF I bet you had more than enough fuel and simply not enough fluids.

In Texas if you head out early in can be 85-90% humidity and it will slowly burn off after noon. I’ve done 80 miles with the humidity very high and stopped sweating near the end. Took some time to dial it in and it turns out I just needed more water. Did the same ride, same loop, took 2x the water (I used a USWE but I could have stopped to refill) and I was still tired from the humidity but my sweat rate didn’t drop and I didn’t feel like rolling death the last few miles getting home.

That’s my $.02

1 Like

Agree that it was likely the heat, especially if you have some prior experience with long rides. This latest heatwave has really been something with the high temps combined with the humidity.

I’ve been doing a bunch of rides in it and also trying to get one or two sauna sessions in a week as well. I still find them very taxing and have had to keep an eye on power on long days. For the most part I’ve managed ok but I’ve found I have to be very careful on the days I ride back to back. On Saturday I hit a pretty easy 55 mile endurance road ride in 90+ temperatures on Saturday afternoon and then went out midday Sunday for a gravel grind and I could really feel a lot of fatigue a few hours in. I still got through it fine, but the RPE was a lot higher than a couple months ago. For the 3+ hour gravel grind I finished off a 2 liter USWE back with Skratch plus most of a 700mL bottle also filled with Skratch.

1 Like

I agree with the other comments. 1 bottle per hour is usually enough for a moderately warm day but when it is nearing 90 and high humidity, made worse if there is little to no shade, then it probably isn’t enough for a ride that long.

1 Like