The one part of this I know I can speak on is that dehydration causes heart rate to go up, the heart having to work harder because the blood is thickening due to lack of fluid. So we can rule that out.

Underfueling (with adequate hydration) will cause HR to drop. This is because you are simply lacking the energy to pump your heart enough. BUT, "underfueling" can also be caused by people eating too much or eating without enough water, and that causes the digestive tract to stall. The net effect is still the same - calories can't get to where they need to go and your HR drops.

Your issue sounds like a combo of underfueling along with something else... If it's blood pressure stuff, then that's out of my wheelhouse. But perhaps somebody could speak about how electrolytes affect blood pressure?

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Zen and the Art of Triathlon. Strava Workout Log
Interviews with Chris McCormack, Helle Frederikson, Angela Naeth, and many more.
http://www.zentriathlon.com

Bonky

Were you walking?

You may have nailed it. I was drinking plenty of water, but maybe not enough gels. I was trying to take 100 calories every half hour and didn't want to overdo it, but I was feeling hungry.

Running aid station to aid station with no more than 30 second walk through. After mile 17 I had to walk for about a mile while I took salt pills, and coke. Eventually bounced back to run the piddle pace to the finish.

Yup, you probably were bonky. By the time you get to the end of an IM, you've burned through all your reserves and now you're running on what you are putting in. 3 gels per hour is more along the lines of what the fast folks do, depending on the gel. Some gels are 100 cals each, some more, some less.

Do this instead - Try 3 gels per hour. If you start burping, that's a sign your stomach has reached overload and things are going the wrong way. Drink only water and skip fueling for at least 15 minutes. If you start farting, then things have reversed and the fuel is now going the correct direction. You can then start fueling again, but now you know that you have reached the max you can take in for the conditions you are in an the water you are drinking it with. Don't be afraid to toss away half a gel or to just sip on a gel like Macca did.

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Zen and the Art of Triathlon. Strava Workout Log
Interviews with Chris McCormack, Helle Frederikson, Angela Naeth, and many more.
http://www.zentriathlon.com

Thank you. Good advice. I've been doing this a long time and never understood the low heart rate. This makes sense.

Yes, what others said. Bonk

Not trying to hijack here... just trying to learn.

So I raced my first full 3 weeks ago. I was starting to fart around mile 15-16. Was taking in 2 gels an hour plus some Gatorade (was trying to get to 240 cal/hr on the run). I felt like my stomach wanted to empty and after each fart there was relief in my stomach. I was worried if I kept taking gels and Gatorade, I would need to stop in the porta potty. So I stopped taking in the gels and Gatorade and went to just water and coke. I had no GI issues the rest of the run but wonder if I would have had a little bit more energy the last few miles if I had taken a gel or 2 the last 10 miles. First half marathon was 1:35, second half was 1:38. I ran really conservative the first half.

Just curious on your thoughts.

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blog

Best explanation i have come across for this subject, Thank you
Had the same issue few weeks back during a marathon, Klm 30 HR plummeted and stayed in the low 100s until the 40 K mark were i went back to my normal 160 -175