Hi, I’m a female masters cyclist, rode 9k mi and climbed 660k’ in 2025 with a long history of amateur racing and other endurance sports; I also have type 1 diabetes. My FTP usually ranges between 190-220W/kg depending on how things are going. Resting HR is 45-48 bpm and max is 187 bpm.
I have struggled with weight for decades and after a couple years of internal debate, started a GLP-1 (tirzepatide) mid December 2025. First impression? This is a POWERFUL drug and should be taken with care and under the close supervision of a doctor. My insulin sensitivity immediately went up (I was sensitive to begin with), and my appetite seemed to normalize without the constant internal battle. I noticed quickly that on my rides, I had to fuel a lot more and it was a lot harder to keep my blood glucose up. I think this has to do with glucagon suppression, reduced gut motility, and heightened insulin sensitivity.
By the end of Dec 2025, I did a routine (easy) ride and had to stop multiple times to catch my breath and my HR was significantly elevated – not just “tired” elevated but abnormally high for trivial efforts. At one point I felt pre-syncope, that darkening before you pass out (but didn’t). Things progressed over the next 6 weeks (including a pulmonary embolism misdiagnosis cleared up 2/2/26), and the shortness of breath, orthostatic blood pressure drops, fatigue and increased heart rate got worse and worse. Instead of just feeling out of breath riding, I found it hard to tolerate even standing up. I was seeing 70-80bpm jumps just walking around my house.
Long story short, the culprit seems to be a “preload impairment” (ie not enough blood volume to fill the ventricles before each beat) and possibly some autonomic dysfunction. Instead of riding comfortably at 150 W and 145 bpm, I had to keep lowering power to stay comfortable. I dropped my FTP on TR by about 80 Watts so I could make it through a workout. If I tried to do anything above recovery/easy endurance, it set me back for several days.
After a thorough workup including bloodwork, echo, a pulmonary function test and CPET (aka VO2max test), the verdict is that when I am upright, in part due to low blood volume, the blood is pooling in my legs and the ventricles do not get full enough to keep my BP up. So my blood pressure drops too much (systolic was dropping >20-30 mmHg standing) and my HR increases to try to compensate. The solution for now is to load up on sodium (6-10g/day) and fluids, to wear compression and do recumbent exercise. I am doing a POTS based rehabilitation program and have been off tirzepatide for 3+ weeks. I have seen significant improvements with these approaches, but am still not recovered and don’t know how long that will take.
Caveat: I have in the past experienced some milder form of this (during rides only) but it was intermittent and I was able to recover. My theory is that tirzepatide was such a shock to my metabolism and because tirz. can cause excess sodium excretion and reduced thirst, that I decompensated. I am now working on rebuilding my aerobic system. I did try very hard to keep my electrolytes and hydration up on tirz., but I was not able to do enough.
If I had it to do over again, I would have taken 2-3 weeks off the bike completely (or reduced volume) as my body adjusted to the medication. I was so worried about the supposed muscle loss that I felt compelled to continue training. (And this had worked for me before when losing weight). It was almost miraculous in how much it helped my diabetes and stabilized my weight, but it was not worth it. I don’t see how this could possibly ever be a performance enhancing tool. I have genetically lower BP to begin with so that may be a big part of the reason it hit me so hard like this. There are reports of people with POTS having similar experiences. (I have not been diagnosed with POTS but am experiencing similar symptoms.)
Also it is possible that the tirzepatide has nothing to do with it. However I got worse about 2 weeks after starting so the timing and the known effects of tirz (reduced thirst, increased sodium excretion) makes it likely they are related.
