It doesn't according to Mark Vanness:
I'm not sure where the lactic acid is supposed to be. I don't think there is good evidence for increased levels of lactic acid in the blood of people with ME/CFS, or for abnormal responses to exercise in terms of lactic acid?
I know there was that n=1 paper by Vink, but I think members' experimentation with lactic acid monitors, including my own, didn't find that issues with lactic acid were a common problem.
Lactate levels 24 hours after an appointment at which I was told I'm just deconditioned and need to exercise. FML
For context, the 'exertion' involved in this was being taken into hospital on a stretcher in an ambulance, lying down for the entirety of the appointment, and then being taken home and not being able to get out of bed since.
Interestingly, my lactate levels were only somewhat elevated (2.4 mmol/l) immediately after the appointment. This huge increase seems to have come later, i.e. some time after the actual activity itself.
I once did a test where i had to sit down and stand back up for 3 minutes at the pneumologists, with the point being to text my blood oxygenation. My oxygenation was perfectly fine but my lactate went from 1,2 mmol/l to 8,0 mmol/l in these 3 min- No one gave a single fuck about this, while it felt like a very clear "this could be something" moment for me. Since i dont have any other clue as to whats going on with me and why im in so much pain during and after any exhaustion.
My lactate was below 1mmol/l only twice, and both times after a shower, 1000 mg ibuprofen and much rest.
It was always way above 1mmol/l the other days. I didnt ever exercise or do anything straining and yet it was always above 2 mmol/l in the morning and on average 3-4mmol/l in the evening. And after exhaustion i noticed 4-5mmol/l HOURS afterwards (example exhaustion at 10am, measuring at 3pm).