Actual lactic acidosis is usually fatal without treatment. Elevated lactic acid is not the same thing, but can become lactic acidosis if it persists. However our lower than normal oxygen utilisation might confer some resistance to the problem. I do know we get elevated lactate, but it has to be continuous to lead to the main problems. In the process it results in the lactic acid version of hypoxia. Now for us the threshold is probably higher than for other people ... we can probably, not reliably, tolerate more lactic acid than regular people. This leads me to speculate that if someone with ME kept pushing and pushing, through a crash, through PEM, and through all the symptoms, then after some days they might wind up with genuine lactic acidosis, which can be fatal. I do wonder though if we might compensate by severely worsening the ME ... its hard to overdo things if you cannot even move. If resting restores the situation, if pacing restores the situation, its probably not lactic acidosis. Endurance athletes can have elevated lactic acid, but marathons and ultra marathons are not several days of continuous running, and often rely on working up to the anaerobic threshold, rather than beyond it. Someone with genuine lactic acidosis would have required urgent hospital treatment, or they would most likely have died. That does not mean we are not at risk, only that its not the usual case. Prolonged elevated lactic acid is a trigger condition for lactic acidosis, but its only the beginning of it.