I cannot speak for the the the entire ME space, but this is pretty clear at least in Long Covid, with multiple papers reporting it. The issue is impaired fatty acid oxidation. Here is one example:
Decreased Fatty Acid Oxidation and Altered Lactate Production during Exercise in Patients with Post-acute COVID-19 Syndrome https://www.atsjournals.org/doi/10.1164/rccm.202108-1903LE
And the issue is related to the impaired cellular oxygen uptake we have, as this is an aerobic process. This is the therapeutic target of the ongoing Bocidelpar trial.
I am assuming we would see an increasing level of circulating free fatty acids in these patients (aka dyslipidemia). This is very easy to spot.
Things like fasting or deep ketosis will actually upregulate the fatty acid oxidation. I know some of the members here benefit from being in that state. It is worth noting though that ketosis will have some undesirable side effects for people with orthostatic intolerance. Electrolyte imbalance being the biggest issue for me. If you can handle being on the edge of dehydration the entire time, this may be a good strategy.
There is also a drug called Cardarine that works like magic in fixing this issue. But the problem is that it is carcinogenic.