Mitochondrial Function in the Clinic — What Actually Changes Practice

I had a patient bring in a direct-to-consumer mitochondrial health panel — CoQ10, urine organic acids, mtDNA copy number. She's on ubiquinol, NAD precursors, high-dose C. She wants to know if it's working.

There's no validated blood or urine test for mitochondrial capacity in a healthy adult. That panel is marketed without clinical evidence for that use. Functional capacity has one good test: a cardiopulmonary exercise test, VO2 max.

So the panel tells us nothing?

Correct. CoQ10 serum levels don't track tissue function. MtDNA copy number — that's the count of mitochondrial genome copies per cell — varies with age and inflammation and isn't actionable in a well person. None of those markers have guideline support outside suspected inherited disease.

And the supplements she's taking?

Mechanistic and animal data only, with one exception. Statin-induced myopathy is the one clinical setting where CoQ10 depletion has a plausible mechanism, but randomized trial evidence for supplementation even there is mixed. The rest — ubiquinol, NMN, NR — no trial evidence for clinical endpoints in healthy adults.

What actually works?

Structured endurance training. Guideline-level evidence, multiple trials, measurable VO2 max improvement within one to three months. Mitochondrial content in skeletal muscle rises with use and falls with disuse. Very few things in aging biology are that responsive to a behavioral input.

She walks four times a week — isn't that enough?

Walking is better than nothing, but it likely doesn't produce the intensity needed to drive mitochondrial biogenesis — the process by which cells generate new mitochondria. You need sustained aerobic effort above lactate threshold. A CPET would quantify exactly where she is.

Okay. Now, different patient — thirty-eight, type 2 diabetes diagnosed at twenty-nine, bilateral sensorineural hearing loss. Mother and maternal aunt have the same combination.

That pedigree is almost diagnostic. Diabetes plus sensorineural hearing loss plus maternal-lineage transmission is the classic presentation of MIDD — maternally inherited diabetes and deafness — caused by the m.3243A>G variant in mitochondrial DNA.

Why maternal lineage specifically?

Mitochondria are inherited almost exclusively through the oocyte, so mtDNA variants track through the mother. Father-to-child transmission is effectively absent. That's what makes the inheritance pattern diagnostically distinctive.

What do I do for him?

Refer to a mitochondrial disease specialist or clinical geneticist. Testing for m.3243A>G is available and straightforward — ideally from urine epithelial cells or blood, though blood sensitivity drops with age. Management of his diabetes may need adjustment; metformin is typically avoided because it inhibits mitochondrial complex I.

That's a meaningful clinical change.

It is. And it affects his children's maternal risk assessment too. These are completely different problems — one is about optimizing capacity, the other is about diagnosing inherited disease. Collapsing them is where the confusion starts.

So what's the one thing to hold onto?

If the question is capacity, the test is a CPET and the intervention is structured exercise — nothing in the supplement market has clinical endpoint evidence. If the pedigree shows maternal-lineage diabetes with hearing loss, refer and check metformin use before you leave the room.