Category: Longevity & Cellular Aging / Mitochondrial Health

What Mitochondrial Function Testing Can Actually Show

Posted on August 24, 2026

Introduction

When energy drops in midlife, it is tempting to blame mitochondria. The idea sounds tidy: if the cell’s power system is struggling, fatigue, stamina, vision, muscle strength, and aging all start to feel connected. The harder question is this: how would a trial prove that mitochondrial function testing changed something a person can actually notice?

SS-31, also called elamipretide or MTP-131, is a useful case study because it has been tested in several human randomized trials. The evidence does not support a broad claim for healthy aging or everyday fatigue. It does show how researchers try to measure mitochondrial-linked outcomes, and why those measurements often disagree.

Research Confidence

★★★★☆

Confidence is high for describing how elamipretide has been tested because multiple randomized human trials exist. It is not five stars because results are mixed, populations were disease-specific, and the FDA’s Barth syndrome approval relied on a surrogate strength endpoint after randomized symptom and walking endpoints were not superior to placebo.

Study Snapshot

• Study type: evidence landscape built around human randomized trials
• Participants: disease-specific groups including primary mitochondrial myopathy, Barth syndrome, heart failure, and dry age-related macular degeneration
• Duration: 4 weeks to 48 weeks, with longer open-label Barth syndrome follow-up
• Measured: walking distance, fatigue scales, cardiac MRI, retinal imaging, low-light vision, and knee extensor strength
• Found: several randomized primary endpoints failed, while exploratory or surrogate measures showed signals in selected settings
• Funding: most key trials were sponsored or funded by Stealth BioTherapeutics

Why This Matters

The word “mitochondria” can become a catch-all for problems that are real but not simple. Poor sleep, iron status, mood, medications, menopause-related vasomotor symptoms, deconditioning, thyroid disease, cardiometabolic change, and rare mitochondrial disorders can all show up as low energy.

That is why mitochondrial function testing in human trials cannot stop at cell biology. A blood marker or imaging signal may be interesting, but a person wants to know whether they can walk farther, climb stairs more easily, read in dim light, or get through the day with less fatigue. Good trials force that question into a measurement. Sometimes the answer is no.

Mitochondrial Function Testing Starts With What A Person Can Feel

Walking distance is one of the plainest endpoints. In the phase 3 MMPOWER-3 randomized, double-blind, placebo-controlled trial, 218 adults with primary mitochondrial myopathy received elamipretide 40 mg per day by subcutaneous injection or placebo for 24 weeks. The co-primary endpoints were the 6-minute walk test and a fatigue symptom score.

The trial missed both. The between-group difference in walking distance was -3.2 metres, with a 95% confidence interval from -18.7 to 12.3, and the fatigue score difference was -0.07. In everyday terms, the largest primary trial did not show that participants walked farther or reported less total fatigue than placebo.

An earlier phase 2 randomized crossover trial in 30 adults used 4-week elamipretide and placebo periods separated by a washout. A crossover design means each participant receives both interventions at different times. The walking difference favoured elamipretide by 19.8 metres but did not meet statistical significance. Some fatigue measures moved in a favourable direction, but injection-site reactions were common, and one participant stopped because of injection-site pain.

Where SS-31 Fits Into Mitochondrial Function Testing

SS-31 entered this literature because elamipretide was designed as a cardiolipin-targeting therapy. Cardiolipin is part of the inner mitochondrial membrane; as an analogy, it is part of the structure that helps the cell’s power machinery stay organized. That mechanism is a rationale, not proof of a felt outcome.

In Barth syndrome, a rare genetic disorder of mitochondrial cardiolipin metabolism, a phase 2/3 randomized crossover trial enrolled 12 male participants. They received elamipretide 40 mg per day by subcutaneous injection and placebo during two 12-week periods, with a 4-week washout. The randomized portion did not meet its primary walking or fatigue endpoints.

The U.S. FDA later granted accelerated approval in 2025 for Forzinity, the clinical elamipretide injection, to improve muscle strength in people with Barth syndrome who weigh at least 30 kg. That is not an approval for general mitochondrial health, healthy aging, fatigue, primary mitochondrial myopathy, heart failure, or age-related macular degeneration.

Why Surrogate Endpoints Can Matter Without Settling Everything

A surrogate endpoint is a stand-in measurement that regulators or researchers believe may predict real benefit. In Barth syndrome, the FDA summary says approval relied on knee extensor muscle strength measured by handheld dynamometry during open-label follow-up. Open-label means participants and researchers knew the drug was being given, and there was no placebo comparison in that extension.

Other elamipretide trials show the same endpoint tension. In a phase 2 randomized trial of 71 adults with heart failure and reduced ejection fraction, 4 mg or 40 mg elamipretide by daily subcutaneous injection for 28 days did not significantly improve left ventricular end-systolic volume on cardiac MRI versus placebo.

In ReCLAIM-2, a phase 2 randomized, double-masked trial in 176 adults aged at least 55 with dry age-related macular degeneration and geographic atrophy, 40 mg elamipretide by daily subcutaneous injection for 48 weeks missed its primary low-light vision and atrophy endpoints. Exploratory retinal-structure and low-light letter-gain signals were reported. Adverse events occurred in 86% of elamipretide participants and 71% of placebo participants, mainly injection-site reactions.

What This Means For You

If a mitochondrial claim is tied to fatigue, stamina, vision, or aging, the first useful question is not “does it affect mitochondria?” It is “what did the trial measure that a person could feel?”

For a clinician conversation, that might mean asking whether an endpoint was walking distance, a validated fatigue scale, muscle strength, imaging, or a lab marker. A validated scale is a questionnaire tested to measure a symptom consistently, not just a casual rating. The SS-31 literature shows why that distinction matters: biology can look plausible while the patient-centred endpoint stays unchanged.

What This Tells Us About Women Specifically

Women were well represented in the primary mitochondrial myopathy trials: the phase 3 trial reported 64% women, and the earlier crossover trial reported 83% female participants. That does not mean the results were analyzed by menopausal status. They were not.

The women’s data become much thinner elsewhere. The pivotal Barth syndrome efficacy cohort was 100% male. The heart-failure trial was 24% female. The AMD trial appears to have enrolled a majority of women, but menopausal status and hormone therapy use were not reported. None of the opened trials provided menopause-specific outcomes, which matters when fatigue and exercise tolerance overlap with midlife symptoms.

Questions This Study Couldn't Answer

• Whether SS-31 improves energy, exercise performance, or healthy aging outcomes in generally healthy midlife or older adults.
• Whether exploratory retinal or fatigue signals would hold up in larger trials designed around those endpoints.
• Whether women in perimenopause or menopause respond differently, because menopausal status was not recorded.
• Whether open-label strength gains in Barth syndrome translate into durable daily-life improvements.
• How much injection-site reactions affect adherence over longer periods.
• How much sponsor funding may have shaped design choices, endpoint selection, or reporting emphasis.

Future Research

The next confidence jump would require larger, independently replicated randomized trials with patient-centred primary endpoints. Trials should prespecify walking, fatigue, strength, vision, or daily function outcomes rather than leaning on exploratory measures after the fact. Sex, age, menopausal status, adverse events, and dropout patterns need to be reported clearly.

Sources

1. Karaa A, Haas R, Goldstein A, et al. Efficacy and Safety of Elamipretide in Individuals With Primary Mitochondrial Myopathy: The MMPOWER-3 Randomized Clinical Trial. Human phase 3 randomized, double-blind, placebo-controlled trial, n=218. Neurology. 2023. DOI: 10.1212/WNL.0000000000207402. https://pubmed.ncbi.nlm.nih.gov/37268435/

2. Karaa A, Haas R, Goldstein A, et al. A randomized crossover trial of elamipretide in adults with primary mitochondrial myopathy. Human phase 2 randomized, double-blind, placebo-controlled crossover trial, n=30. Journal of Cachexia, Sarcopenia and Muscle. 2020;11(4):909-918. DOI: 10.1002/jcsm.12559. https://pubmed.ncbi.nlm.nih.gov/32096613/

3. Thompson WR, Hornby B, Manuel R, et al. A phase 2/3 randomized clinical trial followed by an open-label extension to evaluate the effectiveness of elamipretide in Barth syndrome, a genetic disorder of mitochondrial cardiolipin metabolism. Human phase 2/3 randomized, double-blind, placebo-controlled trial with open-label extension, n=12 randomized. Genetics in Medicine. 2021;23(3):471-478. DOI: 10.1038/s41436-020-01006-8. https://pubmed.ncbi.nlm.nih.gov/33077895/

4. FDA Drug Trials Snapshot: Forzinity (elamipretide). Regulatory approval summary based on human clinical trial data, pivotal Barth syndrome efficacy population n=12. U.S. Food and Drug Administration. Approval date September 19, 2025. https://www.fda.gov/drugs/drug-trials-snapshots/drug-trials-snapshots-forzinity

5. Butler J, Khan MS, Anker SD, et al. Effects of Elamipretide on Left Ventricular Function in Patients With Heart Failure With Reduced Ejection Fraction: The PROGRESS-HF Phase 2 Trial. Human phase 2 randomized, double-blind, placebo-controlled trial, n=71. Journal of Cardiac Failure. 2020;26(5):429-437. DOI: 10.1016/j.cardfail.2020.02.001. https://pubmed.ncbi.nlm.nih.gov/32068002/

6. Ehlers JP, Hu A, Boyer D, et al. ReCLAIM-2: A Randomized Phase II Clinical Trial Evaluating Elamipretide in Age-related Macular Degeneration, Geographic Atrophy Growth, Visual Function, and Ellipsoid Zone Preservation. Human phase 2 randomized, placebo-controlled, double-masked trial, n=176. Ophthalmology Science. 2024;5(1):100628. DOI: 10.1016/j.xops.2024.100628. https://www.aaojournal.org/article/S2666-9145(24)00164-7/fulltext

Research Use And Availability

Nuforme supplies SS-31 in Canada as a research-use-only material, separate from the FDA-approved clinical drug Forzinity. Research-use-only means not for human or veterinary use. Nuforme provides purity information, third-party testing, and documentation for qualified research contexts. Relevant internal references include SS-31 research-use materials, Mitochondrial Research Peptides, and the Nuforme peptide glossary.

Final Thoughts

Mitochondrial function testing is only as useful as the endpoint it chooses. SS-31 trials show a serious human evidence base, but not a simple anti-aging story. The most honest answer is narrower: researchers can measure mitochondrial-linked outcomes, yet walking, fatigue, strength, heart imaging, and vision do not always move together.