MOTS-c and physical decline: what the human studies actually measured

In thirty people, an acute bout of endurance cycling moved humanin in the blood and left MOTS-c with a trend and nothing else. In nineteen men held still for two weeks, heat exposure raised circulating MOTS-c by a measurable amount. Neither study gave anyone MOTS-c to take.

thinPlausible, under-studied. Treat as a hypothesis.Revised 03 Nov 2026

A blood collection tube in a plain metal rack on a stainless steel bench, lit from a window

Two human studies, both about endogenous levels. The gap between measuring a mitokine and giving one is the whole story.

The aging literature has a favourite new character: a short peptide made inside the mitochondria, released during stress, and credited — usually without a trial to point at — with the maintenance of muscle and glucose handling in later life. MOTS-c is that peptide. The human evidence on it is remarkably thin, and it is worth reading precisely because it is thin, in public, with the numbers attached.

Study one: thirty people, one session of cycling

The first human paper, in the Journal of Applied Physiology in 2021, asked whether a single bout of exercise changes the circulating levels of two mitochondrial-derived peptides, humanin and MOTS-c. After a familiarisation session, subjects were randomised to endurance exercise (n = 10, 45 minutes of cycling at 70% of estimated VO2max), resistance exercise (n = 10, four sets of seven repetitions at maximum load on leg press and knee extension), or control (n = 10). Muscle biopsies and blood samples were taken before exercise, at 30 minutes, and at three hours.

The findings are precise and mostly about humanin:

Acute endurance and resistance exercise both produced the expected gene expression changes in muscle for each modality.
Humanin was significantly elevated in the circulation by endurance exercise but not by resistance exercise.
MOTS-c showed a trend to increase after endurance exercise — a trend, not a significant result.
Plasma levels of these peptides were not correlated to fitness outcomes: VO2max, leg strength, or muscle mitochondrial DNA copy number.
At rest, circulating humanin was positively correlated with MOTS-c and with age.

So in the only study that measures MOTS-c during acute exercise in people, the compound is the one with the weaker result, and it does not track fitness.

Study two: nineteen men, two weeks of doing nothing

The 2025 study in Medicine & Science in Sports & Exercise took the opposite approach. 19 physically active men completed four weeks of supervised training and then two weeks of immobilisation of the left ankle, during which they were randomised to repeated heat exposure (n = 9) or a sham condition (n = 10). Muscle thickness was measured by ultrasound, cross-sectional area by magnetic resonance imaging, and mitokine levels were measured in blood and skeletal muscle.

Repeated heat treatment upregulated circulating MOTS-c (p = 0.033) and downregulated skeletal muscle FGF21 (p = 0.027).
Immobilisation alone changed no mitokine level at all.
Immobilisation did reduce muscle thickness (p = 0.012) and cross-sectional area (p < 0.01) of the gastrocnemius medialis.

The authors describe heat stress as producing mitokine changes comparable to, but not identical to, exercise, and call for further work on the mechanism and on the therapeutic potential of heat stress and mitokines in mitigating muscle loss.

What connects the two, and what does not

Put side by side, the record reads this way: thirty exercise bouts and nineteen immobilised legs. In both cases, the intervention was something the participants already had — cycling, or heat — and the measurement was of the body’s own circulating levels of a peptide it makes itself.

That is a coherent and genuinely interesting piece of physiology. It is not a trial of administration, and the two papers do not claim to be one.

The research page on the same peptide lays out the compound side of the file: MOTS-c and the mitochondrial research page.

What this evidence does not show

Nobody was given MOTS-c. Neither study administered the peptide to a human being. Both measured endogenous levels.
No amount, no route and no administration schedule appears anywhere in either paper, because no administration took place.
No clinical outcome. No muscle strength test result, no glucose challenge, no insulin sensitivity measurement as an outcome, no falls, no frailty score. Study one explicitly found no correlation between circulating levels and fitness outcomes.
The MOTS-c result in study one was a trend, and the significant finding in that paper belongs to humanin.
Sample sizes of 10 per arm and 9 or 10 per group are pilot sizes. Neither paper is powered to detect a clinical effect, and neither says it is.
The heat study’s outcome is a biomarker shift, with the authors themselves calling the therapeutic potential a subject for further research.

The line

There is a coherent signal here: mitochondrial stress raises mitokines, and mitokines move with age and with immobilisation. What does not exist yet is the step where someone is given the peptide and something happens to a person. The distance between those two sentences is the entire honest state of the human evidence.