§ Claim under review · Study
"A meta-analysis of 353 studies (~6,000 people) found that training programs built around ~4 weekly sessions and 3–5 hours of easy aerobic work produced a 23% increase in mitochondrial content (Mølmen K.S., Almquist N.W., Skattebo Ø., Sports Med, 2025, PMID: 39390310)." Caption adds: "~13% gains in capillary density," "These adaptations together can raise VO₂max by about 10%," and a practical prescription of "3–5 hours/week of steady, conversational-pace aerobic training, ideally in 45–75 minute sessions."
Verdict
Source exists but framing is misleading
Confidence
HighSummary
The study cited in this post is real. Mølmen, Almquist and Skattebo published a review in Sports Medicine in 2025 covering 353 research articles and about 6,000 participants, and it does report roughly a 23 percent rise in markers of mitochondrial content and about a 13 percent rise in capillary density after low to moderate intensity endurance training. The numbers are copied correctly, but the framing changes their meaning. The 23 percent figure is a statistical average across hundreds of very different training studies, not the result of a specific program of four sessions and three to five hours per week as the post claims. The post also leaves out that the same review found high intensity and sprint training produced equal or slightly larger gains, roughly two to four times faster per hour of training, and that higher weekly frequencies produced better results than four sessions. It also omits the paper's main conclusion, which is that the size of the improvement depends largely on how unfit a person is at the start, with capillary gains seen only in untrained to moderately trained people. The claims about 45 to 75 minute sessions and a 10 percent rise in VO2max could not be confirmed in the source material reviewed, and the post comes from a supplement brand, though the study itself was unfunded and its authors declared no conflicts.
The readings
key figures from the evidencemitochondrial content increase, endurance training (full model)
mitochondrial content increase, HIT and SIT
research articles included in mitochondrial synthesis
Why this verdict
Evidence
The cited paper is real, correctly attributed, and correctly identified by PMID. It is a systematic review and meta-regression, not a conventional effect-size meta-analysis. The authors searched PubMed, Web of Science, and SPORTDiscus with no data restrictions up to 2 February 2022, including exercise training intervention studies of endurance training (ET), high-intensity training (HIT), and sprint interval training (SIT) that had baseline and follow-up measures of at least one marker of mitochondrial content or capillarization. In total, data from 5973 participants in 353 and 131 research articles were included for the mitochondrial and capillary quantitative synthesis respectively.
On the headline number: after adjusting for covariates such as training frequency, number of intervention weeks, and initial fitness level, percentage increases in mitochondrial content increased to a similar extent with ET (23 ± 5%), HIT (27 ± 5%), and SIT (27 ± 7%) (P > 0.138), and were not influenced by age, sex, menopause, disease, or the amount of muscle mass engaged. The full-text results state the same values more precisely: using pooled data from the five mitochondrial content markers and adjusting for covariates (intervention weeks, training frequency, initial fitness level, active muscle mass while training, disease status, sex, and age), all training intensity categories displayed significant increases in mitochondrial content (ET, 22.7 ± 4.6%; HIT, 27.0 ± 5.1%; SIT, 27.0 ± 6.7%; all P < 0.001).
On capillaries: capillaries per fiber increased similarly with ET (15 ± 3%), HIT (13 ± 4%) and SIT (10 ± 11%), while capillaries per mm² only increased after ET (13 ± 3%) and HIT (7 ± 4%), with increases being larger after ET compared with HIT and SIT.
Critically, the paper's own emphasis runs against the post's framing: higher training frequencies (6 > 4 > 2 sessions/week) were associated with larger increases in mitochondrial content, and per total hour of exercise, SIT was about 2.3 times more efficient in increasing mitochondrial content than HIT and about 3.9 times more efficient than ET, while HIT was about 1.7 times more efficient than ET. The stated conclusion is that the magnitude of change in mitochondrial content, capillarization, and VO2max is largely determined by the initial fitness level, with greater changes observed in individuals with lower initial fitness, and the ability to adapt is maintained throughout life irrespective of sex and presence of disease.
Findings
✓ What's accurate 6
- The citation is genuine and accurate. PMID 39390310, correct authors, correct journal, correct year.
- "353 studies" is correct for the mitochondrial synthesis, and "~6,000 people" is a fair rounding of the 5973 participants reported across the review.
- "23% increase in mitochondrial content" matches the reported ET estimate of 23 ± 5% (22.7 ± 4.6% in the full model).
- "~13% gains in capillary density" matches the reported ET increase in capillaries per mm² of 13 ± 3%.
- The general premise that low- to moderate-intensity continuous endurance training meaningfully increases mitochondrial content markers and capillarization in human skeletal muscle is supported by the paper.
- The associated idea that training frequency matters is directionally supported, since higher frequencies were associated with larger increases.
≈ What's misleading 6
- Fabricated dose attribution (omitted qualifier plus unsupported specificity). The post states the 23% was produced by "training programs built around ~4 weekly sessions and 3–5 hours of easy aerobic work." The paper's 23% is a covariate-adjusted marginal mean across hundreds of heterogeneous protocols, explicitly calculated at the mean of continuous covariates. No dose of "four sessions and three to five hours" is presented in the abstract or results as the condition that produced 23%.
- Selective intensity framing (omitted comparator). The post presents the finding as evidence for "easy" aerobic work specifically. The paper found HIT and SIT produced numerically larger increases (27% each) with no statistically significant difference between categories, and found ET to be the least time-efficient of the three per hour trained. Presenting the ET number in isolation reverses the paper's comparative message.
- Frequency claim points the wrong way. The paper reports that 6 sessions per week beat 4, which beat 2. The post converts this into an endorsement of "~4 weekly sessions" as the effective dose, dropping the finding that more was better.
- Omitted moderator (subgroup generalization). The paper's headline conclusion is that effect size is largely determined by baseline fitness, with the largest gains in the least fit, and that capillary gains were seen only in untrained to moderately trained participants. The post presents 23% and 13% as if they were general expectations for any reader.
- Marker versus organelle. "Mitochondrial content" here is a composite of surrogate biochemical markers, mostly enzyme activities. "Measurably reshape cellular infrastructure" overstates what the pooled proxy establishes.
- Commercial context. The graphic is branded content from a supplement account, and the caption routes readers to a link in bio. The cited research is about exercise and has no bearing on any product.
? What's uncertain 5
- The "3–5 hours" weekly volume figure. I could not retrieve Table 1 of the paper, which contains the training characteristics of included studies. It is plausible that ET studies averaged something in this range, but I could not verify it, and in any case the 23% estimate is not conditioned on it.
- The "45–75 minute sessions" recommendation. Not found in the material retrieved. It appears to be the poster's own extrapolation rather than a study finding.
- The "~10% VO₂max" figure. The abstract confirms that VO2max increased similarly with ET, HIT, and SIT, with HIT tending toward greater improvement, but I could not retrieve the specific ET percentage value to confirm or refute "about 10%."
- The causal chain asserted in the caption, that "new mitochondria only reach full capacity when oxygen delivery scales with them," is a mechanistic framing. I did not find it stated as a finding of this paper.
- Search budget was exhausted before Table 1 and the VO2max results section could be retrieved directly.
Sources
3 of 3 linked to recordsMølmen KS, Almquist NW, Skattebo Ø. "Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle: A Systematic Review and Meta-Regression." Sports Med. 2025 Jan;55(1):115-144. PMID 39390310, PMCID PMC11787188, DOI 10.1007/s40279-024-02120-2
Springer Nature publisher record of the same article (full text, methods and results sections)
PubMed record and University of Copenhagen research portal entry (abstract, bibliographic confirmation)