§ Claim under review · Study
"A 2026 meta-analysis (Vabishchevich et al., 2026) of 14 studies and 184 adults found moderate-intensity continuous aerobic exercise increases mitochondrial volume density (SMD=1.04) and VO2max (SMD=0.75), but none of the included studies actually prescribed 'Zone 2' training defined by a 2 mmol/L lactate ceiling or individualized lactate threshold testing — the prescriptions instead used percentages of HRmax, heart rate reserve, VO2peak, or peak power"
Verdict
Partially accurate but misleading
Confidence
MediumSummary
The study being cited is real. Vabishchevich and colleagues published a systematic review and meta-analysis in PLOS ONE on 2 January 2026 covering 14 studies and 184 adults, and the numbers in the post are correct: mitochondrial volume density increased with a pooled effect of 1.04 and VO2max with a pooled effect of 0.75, with the authors rating the overall certainty of evidence as very low to low. The post's main added argument is that none of the included studies used lactate based Zone 2 prescriptions, and this is partly wrong. One included study, Granata and colleagues, prescribed training at 90 to 97.5 percent of the work rate at each participant's individually tested lactate threshold, which the post itself lists on one slide while denying it in the caption. It is fair to say no study capped lactate at 2 mmol per litre, but the post then treats that as evidence the training was not Zone 2, when several prescriptions such as 60 to 70 percent of maximum heart rate fall inside common Zone 2 definitions and no lactate values were reported either way. Two further caveats the post skips: the pooled effects come from before and after changes within training groups with no control group data extracted, and the mitochondrial result rests on only 5 of the 14 studies. The paper does not compare Zone 2 against other intensities, so it cannot show that Zone 2 is overhyped, and the post ends with an offer for a paid-style free product, which is worth noting when weighing the framing.
The readings
key figures from the evidencepooled effect on mitochondrial volume density (MitoVD)
pooled effect on VO2max
participants across 14 studies in the meta-analysis
Why this verdict
Evidence
The paper is real and the headline numbers check out. It is cited as Vabishchevich V, Smith RT, Bittel AJ (2026), Markers of clinical and mitochondrial adaptation in response to moderate intensity continuous training: A systematic review and meta-analysis , published in PLoS One on 2 January 2026 (doi: 10.1371/journal.pone.0339902), received 22 August 2025 and accepted 13 December 2025 .
On sample and certainty: a total of fourteen studies (n = 184) met inclusion criteria, with an overall low to moderate risk of bias and very low to low certainty of evidence . Eligible studies assessed MICT lasting at least 2 weeks in adults, published since 2010, and collected vastus lateralis skeletal muscle biopsies pre and post chronic endurance exercise exposure .
On mitochondrial volume density: five included studies estimated the effect of MICT on mitochondrial volume density , and effect sizes were large, ranging from 0.59 to 1.63, with low heterogeneity (I² = 0%, Cochrane's Q: p = 0.78), giving a significant pooled effect estimate of 1.04 [95% CI: 0.68, 1.40] (z = 5.63, p < 0.00001) .
On VO2max: the pooled effect estimate was 0.75 [95% CI: 0.37, 1.14] (z = 3.87, p < 0.0001) . On other markers: MICT significantly increased MitoVD (p < 0.00001) and VO2max (p < 0.0001), while CS (p = 0.05) and MFN2 showed a modest increase (p = 0.01) , and the pooled citrate synthase estimate was 0.48 [95% CI: 0.01, 0.96] (z = 1.98, p = 0.05) .
On how intensity was prescribed, which is the crux of the post's argument: to establish exercise intensity, studies used measures of heart rate max (HRmax), heart rate reserve (HRR), peak power output (Wpeak), or maximal oxygen intake . Specific examples include 60–80% of HRR in one study, VO2max monitoring in Gunnarsson et al., 60–70% of HRmax in Youssef et al. using a 1-hour treadmill protocol, and 60% VO2peak in Vaccari et al. , plus two studies at 50% Wpeak, three studies at 65% Wpeak, and cycling at 70% of HRmax in Skelly et al. and Gillen et al.
However, one included study was anchored directly to an individually tested lactate threshold: Granata et al. is described as 4 weeks of cycling MICT performed 3x/week at 90–97.5% of work at the lactate threshold .
The "bigger not more" point in the post traces to a real included study: Meinild Lundby et al. (2018), "Exercise training increases skeletal muscle mitochondrial volume density by enlargement of existing mitochondria and not de novo biogenesis," Acta Physiologica 222(1) , which is reference 45 in the meta-analysis.
Findings
✓ What's accurate 8
- The paper exists, with the author, year, journal and topic exactly as stated.
- 14 studies and 184 participants: accurate.
- Mitochondrial volume density pooled effect 1.04 [0.68, 1.40], I² = 0%, p < 0.00001: accurate, based on 5 of the 14 studies.
- VO2max pooled effect 0.75 [0.37, 1.14]: accurate.
- Certainty of evidence described as very low to low: accurate.
- Intensity anchors of HRmax, HRR, VO2peak/VO2max and peak power were indeed used across the included studies, exactly as the review reports.
- Elite athletes were excluded, and vastus lateralis biopsies pre and post were an inclusion requirement.
- The "enlargement, not de novo biogenesis" point reflects a real included study (Meinild Lundby et al. 2018) accurately at the title level.
≈ What's misleading 5
- Factual error within the claim (overstatement): the claim says none of the studies used "individualized lactate threshold testing." At least one did. Granata et al. prescribed training at 90–97.5% of the work rate at the individually determined lactate threshold. That is lactate threshold testing applied to an individual. The post's own slide 4 even lists this prescription while the caption and claim assert no threshold testing occurred. The narrower statement, that no study capped lactate at 2 mmol/L, is not contradicted by anything found, but the broader statement as written is.
- Conflation of prescription method with physiological intensity: the argument treats "not prescribed by lactate" as equivalent to "not Zone 2." Several included prescriptions (70% HRmax, 60–70% HRmax, 60% VO2peak) sit inside the intensity band that most Zone 2 definitions describe, including common heart rate based five-zone models. Whether those sessions produced blood lactate above or below 2 mmol/L is unknown, not disproven. Absence of a lactate ceiling in the prescription is not evidence that the training was above LT1.
- Omitted qualifier: the pooled estimates come from single-arm pre-post changes with no sedentary control group extracted, which the authors state explicitly. The claim reports effect sizes of 1.04 and 0.75 without that caveat, and the MitoVD figure rests on only 5 studies.
- Framing beyond the source: the surrounding post concludes "Zone 2 is waaaay over hyped." The meta-analysis makes no intensity-versus-intensity comparison and did not analyse comparator arms. It cannot rank Zone 2 against anything. If anything the paper is a supportive finding for moderate continuous training. The claim itself is careful on this point, but the post it sits in is not, and the conclusion is attached to a product offer.
- Selective emphasis: the post highlights null results (PGC-1α p=0.10, TFAM p=0.08, DRP1 p=0.22) while the authors' own stated conclusion is that MICT significantly improves MFN2, CS, MitoVD and VO2max.
? What's uncertain 4
- Whether any of the 14 included studies measured or capped blood lactate during training sessions but did not use it as the prescription anchor. The review text lists prescription anchors, not all monitoring performed, and I could not retrieve the full Table 3 or each original trial protocol.
- The exact per-study lactate concentrations elicited by the prescribed intensities. Without those, neither the post nor this report can say whether the training was above or below 2 mmol/L in practice.
- Which lactate threshold definition Granata et al. used (LT1 versus a modified Dmax style threshold), which affects how close that protocol sits to a Zone 2 ceiling. Search capacity was exhausted before this could be resolved.
- Demographic claims made on slide 3 (only 3 of 14 studies included women, 8 studies used sedentary adults, 2 recruited people with obesity, no race or ethnicity reporting) could not be individually verified from retrieved text, though the review does note sex as a potential moderator warranting further research. The exclusion of elite athletes is confirmed.
Sources
4 of 4 linked to recordsVabishchevich V, Smith RT, Bittel AJ (2026). "Markers of clinical and mitochondrial adaptation in response to moderate intensity continuous training: A systematic review and meta-analysis." PLOS ONE 21(1): e0339902
Same article, publisher PDF
Meinild Lundby A-K et al. (2018), Acta Physiologica 222:e12905, cited as reference 45 in the meta-analysis