In plain terms: Zone 2 aerobic training has strong evidence from human clinical trials. The benefit shown in those trials is if that describes your situation.
Zone 2 aerobic training
The evidence as the record states it
Gormley et al. (Med Sci Sports Exerc, 2008; PMID 18580415): 61 healthy adults randomized to moderate (50 percent of VO2 reserve), vigorous or near-maximal cycling or a non-exercising control for 6 weeks; VO2max rose 10.0, 14.3 and 20.6 percent. Church et al. (JAMA, 2007; PMID 17507344), the DREW trial: 464 sedentary postmenopausal women randomized to a non-exercise control or three doses of physical activity at 50 percent of peak VO2 for 6 months; cardiorespiratory fitness (peak oxygen consumption) rose 4.2, 6.0 and 8.2 percent against control. Nuuttila et al. (Scand J Med Sci Sports, 2026; PMID 41543030): meta-analysis of 50 randomized trials of low-intensity endurance training below the first threshold; large effect on VO2max, with higher intensity giving larger gains. Grade A: replicated instrument-measured VO2max.
Why Grade A and not higher
Grade A requires a hard clinical endpoint or objectively measured functional outcome, independently replicated across at least two trials. Zone 2 aerobic training meets that standard based on the trials cited above. A downgrade to Grade B would follow if replication attempts produced null results, if the effect size proved substantially smaller in a broader population, or if the studied dose proved impractical to sustain.
What this grade covers and what it does not
What would move this grade
A null result in a well-powered independent replication trial, or evidence that the effect does not hold at a practically sustainable dose, would prompt downgrade to Grade B.
How this was studied
Form: Supervised aerobic training on a cycle ergometer at moderate intensity, 50 percent of VO2 reserve (Gormley 2008, PMID 18580415); aerobic exercise at the heart rate for 50 percent of peak VO2 (Church 2007, PMID 17507344); low-intensity endurance training below the first lactate or ventilatory threshold, or at most 60 percent of heart rate reserve or VO2max, or at most 75 percent of maximum heart rate (Nuuttila 2026, PMID 41543030). · Dose: 60 minutes, 4 days a week in the moderate group during the final 4 weeks (Gormley 2008, PMID 18580415); 4, 8 or 12 kcal/kg per week, a mean of 72, 136 or 192 minutes per week (Church 2007, PMID 17507344); doses across the pooled trials not stated in the cited abstracts (Nuuttila 2026, PMID 41543030). · Population: 61 healthy young adults matched for sex and VO2max, 55 completing (Gormley 2008, PMID 18580415); 464 sedentary, overweight or obese postmenopausal women with systolic pressure 120 to 159.9 mm Hg (Church 2007, PMID 17507344); healthy adults aged 18 to 65, 50 trials with 824 participants in the training groups (Nuuttila 2026, PMID 41543030). · Duration: 6 weeks (Gormley 2008, PMID 18580415); 6 months (Church 2007, PMID 17507344); at least 3 weeks per trial (Nuuttila 2026, PMID 41543030).