Does an hour of daily exercise build more than half an hour?
In 13 weeks in sedentary overweight men, not clearly: fat oxidation rose with both (+1.2 vs +1.8 mg/kg FFM/min), enzymes and fat loss alike. Markers; from the abstracts.
Rosenkilde and colleagues (University of Copenhagen) randomised young, sedentary, moderately overweight men to 13 weeks of no exercise or to daily aerobic exercise expending 300 kcal (MOD, about 30 min) or 600 kcal (HIGH, about 60 min).
The 2012 report (61 randomised: 18 control, 21 MOD, 22 HIGH) looked at fat loss and energy balance: body weight fell 3.6 kg in MOD and 2.7 kg in HIGH, fat mass 4.0 and 3.8 kg, and the energy balance calculated from body composition (−39.6 vs −34.3 Mcal) did not differ, although HIGH had expended twice the exercise energy.
Against expectation, MOD's deficit was 83% larger than its exercise energy predicted and HIGH's 20% smaller; measured energy intake and non-exercise activity did not change enough to explain either.
The 2015 report (50 men, given in its abstract as randomised: 15 control, 18 MOD, 17 HIGH) looked at the capacity to burn fat during exercise: peak fat oxidation, measured by indirect calorimetry, rose by 1.2 mg/kg fat-free mass/min in MOD and 1.8 in HIGH against controls; citrate synthase, β-hydroxyacyl-CoA dehydrogenase and mitochondrial oxidative-phosphorylation complexes II-V rose similarly with both doses; and in a stepwise regression the gain in peak fat oxidation went with gains in cycling efficiency, fat-free mass and VO2peak rather than with the enzyme changes.
Our reading: in untrained men, doubling daily exercise time from about 30 to about 60 min did not clearly double any of the measured adaptations; the abstracts give no between-dose test for fat oxidation and no VO2peak figures, and the intensity is unknown, so this is a signal of diminishing returns in markers, not a rule for athletes.
Protocol
Key Results
| Outcome (change vs control unless noted) | MOD, 300 kcal/day (~30 min) | HIGH, 600 kcal/day (~60 min) |
|---|---|---|
| Peak fat oxidation, mg/kg FFM/min (2015) | +1.2 (95% CI 0.08 to 2.3), P = 0.03 | +1.8 (0.6 to 2.9), P < 0.001 |
| Citrate synthase, β-HAD, OXPHOS complexes II-V (2015) | increased | increased "similarly" |
| Body weight, kg, within group (2012) | −3.6 (P < 0.001) | −2.7 (P = 0.01) |
| Fat mass, kg, within group (2012) | −4.0 (P < 0.001) | −3.8 (P < 0.001) |
| Accumulated energy balance from body composition, Mcal (2012) | −39.6 | −34.3 (not different from MOD) |
| Energy balance vs that expected from exercise (2012) | 83% more negative than expected | 20% less negative than expected |
Mechanisms
Practical Application
Limitations
How this entry was checked and changed
(from the abstracts) — randomised with a no-exercise control: 61 randomised in the 2012 report and 50 given as randomised (15/18/17) in the 2015 report. The 30-vs-60-min comparison rests on 35 men (18 MOD, 17 HIGH), the rubric's B+ for a parallel RCT of 20-39; held two steps lower, to the rubric's B-, because the outcomes are markers (peak fat oxidation in a laboratory test, muscle enzyme protein, body composition), not performance, and the abstracts give neither the training intensity nor the mode, nor why the 2015 report gives 50 where the 2012 report gives 61. Closed access; no open copy via Europe PMC or OpenAlex, and the publisher page for the 2012 report returned a bot check (not bypassed)