TRAINING INSIGHTS
Cardio for Weight Loss and Body Composition
Cardio Pt.3
By Shane O'Dwyer
If you’ve been following this series, it won’t surprise you that there’s a lot of misinformation about cardio and weight loss. One persistent claim is that cardio barely matters for fat loss — a view often held by physique athletes who weigh their meals to the gram but skip the stairmaster in favor of high-volume leg days. At the other extreme are cardio purists who insist that cardio is the only real tool for weight loss, and avoid lifting anything heavier than a 5-lb dumbbell for fear of “bulking up.” In reality, the relationship between cardio and body composition is more nuanced than either camp assumes.
Why would calorie-conscious, physique-focused athletes believe cardio doesn’t help with fat loss? After all, aside from a few outliers, most of them fully subscribe to the energy balance model — the idea that weight change comes down to calories in versus calories out. If cardio increases energy expenditure, shouldn’t it produce reliable weight loss, the same way cutting calories does? As it turns out, there’s some truth buried in their skepticism.
What Research Says About Cardio and Weight Loss
A study by Broskey and colleagues (2021) examined a subgroup of participants from a larger randomized controlled trial called the E-MECHANIC study (Examination of Mechanisms of Exercise-induced Weight Compensation). The trial tracked changes in weight and total daily energy expenditure in a cohort of over 200 participants, randomly assigned to a control group, a group targeting roughly 800–1,000 kcal of exercise per week (about 8 kcal/kg/week), or a group targeting 2,000–2,500 kcal per week (about 20 kcal/kg/week), over 24 weeks. After the intervention, only the higher-exercise group lost a significant amount of body weight and fat — and even then, the weight loss was just 20–40% of what would have been predicted from that level of energy expenditure. Despite being coached on healthy eating throughout, participants appeared to compensate for the extra activity by unconsciously eating more.
In a sub-analysis, Broskey and colleagues (2021) had 42 participants from the larger trial spend a day in a metabolic chamber — a specialized, sealed room that precisely measures gas exchange and physical activity — at the start and end of the intervention. Researchers could also serve meals with precisely measured energy content, removing any uncertainty about calorie intake.
The original study suggested participants had compensated for increased exercise by unconsciously eating more; in the metabolic chamber, though, participants weren’t given any extra calories to make up for their activity. Even so, the 24-hour energy expenditure of participants in the higher-exercise group dropped by roughly 4% by the end of the study. That’s a striking finding, but don’t panic: these individuals still lost weight overall — just less than projected.
Does Cardio Make Your Body Burn Fewer Calories?
Findings like these (Broskey et al., 2021) feed a narrative that cardio tanks your metabolism and stalls weight loss. One common version of this claim is that the stress of cardio raises cortisol, which then drives fat storage — a claim that’s largely unfounded, and even less supported than the idea (addressed in Part 2) that cardio “kills gains” by stripping muscle. Still, the underlying pattern holds: cardio does burn calories, just not always as many as expected.
These results echo a separate study in which overweight and obese participants expended either 1,500 or 3,000 kcal per week via exercise, spread across five supervised sessions weekly for 12 weeks (Flack et al., 2018). Energy compensation was similar between the two groups — roughly 950–1,000 kcal — but only the higher-volume group lost a significant amount of fat mass and body weight. Unlike the Broskey findings, this study found no change in resting metabolic rate by the end of the intervention (Flack et al., 2018), though it ran for only half as long as the E-MECHANIC trial.
It’s worth noting that neither study (Broskey et al., 2021; Flack et al., 2018) could fully control participants’ food intake throughout. In an ideal world, calorie intake would be tightly controlled over a full 24-week study — but ethics boards, understandably, aren’t in the business of keeping people confined to research facilities for months at a time. Instead, researchers typically rely on nutrition education and food-recall questionnaires. These methods have known limitations, and researchers are well aware of them, but they still provide reasonably reliable estimates of intake over time — including shifts in eating that occur in response to more exercise. So while we can’t fully rule out increased eating as an explanation, it doesn’t appear to be the whole story. Something else seems to be driving an adaptive drop in energy expenditure — beyond what you’d expect just from losing body mass.
How Your Body Adapts to Cardio and Energy Expenditure
While physical activity matters for energy balance, the body has several other metabolically costly jobs that make up a larger share of daily energy use. Supplying energy to vital organs — the brain, liver, heart, kidneys — while regulating temperature and immune function are essential to staying alive, and the body prioritizes them accordingly. There’s likely some adaptive down-regulation of energy use in other tissues to help protect this budget as physical activity increases (Pontzer et al., 2016). Zooming out further: for most of human evolutionary history, energy-dense food wasn’t reliably available, and periods of scarcity or semi-starvation were common. In that context, there’s a clear evolutionary logic to capping energy expenditure within certain limits — it would have helped preserve the ability to keep hunting and foraging even when food was scarce.
As much as we’d like to trust our smartwatch telling us we just burned 600 calories on a 30-minute treadmill jog, the real number is usually lower. Actual energy expenditure tends to run meaningfully below what cardio machines and fitness trackers estimate. A 2026 study by Lee and colleagues compared fitness-tracker accuracy against gold-standard lab measurements during exercise. While today’s trackers do a reasonably good job estimating heart rate, oxygen saturation, and overall activity, they tend to meaningfully overestimate calories burned during a workout.
There’s also an efficiency effect: the more you perform a given type of exercise, the more efficient your body becomes at it, which lowers its energy cost over time. And if you’re losing weight, the energy cost of the same exercise drops further, simply because a lighter body takes less energy to move. On top of all this, Non-Exercise Activity Thermogenesis (NEAT) plays a meaningful role too. NEAT covers all the unconscious movement you do throughout the day — fidgeting, shifting position, even blinking. It tends to quietly decline as physical activity rises, and during periods of sustained dieting or low energy availability — another way the body adapts to protect its energy budget.
The Best Approach to Sustainable Weight Loss
If this is all starting to sound discouraging, don’t despair. Moving from a sedentary lifestyle to moderate activity can produce real weight loss even with no change in diet (Broskey et al., 2021; Flack et al., 2018; Pontzer et al., 2016). The key takeaway isn’t that cardio does nothing for weight loss — it’s that it does less than intuition suggests. That adaptive tendency was a useful survival trait in our evolutionary past; in a modern environment full of cheap, calorie-dense food, it can simply be frustrating.
Trying to lose a large amount of body weight through exercise alone, without adjusting how you eat, is generally an uphill battle. You’ll get fitter and healthier along the way, but weight loss will likely stall at some point. Pairing a moderate calorie reduction with consistent training remains the most reliable path to lasting weight loss. In practice, skipping a post-dinner dessert will usually do more for your calorie deficit than tacking on another 30 minutes of cardio. Whether that’s good news is a matter of perspective — but the most sustainable approach is clear: a calorie-controlled diet that still makes room for foods you enjoy, paired with consistent aerobic and resistance training.
References
Broskey, N. T., Martin, C. K., Burton, J. H., Church, T. S., Ravussin, E., & Redman, L. M. (2021). Effect of aerobic exercise-induced weight loss on the components of daily energy expenditure. Medicine & Science in Sports & Exercise, 53(10), 2164–2172. https://doi.org/10.1249/MSS.0000000000002689
Flack, K. D., Ufholz, K., Johnson, L., Fitzgerald, J. S., & Roemmich, J. N. (2018). Energy compensation in response to aerobic exercise training in overweight adults. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 315(4), R619–R626. https://doi.org/10.1152/ajpregu.00071.2018
Lee, T. H., Jun, D. U., Bae, J. Y., Roh, H. T., & Cho, S. Y. (2026). Comparative validity of smartwatch-derived heart rate and energy expenditure during endurance and resistance exercise. Sensors, 26(8), 2526. https://doi.org/10.3390/s26082526
Pontzer, H., Durazo-Arvizu, R., Dugas, L. R., Plange-Rhule, J., Bovet, P., Forrester, T. E., Lambert, E. V., Cooper, R. S., Schoeller, D. A., & Luke, A. (2016). Constrained total energy expenditure and metabolic adaptation to physical activity in adult humans. Current Biology, 26(3), 410–417. https://doi.org/10.1016/j.cub.2015.12.046