Response: Could a Short Incremental Test Be a Valid Cardiorespiratory Efficiency Assessment Tool in Obese Adults? A Comparison between Two Exercise Testing Protocols (J Obes Metab Syndr 2025;34:282-90)

We thank the authors for their thoughtful letter regarding our article, “Could a short incremental test be a valid cardiorespiratory efficiency assessment tool in obese adults? A comparison between two exercise testing protocols,” published in the Journal of Obesity & Metabolic Syndrome.¹ We appreciate their interest in our work and the opportunity to clarify the rationale for our study and further discuss the clinical and practical implications of maximal fat oxidation intensity (Fat_max) testing in populations with obesity.

As summarized in the letter, our study compared two submaximal incremental treadmill protocols that differed only in stage duration (2 minutes vs. 3 minutes). Although maximal fat oxidation (MFO) was statistically higher with the shorter protocol, heart rate (HR), oxygen uptake (VO₂), and ratings of perceived exertion (RPE) at the intensity corresponding to Fat_max were similar between protocols.¹ This finding represents the central outcome of our investigation.

We fully acknowledge that our study was cross-sectional and was not designed to evaluate long-term clinical outcomes. However, the objective was not to determine whether differences in absolute fat oxidation rates translate into superior metabolic or anthropometric adaptations over time. Rather, our aim was to determine whether two nearly identical testing protocols, differing only in workload-stage duration, would yield comparable physiological markers of exercise intensity at Fat_max, because these markers are primarily used for exercise prescription in clinical and applied settings.

From this perspective, the key result is that exercise intensity at Fat_max, expressed through HR and RPE, did not differ between protocols. Consequently, the exercise intensity prescribed for individuals with obesity would be essentially the same whether the 2- or 3-minute protocol is used. The only variable that differed was MFO, expressed as grams of fat oxidized per minute (g/min). Although this quantitative difference reached statistical significance, its magnitude was modest and, as discussed in our original paper, is unlikely to be clinically meaningful when considered in isolation. This distinction is consistent with the current literature, which indicates that MFO values are sensitive to protocol design and other methodological factors, whereas Fat_max is better conceptualized as a physiological exercise intensity rather than simply a numerical value.^2-4

The relevance of this distinction has been well documented. Fat_max and MFO show day-to-day variability, even under controlled laboratory conditions, and absolute MFO values can be influenced by factors such as stage duration, prior nutritional status, and individual metabolic characteristics.⁴ Therefore, anchoring exercise prescription to robust and reproducible intensity markers, such as HR and perceived exertion at Fat_max, represents a physiologically sound approach, particularly in populations with reduced exercise tolerance, such as individuals with obesity. This point is also conceptually related to the second issue raised in the letter.

The second point raised in the letter concerns the translation of Fat_max testing into real-world practice, particularly through wearable technologies. We agree that this is a promising and timely direction. The widespread availability of HR monitors and consumer-grade fitness trackers offers valuable opportunities to monitor exercise intensity outside the laboratory.

Several studies have shown that wearable devices can provide reasonably accurate HR measurements across a wide range of exercise intensities and in diverse populations, including individuals with overweight and obesity.^5,6

At the same time, it is necessary to recognize what these devices can and cannot do. While HR can be directly and continuously monitored using wearable technology, MFO cannot be measured outside a laboratory environment except in field testing with wearable metabolic systems, otherwise it can only be estimated indirectly. Consequently, fitness trackers may be useful for monitoring adherence to prescribed intensity zones, providing feedback, and supporting motivation and engagement, rather than as instruments for assessing metabolic outcomes.

In this context, our findings support the use of HR for exercise monitoring. Previous work from our group showed that, in women with class I, II, and III obesity, Fat_max corresponds to the individual ventilatory threshold (IVT), suggesting that prescribing exercise at the IVT may improve cardiorespiratory efficiency and fat oxidation.⁷ Moreover, when laboratory testing is not feasible, predictive equations based on field tests, such as the 6-minute walking test, have been developed to estimate HR at the IVT, particularly in populations with obesity. This approach may represent a valid alternative for exercise prescription.⁸

These findings suggest that wearable devices can play an important role in monitoring individualized exercise programs for individuals with obesity, especially in home-based settings and remote coaching. However, accurate determination of metabolic parameters such as MFO requires laboratory-based assessment or direct measurement. Thus, wearable technologies should be used as complementary tools to promote adherence and support exercise monitoring, rather than as substitutes for physiological or functional evaluation.

Finally, we appreciate the authors’ comments and the opportunity to further clarify the aim and implications of our study. The main finding of our work is that two submaximal incremental protocols that differ only in stage duration (2 minutes vs. 3 minutes) identify the same physiological parameters of exercise intensity at Fat_max, despite differences in absolute MFO values. This emphasizes that the observed MFO difference is a protocol-dependent variation rather than a clinically meaningful difference in exercise prescription.

From a clinical perspective, this finding indicates that the two protocols may be used interchangeably for prescribing exercise in adults with obesity, because HR and perceived exertion—the variables used to personalize training intensity—were the same at Fat_max in both protocols. Although longitudinal studies are needed to determine the long-term effects of individualized interventions for weight loss and exercise monitoring, our findings suggest that shorter exercise testing protocols may reduce the risk of injury or excessive burden during assessment in this population without compromising the quality of exercise prescription. Laboratory assessment and direct measurement remain the gold standard for accurate metabolic characterization, whereas wearable technologies may provide valuable support for monitoring and promoting adherence to physical exercise.