In a development that could reshape how scientists measure fat burning in older women, researchers have unveiled a new exercise test protocol specifically designed for postmenopausal females, a population whose metabolic responses to exercise have long been assessed using methods built around younger bodies. The study, published in Physiological Reports, introduces a body-mass-relative FATmax test that stretches exercise stages to four minutes and scales workload to each individual’s weight, and the results suggest that this gentler, longer approach may capture fat oxidation dynamics that traditional tests miss, particularly in women with reduced fat-burning capacity.
The science of maximal fat oxidation sits at the intersection of metabolism, aging, and exercise physiology. Every person has an exercise intensity at which their body burns fat at its highest rate, a point researchers call FATmax, and the peak value itself is termed maximal fat oxidation, or MFO. Finding these values requires an incremental exercise test paired with indirect calorimetry, in which oxygen consumption and carbon dioxide production are measured breath by breath to calculate how much fat and carbohydrate the body is using at any given moment. The classic equations, derived by Frayn in 1983, convert respiratory gas exchange into substrate oxidation rates, providing a non-invasive window into whole-body energy metabolism.
The problem, as the research team behind the new study explains, is that most FATmax protocols were developed and validated in younger or mixed populations. The traditional absolute power FATmax test, or AFT, starts everyone at the same workload of 30 watts and increases the load by 10 watts every 3 minutes and 15 seconds, regardless of body size or fitness. For a small, older woman, 30 watts may already represent a demanding relative effort, pushing her metabolism toward carbohydrate reliance before her fat-burning machinery has even had time to engage. Previous work by some of the same researchers had shown that postmenopausal women display a downward and leftward shift in their substrate oxidation curves compared with younger populations, meaning their peak fat oxidation occurs at lower workloads and lower oxygen consumption values. In addition, prior research has demonstrated that older individuals reach metabolic steady states more slowly, so the standard three-minute stages may simply be too short for their substrate oxidation kinetics to stabilize before the workload jumps again.
The menopausal transition compounds these challenges in distinctive ways. Menopause is marked by a dramatic decline in ovarian hormones, particularly 17β-estradiol and progesterone, alongside alterations in androgen availability and elevated circulating gonadotropins such as follicle-stimulating hormone and luteinizing hormone. These endocrine shifts have been linked to altered substrate utilization, increased visceral adiposity, and reductions in muscle mass and strength, all of which influence energy expenditure and metabolic flexibility, the body’s capacity to shift between fat and carbohydrate oxidation depending on fuel availability. Aging itself is associated with reduced fat oxidation rates, related to declines in mitochondrial content and respiratory capacity, and recent evidence suggests that metabolic flexibility in postmenopausal women may depend more on muscle function than on chronological age per se. Notably, studies have found that declines in mitochondrial content may be more strongly associated with physical inactivity than with aging itself, reinforcing the idea that exercise capacity, not the passage of time, drives much of the metabolic variation seen in this population.
To test whether a redesigned protocol could better serve this group, the researchers recruited 18 physically active postmenopausal women, aged over 60 and moderately active according to the International Physical Activity Questionnaire, all more than 12 months past menstruation and free from chronic disease, hormone replacement therapy, and medications affecting exercise capacity. Seventeen participants, averaging 69.2 years of age with 41.3 kilograms of fat-free mass, completed the full study. The three-day experimental design was rigorous: on day one, baseline measurements including blood pressure, oxygen saturation, anthropometry, and a five-repetition sit-to-stand test of muscle power were taken, along with familiarization on the cycle ergometer; day two included basal metabolic rate assessment by indirect calorimetry; and on days two and three the women performed both FATmax tests in randomized order, separated by at least 48 hours. Nutrition was standardized, with participants arriving after a 12-hour overnight fast following a replicated dinner containing at least 550 kilojoules of carbohydrate, and testing occurred at identical times of day to minimize intraindividual variability.
The novel protocol, called the relative FATmax test or RFT, starts each participant at 0.45 watts per kilogram of body mass and increases the load by 0.15 W/kg every 4 minutes. This means a 60-kilogram woman begins at roughly 27 watts while an 80-kilogram woman starts at 36 watts, with increments scaled proportionally, and every stage lasting 45 seconds longer than in the traditional test. The incremental tests ended when at least two safety or effort criteria were met, such as a respiratory exchange ratio exceeding 1.0, a perceived exertion above 6 on the Borg 0–10 scale, a pain score above 6, or oxygen saturation dipping below 92 percent. Oxygen uptake and carbon dioxide output were measured continuously with a K5 metabolic cart, and fat oxidation was calculated from the final 60 seconds of each stage using Frayn’s equations, with values also expressed relative to fat-free mass to allow fairer comparisons across a heterogeneous sample.
The headline finding was not a dramatic difference in the peak fat oxidation values themselves at the whole-group level, which were statistically similar between protocols at 0.30 and 0.28 grams per minute, but a systematic delay in when that peak occurred. Under the RFT, participants reached FATmax after an average of 765 seconds compared with 401 seconds under the AFT, a statistically significant difference with a medium effect size. The time to maximal carbohydrate oxidation was even more strikingly delayed, at 1553 versus 1170 seconds, with a large effect size, and heart rate during recovery was significantly lower after the RFT, suggesting the longer, gentler protocol was less physiologically stressful at its conclusion despite its longer duration.
The most revealing results emerged when the researchers split the sample into two groups based on a threshold of 0.3 grams per minute of MFO, a cut-off derived from normative data in middle-aged adults and recent mean values of 0.29 grams per minute reported in postmenopausal women. Eight women fell into the low-MFO group and nine into the high-MFO group, and despite very similar age, body composition, blood pressure, and muscle power profiles, their responses to the two protocols diverged. In the low-MFO group, the RFT yielded a higher peak fat oxidation of 0.22 versus 0.19 grams per minute, a difference that did not reach statistical significance in this small sample but carried a moderate effect size of 0.65. Crucially, this higher fat oxidation was achieved at a lower oxygen consumption of 12.8 versus 15.4 milliliters per minute per kilogram, with a moderate effect size of 0.53. In other words, the relative protocol appeared to let these women burn more fat while consuming less oxygen, a pattern the authors attribute to the more gradual progression of workload and the extended fat-predominant phase of the test.
The Bland–Altman agreement analyses added further nuance. The limits of agreement between the two protocols were relatively narrow, indicating overall consistency, but the direction of bias differed between groups. High-MFO women showed a low, negative mean bias across stages, with explained variabilities reaching 33, 59, and 89 percent in the second, third, and fourth stages, respectively, while low-MFO women displayed a small but positive mean bias that grew slightly with each stage. Violin plots of the first four stages, all below a respiratory exchange ratio of 0.90, showed that low-MFO women exhibited greater dispersion in the RFT, with lower RER values and higher fat oxidation rates during the early, fat-dominant phases. The researchers interpret this as evidence that the RFT elicits higher fat oxidation responses at lower intensities precisely in the women who need the most careful assessment.
Why does this matter? The authors argue that the longer stages and smaller, body-relative increments of the RFT create a broader fat-predominant zone, giving slower metabolic systems time to reach steady state and preventing the premature crossover to carbohydrate reliance that fixed absolute workloads can impose. The extended fat oxidation phase, roughly 6 minutes and 30 seconds longer in high-MFO women and 32 percent longer in the low-MFO group, also enables complementary physiological analyses that demand longer, more stable recordings, such as mechanical efficiency and heart rate variability, the latter an established window into autonomic nervous system function. Normalizing power to body mass additionally compensates for the large heterogeneity in body size and composition that characterizes this population, and relative muscle power is itself sensitive to age-related functional decline, making the metric doubly informative.
The authors are candid about the study’s limitations. The design changed two variables simultaneously, stage duration and increment size, so the effects cannot be attributed solely to the relative scaling of workload; workload was scaled to total body mass rather than fat-free mass, which may be more closely tied to skeletal muscle oxidative capacity; peak VO2 max was not obtained; and dichotomizing participants at a single 0.3 grams per minute threshold inevitably simplifies what is physiologically a continuum. The sample of 17, though powered a priori at nearly 80 percent for the primary paired comparison, is modest for subgroup analyses. Even so, the findings point toward a meaningful shift in how exercise metabolism should be probed in older women: not with one-size-fits-all wattages, but with protocols that respect individual body size, slower metabolic kinetics, and the extended warm-up that aging muscles require. Future work, the team suggests, should test whether scaling workload to fat-free mass further refines the assessment, potentially yielding more accurate prescriptions of the exercise intensities at which older women can best train their fat metabolism, with implications for metabolic health, body composition, and healthy aging.
Subject of Research: Comparison of a novel body-mass-relative FATmax exercise test with a traditional absolute power FATmax test for assessing maximal fat oxidation in active postmenopausal females
Subject of Research: Medicine
Article Title: Maximal fat oxidation assessment in active postmenopausal females: A novel relative power FATmax test
Article References: Monferrer‐Marín, J., Roldán, A., Helge, J. W., & Blasco‐Lafarga, C. (2026). Maximal fat oxidation assessment in active postmenopausal females: A novel relative power FATmax test. Physiological Reports, 14(13), Article e70991. https://doi.org/10.14814/phy2.70991
Image Credits: AI Generated
DOI: 10.14814/phy2.70991
Keywords: maximal fat oxidation, FATmax, postmenopausal females, indirect calorimetry, metabolic flexibility, relative power, exercise physiology, mitochondrial function, substrate oxidation, aging, cycle ergometer, VO2
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Ophelia Keating. (September 3, 2026). New relative power test measures fat burning peak in active postmenopausal women. Scienmag. https://scienmag.com/new-relative-power-test-measures-fat-burning-peak-in-active-postmenopausal-women/
Ophelia Keating. “New relative power test measures fat burning peak in active postmenopausal women.” Scienmag, 3 September 2026, https://scienmag.com/new-relative-power-test-measures-fat-burning-peak-in-active-postmenopausal-women/. Accessed 3 September 2026.
Ophelia Keating. “New relative power test measures fat burning peak in active postmenopausal women.” Scienmag. September 3, 2026. https://scienmag.com/new-relative-power-test-measures-fat-burning-peak-in-active-postmenopausal-women/
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