Effects of Low-Carbohydrate and Ketogenic Diets on Aerobic Performance in Trained Athletes: A Systematic Review and Meta-Analysis

ckground/Objectives: While traditional sports nutrition emphasizes high carbohy- drate intake for endurance athletes, trained athletes may achieve metabolic adaptation to low-carbohydrate and ketogenic diets with maintained or improved performance out- comes. This systematic review and meta-analysis synthesize evidence on the effects of low-carbohydrate (≤130 g·day −1 or≤25% total energy) and ketogenic (<50 g·day −1 or <10% total energy) diets on aerobic performance variables in trained athletes. Methods: A comprehensive search of five electronic databases (PubMed, SCOPUS, Web of Science, SPORTDiscus, and Cochrane Central Register of Controlled Trials) identified 33 aerobic- focused studies meeting comprehensive inclusion criteria. Selected studies examined trained athletes (≥6 months structured training, age 18–45 years) randomized to low- carbohydrate, ketogenic, or high-carbohydrate control conditions with outcome data on aerobic performance variables (VO2max, time trial performance, time to exhaustion, and exercise economy) and metabolic markers (fat oxidation and substrate utilization). Quality assessment employed Newcastle-Ottawa Scale methodology. Results: Maximal aerobic capacity (VO2max) was preserved in 50.0% of studies, with 11.1% documenting improve- ments. Submaximal exercise economy showed the greatest sensitivity, with 50.0% docu- menting impaired efficiency. Time to exhaustion demonstrated context-dependent effects, with 69.2% maintaining performance. All 30 studies measuring fat oxidation demonstrated consistent increases (+28% to +200%). Critically, temporal analysis identified a 1-week adap- tation threshold: studies measuring outcomes within≤7 days documented performance impairment, while studies measuring at >1 week consistently demonstrated maintained or improved performance. Conclusions: Low-carbohydrate diets reliably induce metabolic adaptation characterized by dramatically increased fat oxidative capacity. However, aerobic performance responses

performance. All 30 studies measuring fat oxidation demonstrated consistent increases (+28% to +200%). Critically, temporal analysis identified a 1-week adap- tation threshold: studies measuring outcomes within≤7 days documented performance impairment, while studies measuring at >1 week consistently demonstrated maintained or improved performance. Conclusions: Low-carbohydrate diets reliably induce metabolic adaptation characterized by dramatically increased fat oxidative capacity. However, aerobic performance responses are nuanced, with preserved maximal aerobic power, transient submaximal efficiency impairments, and context-dependent endurance effects. Adapta- tion involves initial acute-phase decrements (≤7 days) followed by recovery. Evidence supports periodized carbohydrate strategies balancing metabolic adaptation benefits from low-carbohydrate training phases with carbohydrate requirements during competition. Keywords:fat oxidation; metabolic adaptation; trained athletes; exercise economy; substrate utilization; time to exhaustion; temporal adaptation threshold Nutrients2026,18, 740 https://doi.org/10.3390/nu18050740

Nutrients2026,18, 740 2 of 51 1. Introduction Dietary macronutrient composition has long been recognized as a critical determinant of athletic performance capacity in trained individuals [1,2]. Traditional sports nutrition paradigms have emphasized high carbohydrate intake (typically 6–10 g·kg −1 · day −1 ) as a fundamental requirement for optimizing performance in endurance and high-intensity exercise, based on extensive evidence demonstrating that endogenous muscle and hepatic glycogen stores are crucial substrates for ATP resynthesis during moderate-to-high intensity exercise exceeding 70% VO2max [3–5]. This carbohydrate-centric nutritional framework has been formalized in contemporary sports nutrition guidelines by major professional organizations, establishing high carbohydrate availability as the standard recommendation for competitive athletes [1,5]. However, emerging scientific evidence and practical adoption among elite athletes have challenged this conventional paradigm, demonstrating that trained athletes may adapt to sustained low-carbohydrate dietary interventions with maintained or improved performance outcomes in specific contexts [6–9]. The physiological capacity of trained skeletal muscle to shift substrate oxidation patterns in response to dietary macronutrient availability, a phenomenon termed “metabolic flexibility”, has become an increasingly recognized adaptation mechanism in sports physiology [3,10]. This capacity reflects the fundamental plasticity of mitochondrial enzymatic systems, which can be substantially remodeled through sustained dietary interventions or training manipulations to enhance fat oxidative capacity and reduce carbohydrate dependence [3,11,12]. Low-carbohydrate dietary interventions are defined as dietary patterns that restrict carbohydrate intake to≤25% of total energy intake or≤130 g per day. Ketogenic diets are characterized by severe carbohydrate restriction (less than 50 g per day or less than 10% of total energy intake) and are designed to induce physiological ketosis. These diets operate through distinct metabolic mechanisms that fundamentally reorganize cellular energy substrate utilization [13–15]. These interventions bypass the traditional glucose-glycogen- pyruvate-lactate cycle that dominates carbohydrate-replenishment conditions, instead engaging fat oxidative pathways to a substantially greater degree [7,8]. The mechanistic basis for considering low-carbohydrate and ketogenic diets in athletic populations rests on several theoretical presumptions: enhanced fat oxidation capacity through mitochondrial enzyme upregulation including carnitine palmitoyl transferase-I (CPT-I), hydroxyacyl-CoA dehydrogenase (HADH), and acyl-CoA oxidase-1 (ACOX-1), enabling sustained ATP gener- ation from lipid substrates [3,10,12]; reduced reliance on limited glycogen

fat oxidative pathways to a substantially greater degree [7,8]. The mechanistic basis for considering low-carbohydrate and ketogenic diets in athletic populations rests on several theoretical presumptions: enhanced fat oxidation capacity through mitochondrial enzyme upregulation including carnitine palmitoyl transferase-I (CPT-I), hydroxyacyl-CoA dehydrogenase (HADH), and acyl-CoA oxidase-1 (ACOX-1), enabling sustained ATP gener- ation from lipid substrates [3,10,12]; reduced reliance on limited glycogen stores, potentially benefiting very long-duration endurance efforts (>4 h) where glycogen depletion otherwise becomes performance-limiting [7,8,16]; diminished exercise-induced metabolic pertur- bation and inflammation through reduced carbohydrate-induced insulin responses and postprandial hyperglycemia [17–19]; as well as preserved or enhanced body composition through reduced fat mass and maintained or improved lean tissue mass [6,13,14]. The transition from carbohydrate-dependent to fat-dependent energy provision re- quires substantial metabolic reorganization encompassing mitochondrial remodeling, enzy- matic adaptations, and alterations in muscle fiber-type-specific substrate preferences. The temporal characteristics of this adaptive process have been the subject of considerable scien- tific debate. Early mechanistic investigations demonstrated that substantial increases in fat oxidative enzyme activity (CPT-I, HADH, and ACOX-1) occur within 7–14 days of dietary carbohydrate restriction, with maximum adaptive responses manifesting by 4–6 weeks of sustained intervention [3,11,12]. Mitochondrial structural changes, including increased mitochondrial density and cristae elaboration, have similarly been documented across this 4–6-week timeframe [3,10]. However, the time course of performance consequences of these metabolic adaptations, and critically, whether initial transient performance impairment https://doi.org/10.3390/nu18050740

Nutrients2026,18, 740 3 of 51 during early adaptation phases is followed by recovery or sustained deficit, has remained incompletely characterized in the performance physiology literature. The published literature examining low-carbohydrate and ketogenic diet effects on trained athletes’ performance demonstrates striking heterogeneity in reported findings, cre- ating substantial uncertainty for practitioners attempting to evaluate intervention efficacy. Multiple investigations documenting high-quality randomized controlled trials or crossover designs have reported substantial performance impairments with low-carbohydrate diets, including reduced exercise economy (increased VO2cost of submaximalexercise) [20–22], impaired time trial performance [23,24], and compromised high-intensity interval train- ing capacity [20,23,25]. Notably, elite athlete populations, including competitive race walkers, have shown pronounced performance deficits of 8% reduced interval training pace [23] and 2.3% slower completion time of 10,000 m race walking [20] when consuming low-carbohydrate, high-fat diets. Furthermore, investigations employing shorter adap- tation periods have documented performance impairments that were not subsequently reversed [20–22]. Conversely, other rigorous investigations employing comparable or longer inter- vention periods have documented performance maintenance or improvement with low- carbohydrate and ketogenic diets [6–9,26,27], particularly in ultra-endurance contexts where very long exercise duration (9–36 months of habitual ketogenic diet consump- tion) [7,8], metabolic flexibility optimization [26,27], and perhaps superior fat oxidation capacity enable sustained performance or performance gains [6,7]. Several mechanistic in- vestigations have confirmed robust increases in fat oxidation rates (ranging from 34% to 2.3- fold higher increases) following low-carbohydrate dietary interventions [3,4,7,10,16,28,29], establishing that the fundamental metabolic substrate shift occurs consistently and pre- dictably across populations [3,11,12]. This apparent contradiction between comprehensive evidence of metabolic adapta- tion (universally documented fat oxidation increases) and highly variable performance responses (ranging from impairment to improvement) suggests that performance out- comes depend substantially on intervention characteristics, adaptation duration, individual metabolic responsiveness, or measurement timing relative to adaptive processes. The observation that some investigations measuring performance at 3–4 weeks document impairment [20,21], while others evaluating at 6–12 weeks show maintenance or improve- ment [6,27], suggests that a critical temporal threshold may separate acute perturbation from chronic adaptation and recovery [8,20,21]. Despite these clinical and research ques- tions, no comprehensive review and meta-analysis synthesizing findings across

relative to adaptive processes. The observation that some investigations measuring performance at 3–4 weeks document impairment [20,21], while others evaluating at 6–12 weeks show maintenance or improve- ment [6,27], suggests that a critical temporal threshold may separate acute perturbation from chronic adaptation and recovery [8,20,21]. Despite these clinical and research ques- tions, no comprehensive review and meta-analysis synthesizing findings across diverse interventions, populations, and evaluation time points has been conducted to systematically characterize the magnitude, temporal characteristics, and predictors of low-carbohydrate diet effects on the performance of trained individuals. Emerging evidence suggests substantial interindividual variability in response to low- carbohydrate and ketogenic dietary interventions, with particular populations showing marked response heterogeneity [6,22,25,26,30]. Several investigations have documented divergent performance responses ranging from substantial improvement to severe im- pairment within single-study cohorts [6,7,22], suggesting that population-level summary statistics may obscure clinically meaningful individual differences. Proposed mechanisms underlying individual variability include genetic variation affecting mitochondrial function and fat oxidative enzyme expression [7,22], baseline metabolic health status, including glucose homeostasis and insulin sensitivity [26,31], autonomic nervous system charac- teristics reflecting cardiorespiratory and metabolic stress responses [30], and individual tolerance to metabolic perturbation including gastrointestinal symptoms and subjective well-being [6,22,30]. The potential identification of predictive biomarkers or phenotypes https://doi.org/10.3390/nu18050740

Nutrients2026,18, 740 4 of 51 that stratify athletes into responder categories (optimal, adequate, poor responders) rep- resents a promising avenue for precision metabolic medicine approaches to dietary in- tervention personalization [22,26,30]. However, there is limited evidence from research studies on how individuals respond to low-carbohydrate diets. Only five such studies have been published to date. Inconsistent methodology and outcome measurement prevent the quantitative synthesis required to draw firm conclusions regarding response prediction or responder phenotyping. The low-carbohydrate and ketogenic diet literature is subject to potential publication bias concerns, as researchers with positive findings regarding dietary intervention efficacy may have greater incentive to publish favorable results, while null or negative findings may be suppressed from publication [32,33]. Additionally, funding sources, including diet-related industries, may introduce bias toward favorable outcomes [9,34]. However, previous systematic reviews of low-carbohydrate interventions in athletic populations have not formally evaluated publication bias risk or conducted sensitivity analyses examining ef- fect stability across varying methodological quality tiers or study designs. The high-profile nature of low-carbohydrate diet adoption among elite athletes, combined with intense public and media interest, may paradoxically reduce publication bias by increasing visibil- ity of negative findings, including those documenting performanceimpairments [20,23]; however, this remains empirically unexamined in the systematic review context. A critical and largely unresolved question in the low-carbohydrate diet literature concerns the time course of performance adaptation. Do studies measuring perfor- mance at different time points relative to dietary intervention initiation (ranging from 2 days to 36 months) capture distinct phases of metabolic and performance adaptation with systematically different outcomes? Several individual investigations suggest per- formance impairment during acute adaptation phases (≤7 days) followed by recovery of 3–4 weeks [20,21,35], but comprehensive temporal trajectory analysis across multiple investigations remains absent from the literature. The potential identification of criti- cal adaptation thresholds (e.g., one-week time point) separating acute perturbation from chronic adaptation would have substantial implications for research design, athlete coun- seling, and evidence interpretation. Given the substantial heterogeneity in published findings, limited evidence regarding individual variability and response prediction, unexamined publication bias concerns, and inadequately characterized temporal adaptation trajectories, a comprehensive synthesis of

The potential identification of criti- cal adaptation thresholds (e.g., one-week time point) separating acute perturbation from chronic adaptation would have substantial implications for research design, athlete coun- seling, and evidence interpretation. Given the substantial heterogeneity in published findings, limited evidence regarding individual variability and response prediction, unexamined publication bias concerns, and inadequately characterized temporal adaptation trajectories, a comprehensive synthesis of the available literature is warranted. Accordingly, this systematic review and meta-analysis integrate evidence across diverse low-carbohydrate and ketogenic diet interventions, popu- lations, measurement approaches, and adaptation phases. This approach aims to establish evidence-based conclusions regarding performance effects, identify critical moderating variables, characterize response heterogeneity, and guide future research prioritization. The primary objectives of this systematic review and meta-analysis were to: (1) quan- tify performance effects of low-carbohydrate (≤130 g·day −1 or≤25% energy) and ketogenic (<50 g·day −1 or <10% energy) diets on aerobic performance variables (maximal oxygen uptake, exercise economy, time to exhaustion, time trial performance) in trained athletes (≥6 months structured training experience, age 18–45 years); (2) document metabolic substrate utilization shifts (fat oxidation rates, respiratory exchange ratios, and carbo- hydrate oxidation) accompanying low-carbohydrate dietary adaptation to characterize physiological mechanisms underlying performance responses; (3) identify critical temporal adaptation thresholds by stratifying findings according to intervention duration (acute ≤7 days vs. early chronic 8–31 days vs. mid-chronic 4–12 weeks vs. extended chronic >12 weeks) to determine whether distinct adaptation phases produce systematically differ- ent performance outcomes; (4) characterize individual variability in performance responses https://doi.org/10.3390/nu18050740

Nutrients2026,18, 740 5 of 51 by synthesizing evidence from studies explicitly examining interindividual heterogene- ity, identifying predictive biomarkers (heart rate variability, glucose homeostasis, genetic variants) or responder phenotypes; (5) conduct comprehensive quality assessment and bias evaluation including Newcastle-Ottawa Scale quality ratings, publication bias testing (Egger’s regression, Begg’s test, trim-and-fill analysis), and sensitivity analyses examining findings stability across quality tiers, study designs, and sample sizes; and (6) to formulate evidence-based recommendations for athletes, coaches, and practitioners regarding low- carbohydrate diet implementation timing, expected performance trajectories, individual response considerations, and research priorities. This systematic review and meta-analysis were conducted following the Preferred Reporting Items for Systematic Reviews and Meta- Analyses (PRISMA 2020) guidelines to ensure transparency, rigor, and reproducibility of evidence synthesis and to provide comprehensive characterization of low-carbohydrate and ketogenic diet effects on trained athlete performance. 2. Methods 2.1. Study Design and Search Strategy This systematic review and meta-analysis were conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guide- lines [36]. The review protocol was prospectively registered in the PROSPERO database (CRD420261277181) under the title “Effects of Low-Carbohydrate and Ketogenic Diets on Aerobic Performance in Trained Athletes: A Systematic Review and Meta-Analysis”. A comprehensive semantic search was conducted across five major electronic databases: PubMed/MEDLINE, Web of Science, Scopus, SPORTDiscus, and the Cochrane Central Register of Controlled Trials. Additionally, preprint repositories (bioRxiv, medRxiv) and gray literature sources were screened for relevant unpublished data. However, no preprints meeting our inclusion criteria were identified, and all 33 studies included in the final analysis were peer-reviewed publications. Additionally, preprint repositories (bioRxiv, medRxiv) and gray literature sources were screened for relevant unpublished data. Our team was employed to retrieve the 500 papers most relevant to the research question on low-carbohydrate and ketogenic diets and exercise performance from across 850 academic papers indexed in the database, after which studies were filtered to retain only those reporting aerobic performance and metabolic outcomes in trained athletes. The semantic search query employed standardized Boolean operators and Medical Subject Headings (MeSH) and free-text keywords in various combinations: (“ketogenic diet” OR “low-carbohydrate diet” OR “LCHF” OR “keto

ketogenic diets and exercise performance from across 850 academic papers indexed in the database, after which studies were filtered to retain only those reporting aerobic performance and metabolic outcomes in trained athletes. The semantic search query employed standardized Boolean operators and Medical Subject Headings (MeSH) and free-text keywords in various combinations: (“ketogenic diet” OR “low-carbohydrate diet” OR “LCHF” OR “keto diet” OR “low- CHO” OR “low-carb”) AND (“athletic performance” OR “exercise performance” OR “VO2max” OR “maximal oxygen uptake” OR “lactate threshold” OR “time to exhaustion” OR “running economy” OR “cycling economy”) AND (“athletes” OR “trained” OR “en- durance” OR “competitive” OR “elite”) AND (“randomized” OR “RCT” OR “crossover” OR “trial” OR “intervention”). Boolean operators (AND, OR) were employed to refine search specificity. Database filters restricted results to peer-reviewed studies published in the English language. Manual screening of reference lists from included studies and relevant reviews was performed to identify additional eligible investigations. The search strategy was conducted through October 2025. 2.2. Eligibility Criteria and Study Selection 2.2.1. Population Studies were included if they enrolled trained athletes, defined as individuals pos- sessing≥6 months of regular structured training experience in their respective sport or https://doi.org/10.3390/nu18050740

Nutrients2026,18, 740 6 of 51 discipline. The age range was restricted to 18–45 years to ensure applicability to adult populations. No restrictions were placed on participant sex, and studies including male- only, female-only, or mixed-sex cohorts were eligible for inclusion. Minimum baseline training characteristics included documented competitive participation, training histories, or VO2max values indicating trained status (VO2max > 40 mL·kg −1 · min −1 for endurance athletes). Studies enrolling untrained individuals, recreational exercisers, or sedentary populations were excluded. Studies including mixed populations (trained and untrained) were excluded if separate data for trained subgroups could not be extracted. 2.2.2. Intervention (I) Studies were included if they examined low-carbohydrate dietary interventions de- fined as≤130 g·day −1 total carbohydrate intake or≤25% energy from carbohydrates, or ketogenic diet interventions defined as <50 g carbohydrates·day −1 or <10% energy from carbohydrates with confirmed physiological ketosis (β-hydroxybutyrate≥0.5 mmol·L −1 ). Intervention duration required≥3 days for acute dietary manipulations or≥4 weeks for chronic adaptation protocols. Studies documenting dietary compliance through objective measures (food diaries, urinary ketones, serum ketone bodies, and macronutrient analysis) or well-documented adherence protocols were included. Studies involving supplemen- tation combined with dietary intervention, training modifications concurrent with diet manipulation, or other confounding co-interventions were excluded if dietary effects could not be isolated. 2.2.3. Comparison Studies were included if they incorporated appropriate control or comparison conditions, including high-carbohydrate diets (>60% energy from carbohydrates or >300 g·day −1 ), balanced/mixed diets (45–55% energy from carbohydrates), or habitual dietary intake. Studies employing within-subject comparisons (crossover designs) compar- ing identical individuals under low-carbohydrate and control conditions were included. Studies without explicit control groups were excluded. 2.2.4. Outcomes Studies were included if they measured a minimum of two variables encompassing the following domains. Aerobic performance variables included maximal oxygen uptake (VO2max, VO2peak) measured in mL·kg −1 · min −1 or L·min −1 , lactate threshold power expressed in watts (W), heart rate response (beats·min −1 ), or VO2at threshold, time to exhaustion (TTE) in minutes at standardized exercise intensity, heart rate measures includ- ing resting, submaximal at defined intensity, and maximal responses, running economy or cycling economy quantified as

uptake (VO2max, VO2peak) measured in mL·kg −1 · min −1 or L·min −1 , lactate threshold power expressed in watts (W), heart rate response (beats·min −1 ), or VO2at threshold, time to exhaustion (TTE) in minutes at standardized exercise intensity, heart rate measures includ- ing resting, submaximal at defined intensity, and maximal responses, running economy or cycling economy quantified as VO2cost per unit distance, respiratory exchange ratio (RER) or respiratory quotient (RQ), and time trial performance including 5 km running distances, cycling time trials, and race walking completion times or distances. Metabolic variables included blood lactate concentration (mmol·L −1 ) measured at rest, during submaximal exercise, or at peak exercise intensity, base excess (mmol·L −1 ); fat oxidation rates quantified in g·min −1 or as a percentage of total energy oxidation; carbohydrate oxidation rates in g·min −1 or as a percentage of total energy oxidation; and muscle glycogen concentration expressed in mmol·kg −1 dry muscle mass. 2.2.5. Study Design Eligible study designs included randomized controlled trials (RCT), randomized crossover trials, parallel group designs, and repeated-measures crossover protocols. Mini- mum methodological requirements included: (1) prospective study design; (2) randomized allocation to intervention sequence (for crossover designs); (3) appropriate washout periods https://doi.org/10.3390/nu18050740

The review examines dietary impacts on trained athletes' aerobic performance.