Abstract
ective: Recent work has challenged the notion that preferred substrate oxidation is a key determinant of exercise performance. This investigation tested middle-distance running performance, in the fed state, to control for glycogen and exercise-induced hypoglycemia (EIH) confounders. Methods: In a randomized crossover fashion, all while controlling di- etary intake, activity, and body weight, recreational distance runners completed either a 5K (n= 15; VO2max: 58.3±6.2 mL/kg/min) or a 10K (n= 15; VO2max:54.51±5.9 mL/kg/min )
key determinant of exercise performance. This investigation tested middle-distance running performance, in the fed state, to control for glycogen and exercise-induced hypoglycemia (EIH) confounders. Methods: In a randomized crossover fashion, all while controlling di- etary intake, activity, and body weight, recreational distance runners completed either a 5K (n= 15; VO2max: 58.3±6.2 mL/kg/min) or a 10K (n= 15; VO2max:54.51±5.9 mL/kg/min ) middle-distance run after consuming isocaloric low-carbohydrate high-fat (LCHF) and high-carbohydrate low-fat (HCLF) pre-exercise meals. Time trial (TT) performance (sec), car- bohydrate/fat substrate oxidation, blood metabolites, heart rate (HR), ratings of perceived exertion (RPE), and subjective fullness and thirst were measured throughout. Results: LCHF pre-exercise nutrition reliably altered substrate oxidation and metabolite profiles compared to HCLF, evidenced by significant increases in fat oxidation (77% higher) and reductions in RER (5% lower), with corresponding shifts in carbohydrate oxidation. De- spite distinct preferred substrate oxidation profiles during exercise, the 5 and 10 km TT performances were similar between conditions (p= 0.646/p= 0.118). RER was significantly lower (p= 0.002) after the LCHF condition compared to HCLF. Capillary R-βHB increased modestly after LCHF, while blood glucose increased after HCLF only. The LCHF meal was 35% more filling than the HCLF meal. Preferred substrate oxidation did not significantly modulate middle-distance running performance. Conclusion: This work supports recent findings that substrate oxidation is not a primary determinant of aerobic performance, as previously conceived. Keywords:ketogenic; carbohydrate; time trial; supplementation; energy bar Nutrients2025,17, 2771 https://doi.org/10.3390/nu17172771
Nutrients2025,17, 2771 2 of 14 1. Introduction Pre- and intra-race nutritional strategies can positively alter athletic performance. Acute carbohydrate ingestion can shorten time trial (TT) performance by replenishing the glucose pool and mitigating exercise-induced hypoglycemia (EIH) during endurance events [1]—colloquially referred to as preventing “bonking” [2,3]. Athletes perform well following both acute [4] and chronic [5] carbohydrate manipulations prior to competition; however, the vast majority—often as high as 90%—experience gastrointestinal discomfort that limits their performance if they consume carbohydrates during a race [6]. Several logical countermeasures exist to offset intra-race refueling discomfort and preserve performance. The first strategy stipulates pre-loading, whereby athletes consume most calories hours prior to the race to offset digestion issues and saturate intra-muscular glycogen stores in preparation for high-intensity exercise demands [7]. The second strategy, albeit not mutually exclusive to the first, relies on precision-nutrition. This nutritional framework seeks to identify the optimal carbohydrate and lipid type/quantity intake strategies based on the athlete’s habitual diet, tolerance, and exercise demands, including other surrogates such as blood biomarkers, predictive of peak performance [8,9]. Inexorably linked to aerobic events is enhanced lipid oxidation. Athletes may re- duce the total carbohydrate load to preferentially oxidize lipids during 45–65% VO2max steady-state exercise, commensurable with a decrease in skeletal muscle glycogen availability [10,11] . Although it has been well-demonstrated that carbohydrates supplant the energy requirements at exercise intensities > 65% VO2max(i.e., cross-over concept), emerging research suggests that macronutrient manipulations can plausibly shift the cross- over point to higher VO2maxthresholds than previously conceived [12], an effect that can predictably preserve greater rates of fat oxidation during endurance exercise bouts. While carbohydrate oxidation can arguably provide a superior substrate for exercise performance (i) due to its hypothetical improvements in energy efficiency of carbohydrate per unit of O2 consumed and (ii) its obligatory requirement to maintain exercise performance at very high intensities, we recently demonstrated (i) equivalent exercise performance despite distinct carbohydrate oxidation levels and (ii) maintenance of exercise performance at peak fat oxidation during exercise intensities > 85% VO2max, suggesting that substrate oxidation during exercise may not be a key determinant of exercise success
unit of O2 consumed and (ii) its obligatory requirement to maintain exercise performance at very high intensities, we recently demonstrated (i) equivalent exercise performance despite distinct carbohydrate oxidation levels and (ii) maintenance of exercise performance at peak fat oxidation during exercise intensities > 85% VO2max, suggesting that substrate oxidation during exercise may not be a key determinant of exercise success [1,5]. Recently, we showed that carbohydrate-restricted high-fat diets are a feasible strategy to sustain 5 km running performance [13,14]. Moreover, we also documented that a low- carbohydrate, MCT-containing high-fat (LCHF) pre-exercise meal can elicit similar time to exhaustion during a simulated rucking march (TTE; ~1 h to volitional cessation) relative to isocaloric, high-carbohydrate low-fat (HCLF) nutrition [15]. The original rationale of the HCLF/LCHF pre-exercise nutrition study was to test energy-dense food items in a military-relevant setting to evaluate the utility of portable nutrition for long activities that require optimizing carry load and space to support task-relevant energy expenditure. While controlling for diet (i.e., mixed diet), our most recent work suggests that neither glycogen nor carbohydrate oxidation determines prolonged strenuous exercise bout performance and that minimal carbohydrate dosing (10 g/h) improves exercise performance through mitigating EIH [1]. Based on our prior findings, we were motivated to further probe whether substrate oxidation dictates exercise performance. Two pre-nutrition fueling methods, HCLF and LCHF, designed to elicit distinct CHO and FAT oxidation levels, were tested using TT aerobic events lasting <60 min. We thus conducted a series of studies—referred herein as Study 1andStudy 2—to explore whether acute ingestion of LCHF or HCLF pre-exercise nutrition before a 5 km TT (Study 1) and a 10 km TT (Study 2) can modulate TT performance and intra-race exercise metabolism.
Nutrients2025,17, 2771 3 of 14 2. Methods 2.1. Experimental Design A randomized, single-blind, counterbalanced study was used to assess the effects of LCHF vs. HCLC pre-exercise nutrition on performance during a 5 km TT (Study 1) and 10 km TT (Study 2) in 30 healthy, experienced recreational runners (Figure). The participants completed four sessions. Session 1 included familiarization with standard height, weight, and body composition measures, followed by a graded treadmill exer- cise test (˙Astrand VO2maxtest). Session 2 included performance testing of a baseline TT (without bar consumption). During sessions 3 and 4, the participants consumed either an LCHF or HCLF meal following an overnight fast. Three hours post-LCHF or HCLF meal ingestion, the athletes performed a TT. Blood metabolites (blood lactate, blood glucose, and blood ketones) were assessed prior to ingestion, 30 min post-ingestion (+30 min), 3 h post-ingestion, and immediately post-exercise. Perceptual measures for the rating of perceived effort (RPE) and affect were assessed throughout each TT. HR and respiratory gas exchange were monitored continuously during the graded exercise test and each TT. Before implementation, all procedures received IRB approval (117-2022/124-2023), and all subjects signed an informed consent document. The investigations were conducted in accordance with the principles outlined in the Declaration of Helsinki (1975, revised in 2013). All testing was conducted in the Grove City College Exercise Science Department Human Performance Laboratory. Figure 1.Experimental design. Thirty recreational runners were allocated to participate inStudy 1(n= 15) orStudy 2(n =15). Baseline anthropometry—weight, height, and body composition (electrical impedance)—and VO2maxwas measured during session 1 prior to the experimental visits. Session 2 was designed to allow subjects to practice the 5 km or 10 km time trial (TT) to further familiarize them with the study protocol. Sessions 3 and 4 consisted of two experimental visits, where each subject consumed a low-carbohydrate high-fat (LCHF) and high-carbohydrate low-fat (HCLF) pre-exercise meal, in cross-over fashion, prior to their designated TT bout. There was a 1 week washout between sessions 3 and 4.
of two experimental visits, where each subject consumed a low-carbohydrate high-fat (LCHF) and high-carbohydrate low-fat (HCLF) pre-exercise meal, in cross-over fashion, prior to their designated TT bout. There was a 1 week washout between sessions 3 and 4.
Nutrients2025,17, 2771 4 of 14 2.2. Participants Two independent cohorts (n= 15/15) were recruited and assigned toStudies 1and 2(Table). Participants were recruited directly from local running clubs and through community advertisements. Inclusion criteria stipulated the following: (1) completed a 5 km run in under 28 min (Study 1) and a 10 km run in under 55 min (Study 2) within the last three months, (2) running between 16 and 48 km per week, (3) aged between 18 and 35 years old, (4) had >2 years of running experience, (5) VO2max≥ 40 mL/kg/min, and (6) consuming a Standard American Diet [16]. Participants were excluded from our study if they had (1) a history of smoking, (2) any known metabolic, respiratory, or cardio- vascular diseases, including diabetes, and (3) a presence of orthopedic, musculoskeletal, neurological, psychiatric disorders and/or any medical conditions that prevent exercise. The participants were instructed to refrain from exercise 24 h prior to a session, caffeine and alcohol consumption 48 h prior to a session, and food/drink for 3 h before each training session. Before enrolling in the investigation, participants were fully informed of any risks and discomforts associated with the experiments prior to giving their written informed consent to participate. Table 1.Participant descriptives. Characteristics Study 1(5 km TT) N= 15 Study 2(10 km TT) N= 15 Age (years) 20.8 ±3.5 31.2 ±10.9 Height (cm) 182.0 ±10.1 176.4 ±6.4 Bodyweight (kg) 81.3 ±12.4 78.6 ±7.8 Body Fat (%) 14.0 ±4.6 13.4 ±4.4 Fat Free Mass (kg) 69.3 ±8.7 73.4 ±23.4 Fat Mass (kg) 11.7 ±5.0 12.8 ±8.1 BMI (kg/m 2 ) 24.6±3.5 25.4 ±3.3 Mean exercise/week (min) 377.3 ±195.4 318.8 ±152.1 Exercise experience (years) 10.5 ±5.4 13.7 ±11.6 VO2max(mL/kg/min) 58.3 ±6.2 54.51 ±5.9 Familiarization TT (min) 24.6 ±3.2 45.7 ±5.6 Data shown as mean±SD. BMI, body mass index; TT, time trial; VO2max, maximal oxygen consumption. 2.3. Pretrial Preparation The participants were instructed to maintain their usual training frequency during each study intervention without increasing or decreasing the training load. The participants were instructed to maintain a training log (mode, duration, and intensity of each workout) for 1 week before
±5.6 Data shown as mean±SD. BMI, body mass index; TT, time trial; VO2max, maximal oxygen consumption. 2.3. Pretrial Preparation The participants were instructed to maintain their usual training frequency during each study intervention without increasing or decreasing the training load. The participants were instructed to maintain a training log (mode, duration, and intensity of each workout) for 1 week before the first experimental trial. They were provided with a copy of their pre-trial log and instructed to replicate the training routine during the intervention period. In addition, the participants were asked to record their training every week during the study (mode, duration, and intensity of each workout). To quantify training intensity, the participants were asked to record their session RPE (sRPE) after every training session [17] (pre-trial and within trial) using the OMNI Walk/Run 0–10 Perceived Exertion Scale [18]. The training load for each session was calculated by sRPE x duration of session (min- utes) [19]. The sum of each session’s training load was used to quantify the weekly training load. The training load was assessed each week to measure compliance (Supplemental Table S1).
Nutrients2025,17, 2771 5 of 14 The participants’ habitual pre-trial and within-trial dietary intake was assessed weekly via integrative mobile technology (MyFitnessPal, San Francisco, CA, USA). The use of a mobile app for quantifying food intake has been previously shown to be an effective monitoring tool [20]. The subjects were asked to keep a detailed diary for 7 days prior to the start of the first experimental trial. The participants were provided with a copy of their pre-trial log and instructed to have the same dietary intake during the remainder of the study (weeks 1 and 2). During the familiarization session, the participants were given precise oral and written instructions individually on how to accurately record amounts and types of food and beverages (Supplemental Table S2). 2.4. Familiarization, Anthropometric Measurements, and Maximal Aerobic Capacity The participants underwent an orientation involving TT practice and familiarization with the various measurement instruments, equipment, affect measures, and perceived exertion. Affect was measured using a validated 11-point Feeling Scale [21], with the partic- ipants informed that their responses should reflect the affective or emotional components of the exercise and not the physical sensation of effort or strain. The OMNI Walk/Run Perceived Exertion Scale [18] was used to measure the physical perceptions of exertion for the overall body (RPE-O). Following the orientation session, anthropometric measures were obtained, including height (cm), weight (kg), fat free mass (kg), and fat mass (% and kg). Height (cm) was measured using a physician’s scale (Detecto, Webb City, MO, USA). The participants’ body mass (kg) and body composition (fat and lean mass) were measured using a Tanita bioelectrical impedance analyzer (BIA) (MC-980Uplus, Tanita Corporation of America, Arlington Heights, TL, USA). Finally, the participants performed a graded exercise test to exhaustion on a motorized treadmill (Trackmaster TMX425C treadmill, New- ton, KS, USA). Oxygen consumption (VO2) and carbon dioxide production (VCO2) were measured using an automated metabolic analyzer system (TrueOne 2400, ParvoMedics, Sandy, UT, USA) calibrated prior to each exercise test using standard calibration gases (16% O2and 4% CO2). The participants wore a Polar heart rate monitor (H10, Polar Electro, Kempele, Finland) during exercise
motorized treadmill (Trackmaster TMX425C treadmill, New- ton, KS, USA). Oxygen consumption (VO2) and carbon dioxide production (VCO2) were measured using an automated metabolic analyzer system (TrueOne 2400, ParvoMedics, Sandy, UT, USA) calibrated prior to each exercise test using standard calibration gases (16% O2and 4% CO2). The participants wore a Polar heart rate monitor (H10, Polar Electro, Kempele, Finland) during exercise to measure their heart rate. After a thorough explanation of the experimental procedures, each participant was instructed to walk on the treadmill for 3 min as a warm-up at a self-selected speed (0% grade). Immediately following the 3 min warm-up, the speed was increased to 5–8 mph for 3 min (0% grade) to achieve the participants’ comfortable running pace. After 3 min of running at 0% grade, the grade was increased by 2.5% every 2 min throughout the test protocol, while speed was kept constant. The treadmill test was terminated by the subject at the point of volitional exhaustion. At the end of the test, the highest average VO2value recorded over a 30 s period of exercise was considered the participant’s VO2max. A week after the familiarization visit, subjects returned to the laboratory to complete a baseline (i.e., practice) TT on the treadmill entailing no energy bar consumption. 2.5. Experimental Protocol During the next two visits, the participants reported to the lab after an 8 h overnight fast to consume either an isocaloric low-carbohydrate high-fat (Keto Brick Inc., Bryant, AR, USA) or high-carbohydrate low-fat (2.4 services; ER 2400 Calorie Emergency Food Bar, Saint Louis, MO, USA) pre-exercise meal. The participants were randomly assigned to in- gest ~1000 kcal of either LCHF or HCLF meal three hours before the performance test. The subjects were provided with 500 mL of water to consume ad libitum. Each condition was separated by at least one week. These pre-exercise nutritional meal options were chosen for having similar energy density, flavor profiles, and appearance, but distinct macronu- trient profiles, which we previously demonstrated to produce distinct substate oxidation
consume ad libitum. Each condition was separated by at least one week. These pre-exercise nutritional meal options were chosen for having similar energy density, flavor profiles, and appearance, but distinct macronu- trient profiles, which we previously demonstrated to produce distinct substate oxidation
Nutrients2025,17, 2771 6 of 14 profiles [15]. A complete list of the macro-/micronutrients and their relative amounts is described in the supplement (Supplemental Table S3). Testing was single blinded; that is, the participants did not know the type of nutrition bar consumed. Blinding was achieved by removing the wrappers from the bars and dividing each bar into smaller bite-sized fragments. The LCHF and HCLF pre-exercise meals were similar in appearance (i.e., same color and consistency). We acknowledge that physiological cues such as differences in satiety could have provided indirect hints of meal type, but perceptual responses (RPE and affect) were monitored throughout to evaluate any expectancy influence. 2.6. Blood Sampling Capillary blood samples for R-beta-hydroxybutyrate (R-βHB; Precision Xtra, Abbott Diabetes Care Inc., Almeda, CA, USA), blood glucose (Precision Xtra, Abbott Diabetes Care Inc., Almeda, CA, USA), and lactate concentrations (Lactate Plus, Nova Biomedical) were measured at baseline, 30 min post-meal ingestion (+30 min), 3 h post-meal ingestion (immediately before start of TT), and immediately following the TT. Samples were collected using a lancet following the cleaning of the fingertip with an alcohol swab and then dried. The first droplet was wiped away with a cotton swab to remove any alcohol, and the subsequent droplets were used for analysis. 2.7. 5 Km and 10 Km Running Time Trials To determine exercise performance, the participants performed a 5 km (Study 1) and 10 km (Study 2) running TT on a motorized treadmill (TMX425C treadmill; Trackmaster, Newton, KS, USA). Before the start of the run, the participants completed a 5 min self-paced warm-up run. The participants were instructed to finish the run as fast as possible. The gradient was set at 0.0% grade. The participants were provided with feedback on the distance [at regular 500 m intervals (Study 1) and at regular 1000 m intervals (Study 2)] covered during each TT and were not informed of the overall performance time until completion of the study. During the TT, the participants were permitted to volitionally adjust their speed during the TT via control buttons located on the treadmill. The speed indicator and timing devices
Description
The study examines the impact of substrate oxidation on middle-distance running performance.