Effects of 8-weeks of daily time restricted feeding and aerobic exercise on fat oxidation – A randomized controlled trial
Authors: Hofstaetter Florian , Rausch Linda , Niedermeier Martin , Kopp Martin , Lawley Justin 
Published: September 2024

Introduction & Purpose
Substrate metabolism, especially lipid metabolism and thus fat oxidation, is of special interest to reduce the risk of metabolic diseases (diabetes, high cholesterol or triglycerides, etc), and improve athletic performance (Aird et al., 2018). Thus, patients and athletes are recommended to engage in exercise training at a mild to moderate intensity where fat oxidation is high.
Fasting is known to increase lipolysis (i.e., fat oxidation) and therefore may represent a simple intervention to increase training induced adaptions in fat oxidation (Venables & Jeukendrup, 2008). However, breaking the fast with a carbohydrate meal prior to aerobic training may limit these benefits because carbohydrate consumption is known to prioritize carbohydrate oxidation during exercise (Achten & Jeukendrup, 2003).
Therefore, we tested the hypothesis that 8 weeks of aerobic exercise training at a workload that maximizes fat oxidation would improve the maximal rate of fat oxidation during exercise. Moreover, engaging in a fasting regime would augment the improvements in fat oxidation, but breaking the fast prior to training with a carbohydrate rich snack would attenuate the improvements in fat oxidation.
Methods
Thirty-six participants (28 females, 8 males) were randomized into three groups. 1) One group fasted for at least 14 hours prior to training, 2) One group fasted, but consumed a carbohydrate rich snack 30 minutes prior to training, 3) One exercise only control group where participants could eat ad libitum. Pre-tests included anthropometric measurements, a bio-impedance-analyses, and cycle ergometry combined with indirect calorimetry to identify maximum rates of fat oxidation (fatmax). All participants exercised on a stationary bike 3x/week for 60 min at a heart rate that corresponded to 90-100% of their individualized fatmax values. Pre-test measurements were repeated after the intervention. Rates of fat and carbohydrate oxidation, energy expenditure and heart rate were analysed as workload matched and absolute data using a series of 3 x 2 mixed ANOVAs.
Results
3 x 2 mixed ANOVA showed no group-by-time interactions for any workload-matched data (fatmax: p = 0.371, η²p = 0.058; CHO: p = 0.540, η²p = 0.037; EE: p = 0.470, η²p = 0.045; HR: p = 0.570, η²p = 0.033). No significant group-by-time interactions at the absolute maximal fat oxidation rate were observed (fatmax: p = 0.262, η²p = 0.078; CHO: p = 0.966, η²p = 0.002; EE: p = 0.111, η²p = 0.125; HR: p = 0.618, η²p = 0.029).
Discussion and conclusion
In the current study, the addition of a 16-hour fasting window did not provide any additional improvements in fat oxidation rates beyond an ad libitum control group. This is surprising as both low-intensity training (Achten & Jeukendrup, 2004) and TRF (Jong-Yeon et al., 2002) have independently been shown to improve fat oxidation.
We conclude that fatmax training is an effective lifestyle intervention to improve fat oxidation in young healthy individuals. Collectively, these data suggest fatmax training independent of the fed state might be a useful lifestyle intervention in healthy individuals looking to maintain or improve their metabolic health and avoid future metabolic disease.
References
Achten, J., & Jeukendrup, A. E. (2003). The effect of pre-exercise carbohydrate feedings on the intensity that elicits maximal fat oxidation. Journal of Sports Sciences, 21(12), 1017–1025. https://doi.org/10.1080/02640410310001641403
Achten, J., & Jeukendrup, A. E. (2004). Optimizing fat oxidation through exercise and diet. Nutrition, 20(7-8), 716–727. https://doi.org/10.1016/j.nut.2004.04.005
Aird, T. P., Davies, R. W., & Carson, B. P. (2018). Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scandinavian Journal of Medicine and Science in Sports, 28(5), 1476–1493. https://doi.org/10.1111/sms.13054
Jong-Yeon, K., Hickner, R. C., Dohm, G. L., & Houmard, J. A. (2002). Long- and medium-chain fatty acid oxidation is increased in exercise-trained human skeletal muscle. Metabolism, 51(4), 460–464. https://doi.org/10.1053/meta.2002.31326
Venables, M. C., & Jeukendrup, A. E. (2008). Endurance training and obesity: Effect on substrate metabolism and insulin sensitivity. Medicine & Science in Sports & Exercise, 40(3), 495–502. https://doi.org/10.1249/MSS.0b013e31815f256f