The Impact of Prolonged Fasting on Physical Performance and Metabolism

Fasting is an ancient practice, but how does it affect athletic performance in the modern age?

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A new study (link above) published in Nature Communications delves into the effects of a seven-day water-only fast on physical performance and metabolism during exercise. Here, i'll unpack the key findings and discuss how endurance athletes can use supplements like DeltaG to mitigate the downsides of fasting.

A Schematic representation of the study design illustrating phases of data collection. B Continuous measurement of glucose concentration (Dexcom). Data are reported as means from 8 a.m. to 8 a.m. the following day (n = 12). C Change in bodyweight throughout the week, in absolute terms in kilograms. D Reduction in total body, lean, fat and bone mass during the seven days’ fasting calculated from DXA scan, in absolute values (kg). E Reduction total, lean, fat and bone mass of extremities and total body during the fasting period, relative to baseline values (%). F Daily nitrogen excretion during the fasting period calculated from total urine. Bar diagrams show means ± SEM (Fig. D and E). Figure 1A: “Created in BioRender. Møller, P. (2025) https://BioRender.com/z88l355. Data are from 13 participants except for glucose (n = 12).

Key Findings from the Study

Muscle Strength is Preserved

Despite participants losing 4.6 kg of lean body mass, their maximal leg muscle strength remained unchanged. This finding highlights the body’s ability to conserve strength during periods of nutrient deprivation—a potential evolutionary adaptation for survival.

Results from isometric and isokinetic strength testing and V˙O2peak tests before and after six days of fasting. A Isometric strength in newton metres (Nm). B isokinetic peak torque at three different velocities in Nm. C Mean isokinetic power at three different velocities (D) V˙O2peak in absolute terms; L/min (E) V˙O2peak related to body weight; ml/kg/min. F Peak power output (watt) during V˙O2peak test. G Peak power output during the VO2peak test related to body weight. White and grey bars represent means before and after six days of fasting, respectively. Bars show means. White circles represent individuals before fasting and black after fasting, with dotted lines to illustrate individual change. Two-sided paired t-tests were used for comparisons. N = 13.

High-Intensity Performance is Compromised

• Peak oxygen uptake ( VO₂ max) dropped by 13% after seven days of fasting.

• High-intensity endurance capacity decreased by 15%.

• Muscle glycogen, an essential energy source for high-intensity activities, was reduced by 50%.

Enhanced Fat Metabolism

Fasting induced a significant shift toward fat metabolism. Fat oxidation doubled, contributing up to 73% of total energy at rest. However, this adaptation came at the cost of reduced carbohydrate utilisation, limiting high-intensity performance.

Metabolism, energy expenditure and substrate selection at rest and during the incremental fat oxidation test: A Resting oxygen consumption. B Resting metabolic rate. C RER values during rest. D Energy contribution (% fat) during rest. E V˙O2 during incremental fat oxidation test. F RER during incremental fat oxidation test. G Fat oxidation during incremental fat oxidation test. H Estimated carbohydrate oxidation during incremental fat oxidation test. I Maximal fat oxidation during incremental fat oxidation test, (J) % V˙O2peak where fat max was achieved during incremental fat oxidation test. K Blood lactate during incremental fat oxidation test. White and grey bars represent means before and after six days of fasting, respectively. Bars show means. Error bars show SEM. White circles represent individuals before fasting and black after fasting, with dotted lines to illustrate individual change. Two-sided paired t tests were used for comparisons in figures (A–D and I–K). Repeated ANOVA was used for statistical analyses in figures (E–H and K), with LSD as a post hoc test. *: difference between pre and post. N = 13. Abbreviations: RER: respiratory exchange ratio, RMR: Resting metabolic rate.

Metabolic Shifts

• Pyruvate dehydrogenase kinase 4 (PDK4) expression increased 13-fold, reducing carbohydrate oxidation to conserve glucose.

• Plasma ketones and free fatty acids increased significantly, serving as alternative energy sources.

Plasma metabolites before exercise, after maximal fat oxidation test and V˙O2peak test, before (white bars) and after six days’ fasting (grey bars). White circles represent individuals before fasting and black after fasting, with dotted lines to illustrate individual change: Plasma levels of (A) glucose, (B) insulin, (C) β-hydroxybutyrate, and (D) free fatty acids (FAs). Repeated ANOVA with LSD as post hoc tests were used for statistical analyses. N = 13.

Practical Implications for Athletes

While fasting can improve fat metabolism, it also limits the body’s ability to sustain high-intensity efforts due to reduced carbohydrate oxidation. For endurance athletes, this trade-off may impact performance during events requiring bursts of power or sustained effort near threshold levels.

Effect of seven days’ fasting on glycogen content and glycogen synthase activation. White and grey bars represent means before and after seven days of fasting. White circles represent individuals before fasting and black after fasting, with dotted lines to illustrate individual change. A Intramuscular glycogen content (mmol/kg protein). B Total activity of glycogen synthase (mmol/kg protein/min). C Glycogen synthase fraction velocity (%), phosphorylation of glycogen synthase at site (D) 1a, (E) 2 + 2a, F: 3 + 3b. Bars show means. Abbreviation: GS: glycogen synthase, p: phosphorylation, FV: fraction velocity. Two-sided paired t-tests were used for comparisons. N = 13 except (E) where N = 12.

Effect of seven days’ fasting on activity, phosphorylation and expression of selected proteins. White and grey bars represent means before and after seven days of fasting. White circles represent individuals before fasting and black after fasting, with dotted lines to illustrate individual change. A, B and C Activity of AMPK complex: α1β2γ1, α2β2γ1, and α2β2γ3. D Protein expression of PDH. E Phosphorylation of PDH at site 1. F Phosphorylation of PDH at site 2. G Expression of PDK4. Bars show means. Two-sided paired t tests were used for comparisons. N = 13 for (A–C, F and G) N = 12 for (D and E). Abbreviation: AMPK: 5’-Adenosine monophosphate-activated protein kinase. PDH: pyruvate dehydrogenase, PDK4: pyruvate dehydrogenase kinase-4.

How DeltaG Ketone Supplements Can Help

• DeltaG, a ketone ester supplement, offers a way to harness the benefits of fasting while mitigating its drawbacks. By providing a direct and efficient energy source in the form of ketones, DeltaG can:

• Spare Glycogen Stores: By supplying readily available energy, DeltaG helps preserve muscle glycogen, which is critical for high-intensity efforts.

• Stabilise Energy Supply: Ketones offer a consistent energy source for both the brain and muscles, reducing fatigue and improving endurance.

• Enhance Recovery: Supplementing with DeltaG can help offset the negative effects of reduced carbohydrate availability during prolonged fasting.

Achieve Performance Ketosis™ of 2-3.5 mmol/L blood ketones with DeltaG.

Takeaway for Athletes

Prolonged fasting highlights the human body’s remarkable ability to adapt metabolically. For endurance athletes, fasting can be a tool to enhance fat oxidation and metabolic flexibility. However, the trade-offs include reduced high-intensity performance and glycogen depletion. DeltaG ketone supplements provide a practical solution, enabling athletes to reap the metabolic benefits of fasting while maintaining energy levels for optimal performance.

Whether you’re looking to experiment with fasting or enhance your endurance capabilities, understanding these nuances can help you train smarter and race stronger.