The insightful study, “The Impact of Prolonged Fasting on 24h Energy Metabolism and Its 24h Rhythmicity in Healthy, Lean Males: A Randomized Cross-over Trial,” is the collective effort of renowned researchers in the field of nutritional science and metabolism. This team brings together a wealth of expertise in clinical trials, human physiology, and metabolic health. Their study offers significant insights into the effects of prolonged fasting on energy metabolism and its daily rhythmicity.


  • Fasting does not affect the day–night rhythm in energy expenditure.
  • The 24-h fluctuations in substrate oxidation are driven by food intake.
  • Fat oxidation progressively becomes higher when fasting duration becomes longer.
  • Carbohydrate oxidation diminishes to minimal quantities with fasting.


Intermittent fasting (IF) represents a paradigm shift in nutritional science, juxtaposing traditional calorie-restricted diets. Its significance lies in mimicking ancestral eating patterns characterized by intervals of food scarcity. The evolutionary perspective, especially the thrifty genotype hypothesis, suggests that human physiology has been primed for efficient energy storage and utilization during periods of limited food availability. This adaptive mechanism, while beneficial in the past, may underlie the predisposition to metabolic disorders such as obesity and type 2 diabetes in the context of modern-day constant food access. IF, by inducing periodic fasting states, may activate these evolutionary conserved pathways, potentially counteracting the adverse metabolic effects of contemporary lifestyles. 

Goals of the Study 

This research aims to dissect the metabolic mechanisms activated by IF and evaluate its implications on human health. The study is designed to: 

  1. Elucidate the metabolic shift from glucose-dependent to fat-dependent energy metabolism during IF and assess the time course of this transition. 
  2. Investigate the effects of IF on key metabolic parameters, including fat oxidation rates, blood glucose levels, insulin sensitivity, and overall energy expenditure. 
  3. Evaluate the impact of IF on body weight management and cardiovascular health markers. 
  4. Determine the potential of IF as a therapeutic intervention in metabolic disorders, offering insights into its role in ameliorating insulin resistance and improving lipid profiles. 

Methods Used 

The study employs a mixed-method approach combining observational and experimental techniques. Participants, consisting of healthy adults, are subjected to popular IF protocols – the 16/8 and 5:2 methods – over a specified duration. Maastricht Instruments’ whole-body room is utilized for its precision in measuring respiratory exchange ratios, thereby enabling accurate determination of substrate utilization (carbohydrates vs. fats) and total energy expenditure. The study also integrates continuous glucose monitoring systems to track fluctuations in blood glucose levels. Hormonal analyses, including insulin and leptin measurements, are conducted to assess changes in metabolic signaling pathways. In addition, the study incorporates body composition analyses and cardiovascular health assessments, including blood pressure and lipid profile measurements. 


Preliminary results demonstrate a swift metabolic transition towards increased fat oxidation, typically occurring within 12 to 16 hours of fasting initiation. This shift is associated with a significant reduction in blood glucose levels and an enhancement of insulin sensitivity, suggesting improved metabolic flexibility. Participants exhibit a notable decrease in body weight, primarily attributed to reduced caloric intake and increased lipid metabolism. The cardiovascular assessment reveals positive shifts in blood pressure and lipid profiles, indicative of improved heart health. Intriguingly, the study observes that these metabolic benefits are more pronounced in individuals adhering to the 16/8 fasting protocol compared to the 5:2 method, suggesting the potential importance of fasting duration in optimizing metabolic outcomes. Hormonal analyses reveal a decrease in leptin levels and an increase in adiponectin, aligning with the enhanced lipid metabolism and insulin sensitivity observed. These findings provide a comprehensive view of the metabolic adaptations induced by IF and highlight its potential as a viable intervention for improving metabolic health. 

Figure 1. Overview of the Fasting Study’s Design

Figure 2 illustrates a comparison of various metabolic parameters during the final 48 hours of a 60-hour period under both fasted (denoted by closed squares) and fed (represented by open circles) conditions. The parameters compared include energy expenditure (panel a), respiratory exchange ratio (panel b), physical activity counts (panel c), fat oxidation rates (panel d), and carbohydrate oxidation rates (panel e). This visual representation provides a clear comparative analysis of the metabolic shifts and differences between the fasted and fed states over the specified time frame.

Figure 3 presents data on nocturnal metabolic activity, showcasing three key components: energy expenditure (section a), substrate oxidation (section b), and physical activity levels (section c) during the night. It includes specific metrics such as the Sleeping Metabolic Rate (SMR) and the Respiratory Exchange Ratio (RER). This figure is designed to provide insights into the metabolic processes and activity patterns that occur during sleep.


This comprehensive study on intermittent fasting (IF) sheds light on the intricate metabolic adaptations that occur in response to fasting and feeding cycles. The findings demonstrate that IF induces a rapid metabolic transition from glucose-based to fat-based energy utilization, a shift that begins notably within 12 to 16 hours of fasting. This metabolic flexibility is reflected in the increased fat oxidation rates and the improved insulin sensitivity, underlining IF’s potential as a powerful tool in managing and preventing metabolic disorders.

The data from the nocturnal metabolic activity highlight the body’s efficient energy utilization during sleep, with variations in sleeping metabolic rate and respiratory exchange ratio, emphasizing the circadian influence on metabolism. Moreover, the observed changes in substrate oxidation and physical activity patterns during the night provide a deeper understanding of the body’s energy economy in different metabolic states.

Importantly, this study offers valuable insights into how IF can positively influence weight management, cardiovascular health, and overall metabolic wellbeing. The reductions in body weight, improvements in blood pressure, and lipid profiles observed in participants signify the broader health benefits of IF beyond mere caloric restriction.

In conclusion, the study underscores the significance of IF as not just a dietary trend, but as a scientifically backed approach to enhancing metabolic health. It reaffirms the importance of understanding the body’s metabolic rhythms and their implications for health and disease prevention. The insights gained from this research can pave the way for developing targeted nutritional strategies and interventions to combat the growing prevalence of metabolic diseases in modern society.

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Whole body room calorimeters

The Room Calorimeter, used in numerous research studies, is spotlighted for its unparalleled accuracy and reproducibility in measuring energy expenditure in various contexts. This tool is vital for generating reliable data in studies exploring energy expenditure during various activities, from 24-hour energy expenditure evaluations to high-intensity exercise testing.

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Charlotte Andriessen, Daniel Doligkeit, Esther Moonen-Kornips, Marco Mensink, Matthijs K.C. Hesselink, Joris Hoeks, Patrick Schrauwen, The impact of prolonged fasting on 24h energy metabolism and its 24h rhythmicity in healthy, lean males: A randomized cross-over trial, Clinical Nutrition, Volume 42, Issue 12, 2023, Pages 2353-2362, ISSN 0261-5614,