Abstract
Intermittent fasting (IF), characterized by cyclical periods of eating and voluntary fasting, has emerged as a popular dietary intervention with purported benefits for metabolic health, weight management, and overall well-being. This review aims to comprehensively evaluate the current evidence regarding the impact of different IF protocols on key metabolic markers, body weight, and related health outcomes. The review considers various IF regimens, including time-restricted eating (TRE) and alternate-day fasting (ADF), to provide a nuanced understanding of their effects on human metabolic health.
Introduction
The increasing prevalence of metabolic disorders such as obesity, type 2 diabetes, and cardiovascular disease has spurred interest in lifestyle interventions that can improve metabolic health. Intermittent fasting (IF) has gained considerable attention as a dietary strategy that focuses on the timing of eating rather than strict calorie restriction[1]. IF regimens involve alternating between periods of voluntary fasting and non-fasting, with the goal of eliciting beneficial metabolic adaptations[5]. This review examines the effects of different IF protocols on metabolic markers, weight management, and overall health, providing a comprehensive overview of current evidence.
Intermittent Fasting Protocols
Various IF protocols exist, each with distinct patterns of fasting and eating:
- Time-Restricted Eating (TRE): This involves limiting the eating window to a specific number of hours per day (e.g., 8-hour eating window, 16-hour fast)[1].
- Alternate-Day Fasting (ADF): This protocol alternates between days of normal eating and days of severe calorie restriction (typically ≤25% of usual intake) or complete fasting[2].
- Modified ADF: This variation allows for the consumption of a small number of calories (e.g., 500-600 calories) on fasting days[2].
- Periodic Fasting (PF): This involves fasting for longer periods (e.g., 24 hours or more) on a less frequent basis, such as once or twice per week[5].
- Ramadan Fasting: This involves abstaining from eating and drinking from dawn until sunset for a period of approximately 30 days. Studies show that Ramadan fasting may lead to weight loss and improvement of metabolic parameters during fasting, but the effects are reversed when fasting is finished[4].
Impact of Intermittent Fasting on Metabolic Markers
- Glucose Metabolism and Insulin Sensitivity:
- IF has been shown to improve glucose metabolism and insulin sensitivity[1].
- Studies show a decrease in insulin resistance after an 8-hour TRE[1].
- Significant reductions in fasting insulin have been reported during short-term energy deprivation compared to energy balance[1].
- However, some studies have observed no metabolic changes compared to baseline[1].
- Lipid Profile:
- Time-restricted eating was associated with increases in high-density lipoprotein (HDL) and variations in low-density lipoprotein (LDL) and total cholesterol depending on whether the eating was ad libitum vs. isocaloric[1].
- Some studies showed decreased total cholesterol and triglycerides and an increase in HDL post-TRE[1].
- Divergent effects on lipid profiles have been observed, with increases in HDL as well as in LDL. These mixed results on lipid profiles indicate that intermittent fasting may have variable effects on lipid metabolism, potentially influenced by factors such as fasting duration, diet composition, and individual metabolic responses[1].
- Weight Management and Body Composition:
- Intermittent fasting is more effective than an ad libitum dietary intake, and equally or more effective as continuous energy restriction, for weight management[4].
- TRE has been shown to decrease body mass and fat mass while preserving lean mass[1].
- Intermittent fasting regimens demonstrated 3–8% reductions in body weight after 3–24 weeks[2].
- Markers of Biological Aging:
- Only a few studies have measured markers associated with biological aging[1].
Mechanistic Factors Linking Intermittent Fasting with Health
Intermittent fasting regimens are hypothesized to influence metabolic regulation via effects on circadian biology, the gastrointestinal microbiota, and modifiable lifestyle behaviors[2][5]. Perturbations in these systems can negatively affect metabolic health, predisposing individuals to obesity, diabetes, cardiovascular disease, and cancer[2].
Limitations and Considerations
- Most interventional studies involve interventions with short duration, spanning 4–12 weeks, which limits conclusions about long-term efficacy and safety[1].
- Mixed results on lipid profiles indicate that intermittent fasting may have variable effects on lipid metabolism, potentially influenced by factors such as fasting duration, diet composition, and individual metabolic responses[1].
- There is inconclusive evidence on whether intermittent fasting has a clinically beneficial effect on glucose and lipid metabolism[4].
Conclusion
Intermittent fasting shows promise as a dietary intervention for improving several markers of metabolic health, including glucose metabolism, insulin sensitivity, and weight management[1][3]. Time-restricted eating and other IF protocols can lead to decreases in body mass and improvements in lipid profiles[1]. However, results may vary based on the specific fasting protocol, participant characteristics, and dietary composition[1]. Further research is needed to fully elucidate the long-term effects of IF on metabolic health and to determine the optimal protocols for different populations[2][5]. Future studies should also focus on measuring markers associated with biological aging to better understand the potential long-term benefits of intermittent fasting[1].
Citations:
[1] https://www.nature.com/articles/s44324-024-00041-2
[2] https://pmc.ncbi.nlm.nih.gov/articles/PMC4516560/
[3] https://www.thelancet.com/journals/eclinm/article/PIIS2589-5370(24)00098-1/fulltext
[4] https://onlinelibrary.wiley.com/doi/10.1111/jhn.13253
[5] https://www.annualreviews.org/doi/10.1146/annurev-nutr-071816-064634
[6] https://www.mdpi.com/2077-0383/12/11/3699
[7] https://www.researchgate.net/publication/358306247_Intermittent_Fasting_and_Metabolic_Health
