Abstract
Fasting, a dietary practice with ancient roots, has resurfaced as a popular approach to weight management and metabolic health. Recent research highlights another layer of its benefits: the profound impact of fasting on the gut microbiota. This review explores how various fasting regimens—from intermittent fasting to Ramadan fasting—influence gut microbiota composition, diversity, and function. It also examines the implications of these changes for digestive health, summarizing current knowledge and identifying areas for further research.
Introduction
The gut microbiota, a complex community of microorganisms residing in the digestive tract, plays a crucial role in human health. These microbes influence digestion, immunity, nutrient absorption, and even mental health. Disruptions in the gut microbiota, known as dysbiosis, have been linked to a range of digestive disorders and systemic diseases. As fasting practices gain traction, understanding their effects on the gut microbiota becomes increasingly important. This review investigates how different fasting regimens influence gut microbiota diversity and its implications for digestive health.
Fasting Regimens and Their Impact on Gut Microbiota
- Intermittent Fasting (IF): This approach involves cycling between periods of eating and voluntary fasting on a daily or weekly basis.
- A study of the effects of time-restricted feeding on gut microbiota, showed that time-restricted feeding could regulate circadian rhythms associated with modulation of gut microbiota[1].
- Studies suggest that IF can alter the composition of the human gut microbiome by increasing taxonomic diversity[6].
- One study showed at the phylum level, an increased proportion of Bacteroidetes and decreased Firmicutes were observed after fasting in the Pakistani group, which agrees with previously described effects of caloric restriction on gut microbiota in a mouse model[1].
- Ramadan Fasting (RF): This religious practice involves abstaining from eating and drinking from dawn until sunset for approximately 30 days.
- Ramadan fasting can lead to shifts in gut microbial community composition[1].
- Studies show Ramadan fasting may lead to weight loss and improvement of metabolic parameters during fasting, but the effects are reversed when fasting is finished[1].
- Alternate-Day Fasting (ADF): This involves alternating between days of normal eating and days of severe calorie restriction.
- 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.
Key Microbial Shifts During Fasting
- Increased Bacteroidetes: Several studies have reported an increase in the proportion of Bacteroidetes relative to Firmicutes during fasting[1]. Bacteroidetes are known for their ability to degrade complex polysaccharides, potentially enhancing nutrient availability.
- Decreased Firmicutes: Some studies observed a decrease in Firmicutes during fasting periods[1]. This shift is noteworthy because the ratio of Firmicutes to Bacteroidetes has been linked to various health outcomes, including obesity.
- Changes in Clostridial Firmicutes: Upon fasting, several Clostridial Firmicutes shifted significantly in abundance, with an initial decrease in butyrate producers such as F. prausnitzii, E. rectale and C. comes, which were reverted after 3 months upon refeeding[2].
- Enrichment of Propionate Production Capacity: Fasting enriched for propionate production capacity, mucin degradation gene modules, and diverse nutrient utilization pathways[2].
- Increase Short-Chain Fatty Acids: Intermittent fasting has been reported to induce significant changes in the gut microbiota and increase the production of short-chain fatty acids (SCFAs)[4].
Implications for Digestive Health
- Improved Gut Barrier Function: Fasting-induced changes in the gut microbiota may enhance gut barrier function, reducing intestinal permeability and preventing systemic inflammation.
- Modulation of Gut Immunity: Changes in the abundance of specific microbial taxa can influence immune responses in the gut. For instance, increased butyrate production can promote the development of regulatory T cells, which help maintain immune homeostasis.
- Enhanced Digestive Efficiency: Fasting may improve digestive efficiency by allowing the gut to rest and repair itself. It may also alter the expression of genes that influence circadian rhythm of gut microbiota[1].
- Enrichment of Proteobacteria: Proteobacteria were significantly enriched after fasting, which is noteworthy because enrichment of Proteobacteria has been suggested as a gut microbial signature of dysbiosis[1].
Limitations and Future Directions
- Study Heterogeneity: The studies included are difficult to compare due to variations in study designs and sample sizes.
- Dietary Influences: While fasting may be the primary intervention, dietary composition during eating periods also plays a significant role in shaping the gut microbiota.
- Long-Term Effects: The long-term effects of various fasting regimens on the gut microbiota and digestive health remain unclear. Longitudinal studies are needed to assess the sustainability of these changes.
Conclusion
Fasting, whether intermittent or periodic, has a demonstrable impact on the gut microbiota, influencing its composition, diversity, and function. These changes can have both positive and negative implications for digestive health, underscoring the importance of understanding how different fasting regimens affect the gut microbiome and therefore have various effects on gut microbiome mediated functions in humans[3]. Further research is needed to elucidate the complex interplay between fasting, the gut microbiota, and digestive health. As research continues to evolve, it will be essential to consider individual factors, monitor long-term effects, and tailor fasting protocols to optimize gut health and overall well-being.
Citations:
[1] https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.642999/full
[2] https://www.nature.com/articles/s41467-021-22097-0
[3] https://pmc.ncbi.nlm.nih.gov/articles/PMC10894978/
[4] https://pmc.ncbi.nlm.nih.gov/articles/PMC10495574/
[5] https://www.nature.com/articles/s41522-023-00386-4
[6] https://www.news-medical.net/health/The-Effect-of-Intermittent-Fasting-on-the-Gut-Microbiome.aspx
[7] https://academic.oup.com/nutritionreviews/article/82/6/777/7235070
