Recent advances and health implications of dietary fasting regimens on the gut microbiome

The Beneficial Effects of Dietary Interventions on Gut Microbiota-An Up-to-Date Critical Review and Future Perspectives

Different dietary interventions, especially intermittent fasting, are widely used and promoted by physicians; these regimens have been studied lately for their impact on the gut microbiota composition/function and, consequently, on the general physiopathological processes of the host. Studies are showing that dietary components modulate the microbiota, and, at the same time, the host metabolism is deeply influenced by the different products resulting from nutrient transformation in the microbiota compartment. This reciprocal relationship can potentially influence even drug metabolism for chronic drug regimens, significantly impacting human health/disease. Recently, the influence of various dietary restrictions on the gut microbiota and the differences between the effects were investigated. In this review, we explored the current knowledge of different dietary restrictions on animal and human gut microbiota and the impact of these changes on human health.

Intermittent fasting and bone health: a bone of contention?

Intermittent fasting (IF) is a promising strategy for weight loss and improving metabolic health, but its effects on bone health are less clear. This review aims to summarise and critically evaluate the preclinical and clinical evidence on IF regimens (the 5:2 diet, alternate-day fasting (ADF) and time-restricted eating (TRE)/time-restricted feeding and bone health outcomes. Animal studies have utilised IF alongside other dietary practices known to elicit detrimental effects on bone health and/or in models mimicking specific conditions; thus, findings from these studies are difficult to apply to humans. While limited in scope, observational studies suggest a link between some IF practices (e.g. breakfast omission) and compromised bone health, although lack of control for confounding factors makes these data difficult to interpret. Interventional studies suggest that TRE regimens practised up to 6 months do not adversely affect bone outcomes and may even slightly protect against bone loss during modest weight loss (< 5 % of baseline body weight). Most studies on ADF have shown no adverse effects on bone outcomes, while no studies on the ‘5–2’ diet have reported bone outcomes. Available interventional studies are limited by their short duration, small and diverse population samples, assessment of total body bone mass exclusively (by dual-energy X-ray absorptiometry) and inadequate control of factors that may affect bone outcomes, making the interpretation of existing data challenging. Further research is required to better characterise bone responses to various IF approaches using well-controlled protocols of sufficient duration, adequately powered to assess changes in bone outcomes and designed to include clinically relevant bone assessments.

Intestinal Microbiota Reduction Followed by Fasting Discloses Microbial Triggering of Inflammation in Rheumatoid Arthritis

Rheumatoid arthritis (RA) synovitis is dominated by monocytes/macrophages with inflammatory patterns resembling microbial stimulation. In search of triggers, we reduced the intestinal microbiome in 20 RA patients (open label study DRKS00014097) by bowel cleansing and 7-day fasting (≤250 kcal/day) and performed immune monitoring and microbiome sequencing. Patients with metabolic syndrome (n = 10) served as a non-inflammatory control group. Scores of disease activity (DAS28/SDAI) declined within a few days and were improved in 19 of 20 RA patients after breaking the fast (median ∆DAS28 = -1.23; ∆SDAI = -43%) or even achieved remission (DAS28 < 2.6/n = 6; SDAI < 3.3/n = 3). Cytometric profiling with 46 different surface markers revealed the most pronounced phenomenon in RA to be an initially increased monocyte turnover, which improved within a few days after microbiota reduction and fasting. Serum levels of IL-6 and zonulin, an indicator of mucosal barrier disruption, decreased significantly. Endogenous cortisol levels increased during fasting but were insufficient to explain the marked improvement. Sequencing of the intestinal microbiota indicated that fasting reduced potentially arthritogenic bacteria and changed the microbial composition to species with broader metabolic capabilities. More eukaryotic, predominantly fungal colonizers were observed in RA, suggesting possible involvement. This study demonstrates a direct link between the intestinal microbiota and RA-specific inflammation that could be etiologically relevant and would support targeted nutritional interventions against gut dysbiosis as a causal therapeutic approach.

Intermittent fasting modulates the intestinal microbiota and improves obesity and host energy metabolism

Intermittent fasting (IF) is a promising paradigm for weight loss which has been shown to modulate the gut microbiota based on 16S rRNA gene amplicon sequencing. Here, 72 Chinese volunteers with a wide range of body mass index (BMI) participated in a three-week IF program during which an average loss of 3.67 kg body weight accompanied with improved clinical parameters was observed irrespective of initial anthropometric and gut microbiota status. Fecal samples were collected before and after the intervention and subjected to shotgun metagenomic sequencing. De novo assembly yielded 2934 metagenome-assembled genomes (MAGs). Profiling revealed significant enrichment of Parabacteroides distasonis and Bacteroides thetaiotaomicron after the intervention, with inverse correlations between their relative abundances and parameters related to obesity and atherosclerotic cardiovascular diseases (ASCVD). MAGs enriched after the intervention showed high richness and diversity of carbohydrate-active enzymes, with an increased relative abundances of genes related to succinate production and glutamate fermentation.

Dietary patterns and cardiometabolic health: Clinical evidence and mechanism

For centuries, the search for nutritional interventions to underpin cardiovascular treatment and prevention guidelines has contributed to the rapid development of the field of dietary patterns and cardiometabolic disease (CMD). Numerous studies have demonstrated that healthy dietary patterns with emphasis on food-based recommendations are the gold standard for extending lifespan and reducing the risks of CMD and mortality. Healthy dietary patterns include various permutations of energy restriction, macronutrients, and food intake patterns such as calorie restriction, intermittent fasting, Mediterranean diet, plant-based diets, etc. Early implementation of healthy dietary patterns in patients with CMD is encouraged, but an understanding of the mechanisms by which these patterns trigger cardiometabolic benefits remains incomplete. Hence, this review examined several dietary patterns that may improve cardiometabolic health, including restrictive dietary patterns, regional dietary patterns, and diets based on controlled macronutrients and food groups, summarizing cutting-edge evidence and potential mechanisms for CMD prevention and treatment. Particularly, considering individual differences in responses to dietary composition and nutritional changes in organ tissue diversity, we highlighted the critical role of individual gut microbiota in the crosstalk between diet and CMD and recommend a more precise and dynamic nutritional strategy for CMD by developing dietary patterns based on individual gut microbiota profiles.

Older adults and healthcare professionals have limited awareness of the link between the Mediterranean diet and the gut microbiome for healthy aging

Objectives

Strategies to improve the gut microbiome through consuming an improved diet, including adopting the Mediterranean Diet (MD), may promote healthy aging. We explored older adults' and healthcare professionals' (HCPs) perspectives of the MD, gut health, and microbiome for their role in healthy aging.

Design

Phenomenological qualitative.

Setting

Community-dwelling older adults and HCPs in primary and secondary care in Ireland.

Participants

Older adults (aged 55 + years), recruited through social, retirement and disease-support groups. HCPs recruited through researcher networks and professional associations.

Measurements

Semi-structured 1:1 interviews and focus groups (FGs) conducted remotely with older adults and HCPs separately. Interviews/FGs were recorded, transcribed, and coded using inductive thematic analysis.

Results

Forty-seven older adults were recruited (50% male; 49% aged 60-69 years; 28% 70 +), and 26 HCPs including dietitians (n = 8); geriatricians (n = 6); clinical therapists (n = 4); nurses, pharmacists, catering managers, and meal-delivery service coordinators (n = 2 each). Older adults considered the MD "a nice way to enjoy food," good for cardiovascular health and longevity, but with accessibility and acceptability challenges (increased salads/fish, different food environments, socio-cultural differences). HCPs felt the MD is included in healthy eating advice, but not overtly, mostly through the promotion of mixed-fiber intake. Older adults considered "live" yogurt and probiotics, and to a lesser extent fiber, to maintain a "healthy gut," suggesting the gut has "something to do with" cognitive and digestive health. Overall, microbiota-health effects were considered "not common knowledge" among most older adults, but becoming more topical among both professionals and the public with advancing scientific communication.

Conclusion

While "gut health" was considered important, specific effects of the MD on gut microbiota, and the significance of this for healthy aging, was under-recognized. Future efforts should explain the importance to older adults of maintaining the gut microbiota through diet, while appreciating perspectives of probiotic products and supplements.

Exploratory analysis of one versus two-day intermittent fasting protocols on the gut microbiome and plasma metabolome in adults with overweight/obesity

Nutritional interventions are a promising therapeutic option for addressing obesity and cardiometabolic dysfunction. One such option, intermittent fasting (IF), has emerged as a viable alternative to daily caloric restriction and may beneficially modulate body weight regulation and alter the gut microbiome (GM) and plasma metabolome. This secondary analysis of a larger, registered trial (ClinicalTrials.gov ID: NCT04327141) examined the effect of a four-week intervention comparing one vs. two-consecutive days of IF in combination with protein pacing (IF-P; 4-5 meals/day, >30% protein/day) on the GM, the plasma metabolome, and associated clinical outcomes in overweight and obese adults. Participants (n = 20) were randomly assigned to either a diet consisting of one fasting day (total of 36 h) and six low-calorie P days per week (IF1-P, n = 10) or two fasting days (60 h total) and five low-calorie P days per week (IF2-P, n = 10). The fecal microbiome, clinical outcomes, and plasma metabolome were analyzed at baseline (week 0) and after four weeks. There were no significant time or interaction effects for alpha diversity; however, baseline alpha diversity was negatively correlated with percent body fat change after the four-week intervention (p = 0.030). In addition, beta-diversity for both IF groups was altered significantly by time (p = 0.001), with no significant differences between groups. The IF1-P group had a significant increase in abundance of Ruminococcaceae Incertae Sedis and Eubacterium fissicatena group (q ≤ 0.007), while the IF2-P group had a significant increase in abundance of Ruminococcaceae Incertae Sedis and a decrease in Eubacterium ventriosum group (q ≤ 0.005). The plasma metabolite profile of IF2-P participants displayed significant increases in serine, trimethylamine oxide (TMAO), levulinic acid, 3-aminobutyric acid, citrate, isocitrate, and glucuronic acid (q ≤ 0.049) compared to IF1-P. Fecal short-chain fatty acid concentrations did not differ significantly by time or between groups (p ≥ 0.126). Interestingly, gastrointestinal symptoms were significantly reduced for the IF2-P group but not for the IF1-P group. Our results demonstrate that short-term IF modestly influenced the GM community structure and the plasma metabolome, suggesting these protocols could be viable for certain nutritional intervention strategies.

An urban diet differentially alters the gut microbiome and metabolomic profiles compared with a seed diet in mourning doves

Urbanization influences food quality and availability for many avian species, with increased access to human refuse and food subsidies in built environments. In relation to such nutritional intakes and their presumed impact on microbes harbored in the intestinal tract and metabolic profiles of host physiological systems, our overall knowledge of the role of gut microbiome (GM) and metabolomic expression in the avian host lags far behind our understanding of mammals. Therefore, the objective of this investigation was to examine the potential differential effect of an urban modeled versus control (i.e., bird seed) diet on the GM, the metabolic profiles of plasma, liver, adipose, kidney, and muscle tissues, and circulating endotoxin and inflammatory factors in urban-caught mourning doves (Zenaida macroura). We hypothesized that the urban diet would differently impact the profiles of the GM and tissue metabolomes and increase plasma lipopolysaccharide (LPS) and proinflammatory factors compared with animals fed a seed diet. After a 4-wk-diet period, contents of the large intestine were sequenced to profile the microbiome, metabolomic analyses were performed on plasma and tissue homogenates, and circulating LPS and inflammatory markers were assessed. The composition of the GM was significantly dissimilar between diets, with greater abundance of Erysipelatoclostridiaceae, Sanguibacteraceae, Oribacterium, and Sanguibacter and decreased circulating LPS in the urban-fed birds. These differences were largely not reflected in the surveyed metabolomes and plasma inflammatory markers. This research supports the notion that the microbial composition in urban doves is impacted by diet, though may only weakly associate with host physiology.

The microbiota composition drives personalized nutrition: Gut microbes as predictive biomarkers for the success of weight loss diets

The investigation of the human gut microbiome during recent years has permitted us to understand its relevance for human health at a systemic level, making it possible to establish different functional axes (e.g., the gut-brain, gut-liver, and gut-lung axes), which support the organ-like status conferred to this microecological component of our body. The human gut microbiota is extremely variable but modifiable via diet, a fact that allows targeting of microbes through defined dietary strategies to uncover cost-effective therapies to minimize the burden of non-communicable diseases such as pandemic obesity and overweight and its metabolic comorbidities. Nevertheless, randomly controlled dietary interventions regularly exhibit low to moderate degrees of success in weight control, making their implementation difficult in clinical practice. Here, we review the predictive value of the baseline gut microbiota configurations to anticipate the success of dietary interventions aimed at weight loss, mostly based on caloric restriction regimes and oral fiber supplementation. This emergent research concept fits into precision medicine by considering different diet patterns and adopting the best one, based on the individual microbiota composition, to reach significant adiposity reduction and improve metabolic status. We review the results from this fresh perspective of investigation, taking into account studies released very recently. We also discuss some future outlooks in the field and potential pitfalls to overcome with the aim of gaining knowledge in the field and achieving breakthroughs in personalized nutrition.

Association of food insecurity on gut microbiome and metabolome profiles in a diverse college-based sample

Voluntary caloric restriction (e.g., eating disorders) often results in alterations in the gut microbiota composition and function. However, these findings may not translate to food insecurity, where an individual experiences inconsistent access to healthy food options. In this study we compared the fecal microbiome and metabolome of racially and ethnically diverse first year college students (n = 60) experiencing different levels of food access. Students were dichotomized into food secure (FS) and food insecure (FI) groups using a validated, 2-question screener assessing food security status over the previous 30 days. Fecal samples were collected up to 5 days post survey-completion. Gut microbiome and metabolome were established using 16S rRNA amplicon sequencing, targeted liquid chromatography-tandem mass spectrometry, and gas chromatography-mass spectrometry. FI students experienced significantly greater microbial diversity with increased abundance of Enterobacteriaceae and Eisenbergiella, while FS students had greater abundance of Megasphaera and Holdemanella. Metabolites related to energy transfer and gut–brain-axis communication (picolinic acid, phosphocreatine, 2-pyrrolidinone) were elevated in FI students (q < 0.05). These findings suggest that food insecurity is associated with differential gut microbial and metabolite composition for which the future implications are unknown. Further work is needed to elucidate the longitudinal metabolic effects of food insecurity and how gut microbes influence metabolic outcomes.

Alternate-Day Ketogenic Diet Feeding Protects against Heart Failure through Preservation of Ketogenesis in the Liver

As heart failure develops, the heart utilizes ketone bodies at increased rates, indicating an adaptive stress response. Thus, increasing ketone body availability exerts protective effects against heart failure. However, although it is the widely used approach for increasing ketone body availability, the ketogenic diet shows limited cardioprotective effects against heart failure. This study was aimed at examining the effects of the ketogenic diet on heart failure and the underlying mechanisms. Pressure overload-induced heart failure was established by transverse aortic constriction (TAC) in mice. Continuous ketogenic diet feeding for 8 weeks failed to protect the heart against heart failure. It showed no significant effects on cardiac systolic function and fibrosis but aggravated cardiac diastolic function in TAC mice. Specifically, it induced systemic lipid metabolic disorder and hepatic dysfunction in TAC mice. It decreased the content of 3-hydroxy-3-methylglutaryl-CoA lyase (HMGCL), a key enzyme in ketogenesis, and impaired the capacity of hepatic ketogenesis in TAC mice. It preserved the capacity of hepatic ketogenesis and exerted cardioprotective effects against heart failure, increasing cardiac function and decreasing cardiac fibrosis, in liver-specific HMGCL-overexpressed TAC mice. Importantly, we found that alternate-day ketogenic diet feeding did not impair the capacity of hepatic ketogenesis and exerted potent cardioprotective effects against heart failure. These results suggested that alternate-day but not continuous ketogenic diet protects against heart failure through preservation of ketogenesis in the liver.

Intermittent fasting two days versus one day per week, matched for total energy intake and expenditure, increases weight loss in overweight/obese men and women

Background

Intermittent fasting (IF), consisting of either a one-day (IF1) or two consecutive days (IF2) per week, is commonly used for optimal body weight loss. Our laboratory has previously shown an IF1 diet combined with 6d/week of protein pacing (P; 4–5 meals/day evenly spaced, ~ 30% protein/day) significantly enhances weight loss, body composition, and cardiometabolic health in obese men and women. Whether an IF1-P or IF2-P, matched for weekly energy intake (EI) and expenditure (EE), is superior for weight loss, body composition, and cardiometabolic health is unknown.

Methods

This randomized control study directly compared an IF1-P (n = 10) versus an IF2-P (n = 10) diet on weight loss and body composition, cardiovascular (blood pressure and lipids), hormone, and hunger responses in 20 overweight men and women during a 4-week weight loss period. Participants received weekly dietary counseling and monitoring of compliance from a registered dietitian. All outcome variables were assessed pre (week 0) and post (week 5).

Results

Both groups significantly reduced body weight, waist circumference, percent body fat, fat mass, hunger, blood pressure, lipids, glucose, and increased percent fat-free mass (p < 0.05). However, IF2-P resulted in significantly greater reductions in body weight (-29%) and waist circumference (-38%) compared to IF1-P (p < 0.05), and showed a strong tendency for greater reductions in fat mass, glucose, and hunger levels (p < 0.10) despite similar weekly total EI (IF1-P, 9058 ± 692 vs. IF2-P, 8389 ± 438 kcals/week; p = 0.90), EE (~ 300 kcals/day; p = 0.79), and hormone responses (p > 0.10).

Conclusions

These findings support short-term IF1-P and IF2-P to optimize weight loss and improve body composition, cardiometabolic health, and hunger management, with IF2-P providing enhanced benefits in overweight women and men.

Trial registration

This trial was registered March 03, 2020 at www.clinicaltrials.gov as NCT04327141.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12937-022-00790-0.

Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials

Intermittent fasting is a popular diet for weight loss, but concerns have been raised regarding the effects of fasting on the reproductive health of women and men. Accordingly, we conducted this literature review to clarify the effects of fasting on reproductive hormone levels in humans. Our results suggest that intermittent fasting decreases androgen markers (i.e., testosterone and the free androgen index (FAI)) while increasing sex hormone-binding globulin (SHBG) levels in premenopausal females with obesity. This effect was more likely to occur when food consumption was confined to earlier in the day (eating all food before 4 pm). In contrast, fasting did not have any effect on estrogen, gonadotropins, or prolactin levels in women. As for men, intermittent fasting reduced testosterone levels in lean, physically active, young males, but it did not affect SHBG concentrations. Interestingly, muscle mass and muscular strength were not negatively affected by these reductions in testosterone. In interpreting these findings, it is important to note that very few studies have been conducted on this topic. Thus, it is difficult to draw solid conclusions at present. From the limited data presented here, it is possible that intermittent fasting may decrease androgen markers in both genders. If this is the case, these results would have varied health implications. On the one hand, fasting may prove to be a valuable tool for treating hyperandrogenism in females with polycystic ovarian syndrome (PCOS) by improving menstruation and fertility. On the other hand, fasting may be shown to decrease androgens among males, which could negatively affect metabolic health and libido. More research is warranted to confirm these preliminary findings.

Lipopolysaccharide and the gut microbiota: Considering structural variation

Systemic inflammation is associated with chronic disease and is purported to be a main pathogenic mechanism underlying metabolic conditions. Microbes harbored in the host gastrointestinal tract release signaling byproducts from their cell wall, such as lipopolysaccharides (LPS), which can act locally and, after crossing the gut barrier and entering circulation, also systemically. Defined as metabolic endotoxemia, elevated concentrations of LPS in circulation are associated with metabolic conditions and chronic disease. As such, measurement of LPS is highly prevalent in animal and human research investigating these states. Indeed, LPS can be a potent stimulant of host immunity but this response depends on the microbial species' origin, a parameter often overlooked in both preclinical and clinical investigations. Indeed, the lipid A portion of LPS is mutable and comprises the main virulence and endotoxic component, thus contributing to the structural and functional diversity among LPSs from microbial species. In this review, we discuss how such structural differences in LPS can induce differential immunological responses in the host.

Microbiota and body weight control: Weight watchers within?

Background

Despite several decades of research, managing body weight remains an unsolved clinical problem. Health problems associated with dysregulated body weight, such as obesity and cachexia, exhibit several gut microbiota alterations. There is an increased interest in utilising the gut microbiota for body weight control, as it responds to intervention and plays an important role in energy extraction from food, as well as biotransformation of nutrients.

Scope of the review

This review provides an overview of the role of the gut microbiota in the physiological and metabolic alterations observed in two body weight dysregulation-related disorders, namely obesity and cachexia. Second, we assess the available evidence for different strategies, including caloric restriction, intermittent fasting, ketogenic diet, bariatric surgery, probiotics, prebiotics, synbiotics, high-fibre diet, and fermented foods – effects on body weight and gut microbiota composition. This approach was used to give insights into the possible link between body weight control and gut microbiota configuration.

Major conclusions

Despite extensive associations between body weight and gut microbiota composition, limited success could be achieved in the translation of microbiota-related interventions for body weight control in humans. Manipulation of the gut microbiota alone is insufficient to alter body weight and future research is needed with a combination of strategies to enhance the effects of lifestyle interventions.

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The gut microbiota is involved in the control of nutrient availability, appetite, and body weight.

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Both obesity and cachexia are associated with altered gut microbiota.

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Specific dietary and surgical approaches positively impact body weight and gut microbiota.

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Manipulation of the gut microbiota alone is insufficient to alter body weight in humans.