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In healthy adults, a 5-day fasting intervention acutely reduced BMI and altered gut microbiome and metabolite profilesCan five days without food change your body and gut bacteria for weeks?

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Key Takeaway
Consider that 5-day fasting acutely reduces BMI and alters microbiome/metabolites in healthy adults, but data are limited.

This randomized, waitlist-controlled trial investigated the metabolic and biological effects of a 5-day fasting intervention in a cohort of 38 healthy adults. The study assessed body mass index, body composition, gut microbiome composition, plasma and fecal metabolites, and 12-week body weight response. Safety data, including adverse events and tolerability, were not reported in the provided evidence.

Regarding primary outcomes, body mass index was reduced acutely, with partial persistence observed at the 12-week follow-up. Body composition changes were predominantly driven by loss of fat mass. Secondary outcomes revealed marked shifts in gut microbiome composition and plasma and fecal metabolites. Additionally, longer-term changes in microbial diversity were associated with baseline microbiome diversity, and a model combining baseline microbiome and clinical variables predicted the 12-week body weight response.

Key limitations include the controlled nature of the data, the specific population of healthy adults, and the lack of longer follow-up or multi-omics profiling in broader contexts. The practice relevance supports the development of individualized fasting-based interventions. However, caution is warranted given the small sample size and the absence of reported safety data. Causality and certainty notes were not reported.

Imagine giving your body a complete rest from food for five days. In this small study, thirty-eight healthy adults did exactly that. The goal was to see how this intense break from eating changes your weight, body composition, and the tiny organisms living in your gut. The results showed a clear drop in body mass index right away. Most of that weight loss came from losing fat, not muscle. Some of these effects lasted for twelve weeks, though not completely.

Beyond the scale, the study found big shifts in your gut microbiome and the chemicals floating in your blood and stool. These changes were significant, but the study also noted that how much you change depends on what your gut looked like before you started fasting. Researchers used a special model to predict who would respond better based on their starting point. This suggests that one size does not fit all when it comes to fasting.

The researchers were careful to point out that this data comes from a controlled setting with healthy volunteers. We simply do not have enough information yet to say if this works for people with chronic diseases or if the long-term effects are safe for everyone. While this supports the idea of personalized fasting plans, we must wait for more research before making big changes to our diets based on these early findings.

What this means for you:
Five days of fasting changes body fat and gut bacteria in healthy adults, but results vary and more research is needed.

Study Details

Study typeRct
EvidenceLevel 2
PublishedMar 2026
View Original Abstract ↓
BackgroundProlonged fasting may improve metabolic health, but controlled data in healthy adults with longer follow-up and multi-omics profiling are limited. We investigated short- and long-term effects of a 5-day fasting intervention on body composition, gut microbiome, and circulating and fecal metabolites, and assessed whether baseline characteristics predict individual weight-loss response. MethodsIn a randomized, waitlist-controlled trial, 38 healthy adults completed a 5-day fasting intervention with 12-week follow-up (LEANER study). Outcomes included body mass index and body composition, gut microbiome composition, and plasma and fecal metabolites. Changes over time and between groups were evaluated using regression-based models and paired non-parametric tests, as appropriate. Additionally, permutation-based multivariate testing was performed on microbiome and metabolome data. Twelve-week body weight response was predicted using data-driven machine learning with cross-validation, followed by external validation in three independent cohorts undergoing prolonged fasting protocols. ResultsFasting reduced body mass index acutely, predominantly driven by loss of fat mass, and these improvements partially persisted at 12 weeks. Fasting induced marked shifts in gut microbiome composition and in plasma and fecal metabolites. Post-fasting and longer-term changes in microbial diversity were associated with baseline microbiome diversity. A model combining baseline microbiome and clinical variables predicted body mass index response at 12 weeks; prominent predictors included an unclassified Faecalibacterium species, Oscillibacter sp. 50_27, low-density lipoprotein cholesterol, and systolic blood pressure. The model generalized to three independent cohorts, including individuals with metabolic syndrome, patients with multiple sclerosis exposed to repeated fasting, and healthy volunteers fasting for 6-12 days. ConclusionsIn healthy adults, a 5-day prolonged fasting intervention produces robust short-term metabolic changes with partial persistence and consistent remodeling of the gut microbiome and metabolite profiles. Baseline microbiome and clinical characteristics can help stratify expected longer-term responses, supporting the development of individualized fasting-based interventions. Trial registrationClinicalTrials.gov, NCT04452916. Registered on June 29, 2020
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