The impact of intermittent fasting on body composition and cardiometabolic outcomes in overweight and obese adults: a systematic review and meta-analysis of randomized controlled trials.

This systematic review and meta-analysis investigated the effects of various intermittent fasting (IF) diets on body composition and cardiometabolic health. The researchers compared different forms of IF against control diets (maintaining daily eating habits, including exercise groups considered as control) in overweight and obese adults.

The specific intermittent fasting regimens included:

• Intermittent Energy Restriction (IER): Broadly defined as significant calorie reduction during specific fasting periods, while maintaining normal eating during non-fasting periods.
  • Alternate Day Fasting (ADF): Alternating "fasting days" (complete restriction or significant reduction in food intake) with "feeding days" (free consumption).
  • 5:2 Fasting (Twice-Weekly Fasting): Restricting calorie intake on two chosen days of the week, with normal eating on the other five days.
  • 4:3 Diets: Similar to 5:2 but with three fasting days.
• Time-Restricted Eating (TRE) / Time-Restricted Feeding (TRF): Condensing daily eating into a fixed window (e.g., 8-12 hours) and fasting for the remaining hours (e.g., 16 hours).

The primary outcome measures assessed were:

• Body Composition:
  • Body Weight (BW)
  • Body Mass Index (BMI)
  • Waist Circumference (WC)
  • Waist-to-Hip Ratio (WHR)
• Lipid Profiles (Blood Fats):
  • Total Cholesterol (TC)
  • Triglycerides (TG)
  • Low-Density Lipoprotein (LDL) – often called "bad" cholesterol
  • High-Density Lipoprotein (HDL) – often called "good" cholesterol
• Blood Glucose Control:
  • Fasting Plasma Glucose (FPG)
  • Hemoglobin A1C (HbA1c) – a measure of average blood sugar over 2-3 months
• Blood Pressure:
  • Systolic Blood Pressure (SBP)
  • Diastolic Blood Pressure (DBP)

This meta-analysis synthesized data from 15 randomized controlled trials (RCTs), involving a total of 758 participants. The studies included adults aged 18 years or older who were classified as overweight or obese, defined by a Body Mass Index (BMI) of 25 kg/m² or higher. The participants' biological sex and health status (with or without comorbidities) were not restricted, meaning the findings apply to a broad population of overweight and obese adults. The studies were conducted in various settings, as is typical for a meta-analysis drawing from multiple RCTs globally.

The researchers pooled data from the included studies, which measured various body composition and cardiometabolic markers. While the specific instruments used in each individual study are not detailed in the abstract, the outcome measures themselves imply standard clinical assessment methods:

• Body Weight (BW): Typically measured using calibrated scales.
• Body Mass Index (BMI): Calculated from height and weight (weight in kilograms divided by the square of height in meters).
• Waist Circumference (WC) and Waist-to-Hip Ratio (WHR): Measured using a tape measure at specific anatomical landmarks.
• Total Cholesterol (TC), Triglycerides (TG), Low-Density Lipoprotein (LDL), High-Density Lipoprotein (HDL): Measured from blood samples, typically after an overnight fast, using standard laboratory assays.
• Fasting Plasma Glucose (FPG): Measured from a blood sample after an overnight fast, using standard laboratory assays.
• Hemoglobin A1C (HbA1c): Measured from a blood sample, reflecting average blood glucose levels over the preceding 2-3 months, using standard laboratory assays.
• Systolic Blood Pressure (SBP) and Diastolic Blood Pressure (DBP): Measured using a sphygmomanometer (blood pressure cuff) and stethoscope or an automated blood pressure device.

This study was a systematic review and meta-analysis of randomized controlled trials (RCTs). This design is considered one of the highest levels of evidence in medical research because it systematically combines and analyzes data from multiple high-quality studies, reducing the risk of bias inherent in single studies and increasing the statistical power to detect effects.

How they ran the study:

1. Search Strategy: The researchers conducted a comprehensive search across three major electronic databases: PubMed, Embase, and Web of Science. The search covered studies from their inception up to March 2025. They used specific keywords related to intermittent fasting (e.g., "Intermittent Fasting," "Alternate Day Fasting," "Time-Restricted Eating") and obesity (e.g., "obesity," "overweight"). This systematic approach ensures that as many relevant studies as possible are identified.
2. Study Selection: The selection process followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and the Cochrane Handbook for Systematic Reviews of Interventions, which are international standards for conducting and reporting systematic reviews. Initial records were screened by title and abstract, followed by full-text review.
   • Inclusion Criteria: Studies were included if they were:
     • Randomized Controlled Trials (RCTs) with parallel groups.
     • Involving participants aged 18 years or older with a BMI ≥ 25 kg/m² (overweight or obese).
     • Comparing various forms of intermittent fasting (IER, ADF, 5:2, 4:3, TRF, TRE) against a control group (maintaining daily eating habits, including exercise groups).
     • Reporting at least one of the specified primary outcome indicators (body weight, BMI, waist circumference, waist-to-hip ratio, total cholesterol, triglycerides, LDL, HDL, fasting plasma glucose, HbA1c, systolic blood pressure, diastolic blood pressure).
     • Available in full-text, peer-reviewed, and written in English.
   • Exclusion Criteria: Studies were excluded if they were:
     • Non-original research (e.g., reviews, editorials).
     • Animal studies.
     • Non-randomized trials.
     • Studies comparing IF only with a continuous energy restriction (CR) group, or with no overweight/obese participants, or using a Mediterranean diet as the comparison group. If a study included IF, CR, and CON (control), only the IF vs. CON comparison was used.
3. Data Extraction: Two independent authors extracted data from the selected studies, including details on the first author, publication year, study design, number of groups, participant characteristics (sample size, sex, age, BMI, health status), intervention characteristics (IF and control dietary patterns, duration, protocols), and all outcome indicators. Disagreements were resolved through discussion.
4. Statistical Analysis: Data were pooled using a random-effects model. This model is appropriate when there is expected heterogeneity (variability) between the included studies, which is common in dietary intervention research due to differences in populations, specific IF protocols, and study durations. The random-effects model accounts for both within-study and between-study variability, providing a more conservative estimate of the overall effect.
5. Subgroup Analyses: To explore potential sources of heterogeneity and provide more nuanced insights, subgroup analyses were performed based on:
   • Intervention duration: Short-term (≤ 12 weeks) versus long-term (> 12 weeks).
   • Intermittent fasting form: Alternate Day Fasting (ADF) versus Time-Restricted Eating (TRE).

Why this design matters:

• Systematic Review: By systematically searching and selecting studies, the researchers minimize selection bias and provide a comprehensive overview of the existing evidence.
• Meta-analysis: Combining data from multiple studies increases the statistical power, making it more likely to detect true effects if they exist, and provides a more precise estimate of the effect size than any single study alone.
• Randomized Controlled Trials (RCTs): The inclusion of only RCTs is crucial.