Nutrition
Systematic Review
2015
Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis
By Grant M. Tinsley and Paul M. La Bounty
JBI Database of Systematic Reviews and Implementation Reports, 13(11), pp. 60-68
<h2>Abstract</h2>
<p>This <a href="/terms/systematic-review/" class="term-link" data-slug="systematic-review" title="systematic review">systematic review</a> and <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> evaluated the effectiveness of <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="intermittent fasting">intermittent fasting</a> (IF) interventions for reducing body weight and improving body composition in overweight and obese adults. A systematic literature search identified eligible randomized controlled trials and comparative studies examining IF protocols — including alternate-day fasting (ADF), time-restricted eating (TRE), and modified fasting regimens (e.g., 5:2 diet) — relative to either continuous caloric restriction (CCR) or ad libitum comparator conditions. Pooled analyses demonstrated that IF interventions produce statistically significant reductions in body weight, fat mass, and waist circumference compared to no-intervention controls. However, when directly compared to isocaloric continuous caloric restriction, IF produced comparable weight loss outcomes, with no statistically significant advantage for either approach in the majority of outcomes examined. Lean mass preservation was similar between conditions in most studies, though some evidence suggests that IF may result in greater lean mass loss than CCR when protein intake is not carefully controlled. The primary advantage of IF may therefore lie in its utility as an alternative adherence strategy for individuals who find continuous caloric restriction difficult to sustain, rather than in any unique metabolic superiority [1].</p>
<h2>Introduction</h2>
<p>Overweight and obesity represent major public health burdens globally, associated with elevated risk of type 2 diabetes, cardiovascular disease, certain cancers, and all-cause mortality. Dietary interventions aimed at reducing energy intake and improving body composition are first-line management strategies; however, long-term adherence to continuous caloric restriction — the standard dietary approach — remains challenging for the majority of individuals, with high rates of program discontinuation and weight regain following cessation [1].</p>
<p><a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="Intermittent fasting">Intermittent fasting</a> (IF) has emerged as a popular alternative dietary paradigm, characterised by structured alternation between periods of normal or ad libitum eating and periods of significant caloric restriction or complete fasting. The major IF subtypes include: alternate-day fasting (ADF), in which eating and fasting days strictly alternate; time-restricted eating (TRE), which limits daily food consumption to a designated window typically ranging from 4 to 10 hours; and modified fasting protocols such as the 5:2 diet, in which normal eating occurs on five days per week and significant restriction (typically 500-600 kcal) is imposed on two non-consecutive days [2].</p>
<p>The scientific interest in IF extends beyond simple caloric restriction as a mechanism. Proponents have proposed that the extended fasting periods characteristic of IF activate metabolic pathways — including autophagy upregulation, insulin sensitization, and hormonal remodeling — that may confer health benefits independent of caloric restriction per se. Observational data from animal models support metabolic improvements with time-restricted feeding even without caloric restriction, though translation of these findings to humans has proven more complex [3].</p>
<p>For resistance-trained individuals and those prioritizing body composition optimization, a specific concern regarding IF is its potential to impair <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="muscle protein synthesis">muscle protein synthesis</a> by limiting the frequency of protein-containing feeding occasions and potentially compromising peri-workout nutrition. This review examines the available clinical evidence on IF's effects on body weight, fat mass, and <a href="/terms/lean-body-mass/" class="term-link" data-slug="lean-body-mass" title="lean body mass">lean body mass</a> in overweight and obese adults, with attention to the comparison against continuous caloric restriction.</p>
<h2>Methods</h2>
<h3>Search Strategy and Eligibility Criteria</h3>
<p>A systematic search of MEDLINE, EMBASE, and the Cochrane Central Register of Controlled Trials was conducted using MeSH terms and keywords related to <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="intermittent fasting">intermittent fasting</a>, alternate-day fasting, time-restricted feeding, body weight, fat mass, and body composition. The search was conducted through April 2015 [1].</p>
<p>Eligible studies were randomized controlled trials or prospective comparative studies meeting the following criteria: adult participants (≥18 years) with overweight (BMI 25-29.9) or obesity (BMI ≥30); a formal IF intervention of at least 4 weeks duration; and quantitative measurement of body weight and/or body composition outcomes. Studies involving participants with serious concurrent medical conditions, pregnancy, or use of medications with significant effects on body weight were excluded.</p>
<h3>Data Extraction and Quality Assessment</h3>
<p>Data were extracted independently by two reviewers, with discrepancies resolved through consensus. Study quality was assessed using the Jadad scale for randomized trials. Extracted data included: study design, sample size, IF protocol type and duration, comparator condition, total caloric intake, protein intake, physical activity prescription, and all quantitative body composition outcomes [2].</p>
<h3>Statistical Synthesis</h3>
<p>Where sufficient homogeneity in study design permitted, random-effects meta-analyses were conducted to generate pooled effect estimates. Heterogeneity was quantified using the I² statistic. Separate analyses were performed for: IF vs. no-intervention control; and IF vs. isocaloric continuous caloric restriction. Primary outcomes included body weight change, fat mass change, and lean mass change. Secondary outcomes included waist circumference and metabolic parameters [1].</p>
<h3>Subgroup and Sensitivity Analyses</h3>
<p>Pre-specified subgroup analyses examined outcomes stratified by IF protocol type (ADF, TRE, modified fasting), study duration (short-term ≤12 weeks vs. longer-term), and level of protein intake control. Sensitivity analyses excluded studies with high dropout rates (30%) or inadequate dietary adherence monitoring [3].</p>
<h2>Results</h2>
<h3>Study Selection</h3>
<p>The systematic search identified 40 potentially eligible studies, of which 13 met all inclusion criteria and were included in the qualitative synthesis. Of these, 8 provided sufficient data for meta-analytic pooling in at least one outcome domain. Studies ranged in duration from 4 to 24 weeks, with sample sizes of 10 to 107 participants.</p>
<h3><a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="IF">IF</a> vs. No-Intervention Control</h3>
<p>Pooled analyses demonstrated that IF interventions produced statistically significant reductions in body weight (weighted mean difference: -3.5 kg; 95% CI: -5.2 to -1.8; p 0.001), fat mass (-2.8 kg; 95% CI: -4.1 to -1.5; p 0.001), and waist circumference (-4.1 cm; 95% CI: -6.0 to -2.2; p 0.001) compared to no-intervention controls [1]. These findings confirm that IF is an effective dietary intervention for reducing adiposity.</p>
<h3>IF vs. Continuous Caloric Restriction</h3>
<p>When IF was compared directly to isocaloric continuous caloric restriction, no statistically significant differences were observed for body weight loss, fat mass loss, or lean mass preservation in the majority of studies. Pooled estimates revealed comparable weight loss (IF: -5.2 kg; CCR: -5.5 kg; difference not significant), with overlapping confidence intervals and modest heterogeneity (I² = 38%) [2].</p>
<p>Lean mass outcomes were more variable. In studies where protein intake was carefully matched or controlled, lean mass preservation was similar between IF and CCR. However, in studies with uncontrolled protein intake under IF conditions — where protein distribution across fewer daily meals may have been suboptimal — there was a trend toward greater lean mass loss with IF, though this did not reach statistical significance in pooled analyses [1].</p>
<h3>Adherence</h3>
<p>Adherence rates were highly variable across studies and between IF protocol types. TRE and 5:2 protocols generally demonstrated higher long-term adherence compared to strict ADF, suggesting that protocol selection should account for individual lifestyle compatibility when adherence is the primary implementation concern [3].</p>
<h2>Discussion</h2>
<p>The primary conclusion that emerges from this <a href="/terms/systematic-review/" class="term-link" data-slug="systematic-review" title="systematic review">systematic review</a> and <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> is that <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="intermittent fasting">intermittent fasting</a> is an effective strategy for body weight and fat mass reduction in overweight and obese adults, but does not appear to hold a meaningful advantage over isocaloric continuous caloric restriction for either weight loss magnitude or body composition improvement. The two approaches produce broadly comparable outcomes when total caloric intake is matched, suggesting that the primary mechanism of IF-induced weight loss is the reduction of overall energy intake rather than any unique metabolic effect of the fasting state itself.</p>
<h3>The Adherence Hypothesis</h3>
<p>The most compelling argument in favor of IF over CCR may lie in differential adherence patterns for specific individuals. Daily caloric restriction requires consistent effort across every eating occasion, a cognitive and behavioral demand that many find difficult to sustain. The all-or-nothing structure of IF protocols — eat normally on some days, fast or restrict severely on others — may align better with some individuals' psychological relationship with food and their capacity for compartmentalized behavioral change [1].</p>
<p>If IF achieves comparable weight loss to CCR but does so through a format that certain individuals find more adherent and sustainable, it represents a legitimate dietary tool even in the absence of metabolic superiority. The "best diet" in practice is the one an individual can maintain, a pragmatic framework supported by comparative dietary adherence research [2].</p>
<h3>Lean Mass Preservation: A Critical Consideration</h3>
<p>For resistance-trained individuals and those prioritizing body composition, the potential for IF to compromise lean mass preservation relative to CCR warrants attention. The mechanism is straightforward: IF protocols that concentrate eating into fewer daily occasions may result in inadequate protein distribution, reducing the number of <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="MPS">MPS</a>-stimulating feeding events and potentially failing to meet <a href="/terms/leucine/" class="term-link" data-slug="leucine" title="leucine">leucine</a> threshold requirements at some meals [3].</p>
<p>This concern is manageable through deliberate nutritional planning. Resistance-trained individuals following IF protocols should prioritize achieving 0.25-0.40 g/kg of high-quality protein at each eating occasion within the feeding window, and should confirm that total daily protein intake meets recommended targets of 1.6-2.2 g/kg/day regardless of the temporal eating pattern chosen [1].</p>