Effects of Popular Diets without Specific Calorie Targets on Weight Loss Outcomes: Systematic Review of Findings from Clinical Trials

Amid the obesity epidemic and poor long-term maintenance of weight loss from calorie-restricted diets, alternative dietary approaches have gained interest. Historical dietary guidelines recommended 45–65% of energy from carbohydrates, 20–35% from fats, and 10–35% from protein. Over recent decades, U.S. carbohydrate intake rose (from 39% in 1971 to 51% in 2011) alongside increasing overweight prevalence. Popular diets (e.g., Atkins, Zone) argue that high-carbohydrate intake may hinder weight loss. Despite popularity, the efficacy of many such diets has been questioned. A prior meta-analysis (Johnston et al., 2014) concluded calorie reduction primarily drives weight loss and differences among macronutrient-defined diets are small. However, that review included studies instructing explicit caloric reduction and/or structured exercise, potentially confounding effects of the diets themselves. The current review aimed to evaluate widely recognized popular diets as recommended (without specific calorie targets, meal replacements, supplementation, or structured exercise) on short-term (≤6 months) and long-term (≥12 months) weight loss outcomes in overweight and obese adults.

The authors reference prior work indicating most calorie-reducing diets yield clinically important weight loss if maintained (Johnston et al., 2014) and note U.S. Dietary Guidelines’ stance that diets with <45% carbohydrates are not more successful for long-term weight loss. They highlight trends in increased carbohydrate intake coinciding with rising overweight prevalence and discuss critiques of popular diets by researchers and clinicians. The review positions itself to isolate effects of named diets absent explicit caloric restriction or structured exercise, addressing a gap in previous syntheses.

Design: Systematic review following PRISMA guidelines; protocol registered with PROSPERO (CRD42017056770). Scope: Among the 38 diets in the 2016 U.S. News & World Report “Best Weight-Loss Diets,” the review included those without specific calorie targets, meal replacements, supplementation with commercial products, and not categorized as low-calorie. Twenty diets met criteria for eligibility screening. Data sources: PubMed, Cochrane Database of Systematic Reviews, Cochrane Central Register of Controlled Trials, and Web of Science searched through September 2016. Search strategy: (diet name) AND (weight OR body mass OR body mass index) AND (overweight OR obesity); limits: humans, clinical trial, adult, English; years 1980–September 2016. Inclusion criteria: interventional clinical trials; ≥15 participants per group; intervention duration ≥12 weeks; adults >18 years; BMI ≥25 kg/m²; objective pre- and post-intervention body weight; English language. Short-term defined as 3–6 months; long-term ≥12 months. Clinically meaningful weight loss defined as ≥5% of baseline body weight. Exclusion criteria: modified diet versions deviating from official guidelines; explicit calorie targets; structured/supervised physical activity components; meal replacement programs; lack of full text; ineligible populations; inadequate duration or sample size; lack of weight outcomes; non-original studies. Study selection and data extraction: Three authors independently extracted data; discrepancies resolved by consensus with senior author. Extracted outcomes included mean weight change and 95% CI; where only baseline and post means were available, SDs and CIs were estimated. Risk of bias: Assessed independently by three authors using Cochrane Handbook criteria (selection, performance, detection, attrition, reporting, other biases), with consensus resolution. PRISMA flow: 2186 records identified; 755 duplicates removed; 1431 screened against criteria; 16 articles included. Trials covered seven diets: Atkins, DASH, Glycemic Index, Mediterranean, Ornish, Paleolithic, and Zone.

- Of 38 popular diets, 20 met criteria for evaluation; 16 clinical trials across 7 diets were eligible: Atkins (10 trials), DASH (1), Glycemic Index (2 short-term, 1 long-term), Mediterranean (1 short-term, 2 long-term), Ornish (2), Paleolithic (1 short-term, 1 long-term), Zone (1 short-term, 2 long-term). - Atkins Diet: 9/10 trials showed clinically meaningful short-term weight loss; 6/8 long-term trials supported long-term weight loss. Examples: Foster et al. reported ~−9.6 kg (−9.7%) at 6 months and −7.2 kg (−7.3%) at 12 months; Foster et al. (2010) observed −12.2 kg at 6 months and −10.9 kg at 12 months; Shai et al. reported −6.3 kg at 6 months and −5.2 kg at 12 months. - DASH Diet: In Blumenthal et al. (4 months), DASH alone led to weight maintenance (−0.3 kg; −0.3%). - Glycemic Index Diet: Ebbeling et al. showed −4.5 kg (−4.3% to −4.4%) at 6 months and −3.0 kg (−2.9%) at 12 months in obese young adults. Melanson et al. found −3.4 kg (−4.0%) at 3 months. - Mediterranean Diet: Elhayany et al. reported −7.4 kg (−8.7%) at 12 months (traditional Mediterranean) and −8.9 kg (−10.3%) at 12 months (low-carbohydrate Mediterranean). Austel et al. found −6.1 kg (−7.2%) at 3 months and −4.2 kg (−4.9%) at 12 months. - Ornish Diet: Dansinger et al. found −3.6 kg (−3.5%) at 6 months and −3.3 kg (−3.2%) at 12 months. Gardner et al. observed −2.5 kg (−2.9%) at 6 months and −2.2 kg (−2.6%) at 12 months. - Paleolithic Diet: Mellberg et al. showed −7.9 kg (−9.0%) at 6 months and approximately −9.2 kg (−10.6%) by 12–24 months. - Zone Diet: McAuley et al. reported −6.9 kg (−7.4%) at 6 months. Dansinger et al.: −3.4 kg (−3.4%) at 6 months and −3.2 kg (−3.2%) at 12 months. Gardner et al.: −2.0 kg (−2.4%) at 6 months and −1.5 kg (−1.8%) at 12 months. - Evidence base disparity: Atkins had the most and strongest evidence for both short- and long-term outcomes among diets assessed. Many popular diets lacked clinical trials meeting criteria. - Risk of bias and adherence: Completion rates varied widely (16%–97%); some studies lacked detailed randomization reporting; intention-to-treat not consistently used; few control diets included.

The review addressed whether popular diets, implemented without explicit calorie targets or structured exercise, produce clinically meaningful weight loss. Only seven of twenty eligible popular diets had qualifying clinical trials, indicating limited empirical testing of most named diets in their recommended form. Across available evidence, the Atkins Diet had the strongest support for both short- and long-term weight loss, with most trials achieving ≥5% body weight loss. Findings contrast with dietary guidelines suggesting low-carbohydrate diets (<45% energy from carbs) are not superior long term, as Atkins and a low-carbohydrate Mediterranean or Paleolithic patterns showed substantial losses. Potential mechanisms discussed include macronutrient-driven effects on energy expenditure and satiety; prior work indicates low-carbohydrate, higher-fat diets may attenuate reductions in energy expenditure during weight maintenance compared with low-fat, high-carbohydrate diets. Overall, the results suggest that following certain popular diet prescriptions without explicit caloric restriction can yield clinically meaningful weight loss, though safety and broader cardiometabolic outcomes warrant further evaluation.

Among current popular diets evaluated without specific calorie targets, the Atkins Diet has the most robust evidence for achieving clinically meaningful weight loss both short term (≤6 months) and long term (≥12 months). Evidence for other diets is limited and heterogeneous, underscoring the need for more rigorous, comparative clinical trials to assess efficacy and sustainability of named diets for weight loss over both short and long durations.

- Limited number of eligible clinical trials for most popular diets; precluded statistical comparisons between diets. - Adherence not consistently measured or reported; some studies had high attrition (>40%), potentially biasing outcomes. - Some trials lacked detailed reporting of recruitment/randomization; intention-to-treat analyses were not uniformly applied; few included control diets. - Outcomes focused solely on body weight change from baseline rather than differences versus controls; other relevant outcomes (waist circumference, body composition, cardiometabolic markers, safety) were not synthesized. - The stringent eligibility criteria (excluding explicit calorie targets and structured exercise) may have omitted informative studies and limited generalizability.