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

The obesity epidemic necessitates exploring alternative weight loss strategies beyond calorie restriction, which many struggle to maintain. While previous dietary guidelines emphasized a balance of macronutrients, the rise in obesity despite increased carbohydrate consumption has prompted interest in diets modifying macronutrient ratios. Many popular diets, often promoted by experts, claim benefits not consistently supported by research. A previous meta-analysis showed calorie reduction as the main driver of weight loss, but it included studies with additional calorie reduction or exercise components. This systematic review examines the weight loss effects of popular diets (from the 2016 U.S. News & World Report) without pre-defined calorie targets, meal replacements, supplements, or structured exercise. The aim was to evaluate the effectiveness of these diets on short-term (≤6 months) and long-term (≥1 year) weight loss in overweight and obese individuals based on clinical trial data.

The introduction mentions a previous meta-analysis by Johnston et al. (2014) that found calorie reduction to be the primary factor in weight loss, but that analysis included studies with additional interventions (calorie restriction and/or structured exercise). This study aims to address this limitation by focusing on studies that solely evaluated popular diets without these additional components. The efficacy of popular diets for weight loss has been debated among researchers, nutrition experts, and healthcare professionals, highlighting a gap in the literature that this study addresses.

This systematic review adhered to PRISMA guidelines and PROSPERO registration (CRD42017056770). Thirty-eight diets from the 2016 U.S. News & World Report's "Best Weight-Loss Diets" were screened, and 20 met inclusion criteria (no specific calorie targets, meal replacements, supplements, or low-calorie categorization). PubMed, Cochrane Library, and Web of Science were searched (until September 2016). Inclusion criteria involved interventional clinical trials with ≥15 participants per group, ≥12-week intervention, adults (BMI ≥25 kg/m²), objective weight measures, and English-language articles. Short-term was defined as 3–6 months, and long-term as ≥1 year. Clinically meaningful weight loss was ≥5% of baseline weight. Two independent reviewers extracted data, with discrepancies resolved by a senior author. Risk of bias was assessed using the Cochrane Collaboration Handbook guidelines. The main outcome was mean weight change and its 95% confidence interval (CI).

Sixteen articles met the criteria, representing seven diets: Atkins, DASH, Glycemic-Index, Mediterranean, Ornish, Paleolithic, and Zone. The Atkins diet had the most supporting evidence: nine of ten short-term trials showed clinically significant weight loss; six of eight long-term trials demonstrated similar results. The DASH diet showed weight maintenance in one study. The Glycemic-Index diet showed weight loss in two short-term studies and one long-term study. The Mediterranean diet demonstrated weight loss in one short-term and two long-term trials. The Ornish diet showed weight loss in two trials, and the Paleolithic diet showed weight loss in one short-term and one long-term trial. The Zone diet had mixed results across three studies.

The findings challenge current Dietary Guidelines Advisory Committee recommendations suggesting that diets with <45% calories from carbohydrates are not superior for long-term weight loss. This review found the Atkins and Paleolithic diets (both low-carbohydrate) to be effective for short- and long-term weight loss. The study highlights the need for critical examination of the safety of low-carbohydrate, high-fat diets. Weight loss in these studies was comparable to that achieved with calorie-restricted diets, potentially due to the effect of macronutrient manipulation on overall calorie intake. A discrepancy with previous meta-analyses may be due to differences in inclusion criteria; this review excluded studies with additional calorie restriction or exercise.

The Atkins diet demonstrated the most robust evidence for clinically meaningful short-term and long-term weight loss among the tested diets. However, the limited number of studies for most diets highlights a significant research gap. Further comparative research is crucial to better understand the efficacy of various popular diets for both short- and long-term weight loss. Future studies should also consider body composition, metabolic, and cardiovascular outcomes, and address adherence to diet plans.

The primary limitation was the limited number of clinical trials available that met the strict inclusion criteria. This limited the ability to conduct statistical comparisons between diets. High attrition rates in some studies suggest adherence challenges. The analysis focused only on weight change from baseline, not from a control group; other outcomes (waist circumference, BMI, body composition) were not included. The study lacked sufficient data for many popular diets.