Effect of Herbal Medicines in Obesity and Metabolic Syndrome: A Systematic Review and Meta-Analysis of Clinical Trials
Abstract
Obesity, resulting from an imbalance between energy intake and expenditure, is a growing global health issue. This systematic review and meta-analysis evaluated the efficacy, safety, and mechanisms of herbal medicines in managing obesity and metabolic syndrome in adults. Clinical trials up to May 2019 were systematically identified from Web of Science, Scopus, PubMed, and the Cochrane database. Data extraction and quality assessment were conducted independently by two experts using the CONSORT checklist. The main outcomes were changes in anthropometric indices and metabolic syndrome components. A total of 279 clinical trials were included. Herbal medicines containing green tea, Phaseolus vulgaris, Garcinia cambogia, Nigella sativa, puerh tea, Irvingia gabonensis, Caralluma fimbriata, and their active ingredients were found effective for obesity and metabolic syndrome management. Additionally, C. fimbriata, flaxseed, spinach, and fenugreek reduced appetite. Meta-analysis revealed that green tea significantly improved weight (SMD: -0.75), BMI (SMD: -1.2), waist circumference (SMD: -1.71), hip circumference (SMD: -0.42), and total cholesterol (SMD: -0.43). P. vulgaris and N. sativa also showed significant improvements in weight and triglycerides, respectively. High-quality trials are still needed to confirm the clinical efficacy of these plants.
Keywords: herbal medicines, meta-analysis, metabolic syndrome, systematic review, obesity, randomized controlled trial
Introduction
Obesity is a complex metabolic disorder characterized by excessive body fat accumulation due to an imbalance between energy intake and expenditure. Its prevalence is increasing worldwide, with the World Health Organization reporting that in 2016, 39% of adults were overweight and 13% were obese. Obesity is associated with poor quality of life and increased risk for diseases such as hypertension, dyslipidemia, type 2 diabetes, osteoarthritis, kidney disease, sleep disorders, and certain cancers.
The pathophysiology of obesity involves genetic, environmental, and behavioral factors. Environmental contributors include sedentary lifestyles and high-calorie diets. While conventional treatments such as diet, exercise, behavioral changes, and anti-obesity drugs exist, these approaches often have limited effectiveness and may cause adverse effects. As a result, there is growing interest in complementary and alternative treatments, including herbal medicines.
Increased lipolysis and decreased lipogenesis
Despite their popularity, the efficacy and safety of herbal medicines remain debated, necessitating rigorous clinical trials. This systematic review and meta-analysis aimed to provide high-level evidence on the efficacy, safety, and mechanisms of herbal medicines for obesity and metabolic syndrome.
Methods
Search Strategy and Data Collection
A comprehensive search was conducted in PubMed, Scopus, Web of Science, and the Cochrane database for clinical trials on herbal medicines for obesity and metabolic syndrome, including studies up to May 2019. References of relevant reviews were also screened. Data extraction and quality assessment were performed independently by two reviewers using the CONSORT checklist adapted for herbal interventions.
Inclusion Criteria
Study design: Randomized and nonrandomized clinical trials, including double- and single-blind, parallel, and crossover designs.
Participants: Adults (≥18 years) who were overweight (BMI ≥25) or had metabolic syndrome (per ATP III or IDF criteria).
Interventions: Raw or refined herbal products derived from plants or plant parts, including single or mixed herbs and their active ingredients.
Comparisons: Herbal medicine vs. no treatment, placebo, exercise, or synthetic medicine.
Outcomes: Primary-body weight, BMI, waist circumference, waist-to-hip ratio, body fat, and appetite. Secondary-cholesterol, blood pressure, triglycerides, and blood glucose.
Data Extraction and Quality Assessment
Data were extracted on study characteristics, participants, interventions, controls, outcomes, adverse events, follow-up, and mechanisms. Quality was assessed using the CONSORT Herbal Medicinal Interventions checklist (max score: 34).
Statistical Analysis
Standardized mean difference (SMD) was used to assess the effect of herbal medicines on obesity and metabolic syndrome outcomes. Heterogeneity was evaluated using Q-test and I² statistics. Random-effects or fixed-effects models were applied as appropriate. Publication bias was assessed by Egger’s test. Meta-analyses were conducted for herbs with at least three studies.
Results
Study Selection and Characteristics
From 12,001 identified articles, 594 were reviewed, and 279 clinical trials were included. Most studies came from the United States, Korea, Japan, Iran, China, Italy, Australia, and India. Both single-herb and combination interventions were included.
Quality Assessment
The average CONSORT score was 23.78 ± 3.51. Most studies clearly reported eligibility criteria and settings, but fewer described herbal product characteristics in detail.
Effects on Anthropometric Indices
Body Weight and BMI
Significant reductions in body weight and BMI were observed with:
Green tea (doses: 300–6,000 mg/day; catechins: 150–1,200 mg/day)
Phaseolus vulgaris (445–3,000 mg/day)
Garcinia cambogia (1,667–3,000 mg/day; 1,000–1,500 mg hydroxycitric acid)
Nigella sativa (1,500–3,000 mg/day)
Puerh tea (1,000 mg/day)
Irvingia gabonensis (150–3,150 mg/day)
Caralluma fimbriata (1,000 mg/day)
Other effective herbs included cumin, flaxseed, Hibiscus sabdariffa, rosehip, Lycium barbarum, cinnamon, and others.
Waist and Hip Circumference
Significant decreases in waist and hip circumferences were reported for green tea, P. vulgaris, N. sativa, I. gabonensis, and C. fimbriata.
Body Fat
Green tea, I. gabonensis, H. sabdariffa, P. vulgaris, G. cambogia, Ecklonia cava, cumin, Coleus forskohlii, sorghum tea, Gynostemma pentaphyllum, and cinnamon significantly reduced body fat percentage and fat mass.
Appetite and Food Intake
Appetite suppression was observed with P. vulgaris, C. fimbriata, spinach, flaxseed, fenugreek, G. cambogia, green coffee, and several herbal
combinations.
Energy Expenditure
Herbs such as Kaempferia parviflora, Aframomum melegueta, oolong tea, green tea, and various combinations increased energy expenditure.
Hormonal Effects
Herbal medicines increased secretion of GLP-1 (e.g., green tea combinations), adiponectin (green tea, I. gabonensis, sea buckthorn), and decreased leptin (I. gabonensis, P. vulgaris, G. cambogia, rapeseed oil).
Lipid Profile
Green tea, N. sativa, H. sabdariffa, I. gabonensis, flaxseed, G. cambogia, puerh tea, E. cava, cinnamon, rice bran, and various combinations significantly reduced triglycerides, total cholesterol, and LDL, and increased HDL.
Glycemic Indices
Green tea, H. sabdariffa, puerh tea, I. gabonensis, P. vulgaris, N. sativa, chia, E. cava, cinnamon, carob, brown rice, and several combinations significantly improved fasting blood glucose and, in some cases, hemoglobin A1c and insulin secretion.
Inflammatory Markers
Herbs such as I. gabonensis, pomegranate, Palmaria palmata, puerh tea, Opuntia ficus-indica, cardamom, and Salba-chia reduced C-reactive protein, TNF-α, and IL-6.
Meta-Analysis Findings
Green tea: Significant reductions in weight (SMD: -0.75), BMI (SMD: -1.2), waist circumference (SMD: -1.71), hip circumference (SMD: -0.42), fat mass (SMD: -0.4), body fat % (SMD: -1.96), total cholesterol (SMD: -0.43), and LDL (SMD: -0.21).
Phaseolus vulgaris: Significant reduction in weight (SMD: -0.88).
Nigella sativa: Significant reduction in triglycerides (SMD: -1.67) and LDL (SMD: -0.86).
Irvingia gabonensis: Significant reduction in weight (SMD: -0.66).
Caralluma fimbriata and Garcinia cambogia: No significant effect on weight or BMI in meta-analysis.
No significant publication bias was found. Most studies reported no serious adverse effects, though some mild gastrointestinal symptoms and headaches were noted with certain herbs.
Discussion
Herbal medicines, especially green tea, P. vulgaris, G. cambogia, N. sativa, puerh tea, I. gabonensis, and C. fimbriata, show promise for managing obesity and metabolic syndrome. Their mechanisms include appetite suppression, increased energy expenditure, inhibition of fat absorption, increased lipolysis, and improved lipid and glycemic profiles. However, the effectiveness and mechanisms remain somewhat controversial, and more high-quality, standardized clinical trials are needed.
Conclusion and Implications for Future Research
Herbal medicines are a promising complementary approach for weight and body fat reduction. Green tea, P. vulgaris, G. cambogia, N. sativa, puerh tea, I. gabonensis, and C. fimbriata demonstrated significant anti-obesity and metabolic effects in controlled clinical trials. However, further well-designed trials are necessary to determine optimal candidates, dosages, and long-term safety. Standardization of herbal interventions and more research into mechanisms and Gambogic adverse effects are recommended.