Introduction

Obesity is becoming one of the most prevalent health concerns among all populations and age groups worldwide, resulting into a significant increase in mortality and morbidity related to coronary heart diseases, diabetes type 2, metabolic syndrome, stroke and cancers [1–3]. Prevention and treatment of this problem are an important deal for health systems, whose aim is to reduce the obesity and overweight prevalence, and related complications over the world [4]. Both lifestyle and pharmacotherapy interventions have been considered by physicians and other health care professionals as obesity treatment modalities. Studies show that only 5-10% subjects can maintain their weight loss over the years [5]. The complex pathogenesis of obesity indicates the need of different intervention strategies to confront this problem with a simple drug therapy which is more acceptable to patients [4]. Disappointing results, after cessation the lifestyle modification or pharmacotherapy indicated the need of other treatment modalities to produce better and long-lasting results, in terms of weight loss [6]. Herbal supplements and diet-based therapies for weight loss are among the most common n complementary and alternative medicine [CAM] modalities [7]. A vast range of these natural products and medicinal plants, including crude extracts and isolated compounds from plants can be used to induce weight loss and prevent diet-induced obesity. In the recent decades, these have been vastly used in management of obesity [4, 8] due to containing a large variety of several components with different anti-obesity and anti-oxidant effects on body metabolism and fat oxidation. Medicinal plants have been investigated and reported to be useful in treatment of obesity, diabetes and other chronic diseases [9, 10].

To date, some reviews on anti-obesity agents have been accomplished including, our systematic review on efficacy and safety of herbal plants in the treatment of obesity that published 4 years ago [11]. Because of the increasing number of randomized clinical trials conducted in the recent years, we felt the need for a new systematic review on this topic with a special focus on clinical trials. Therefore, the aim of the present review was to update data on potential anti-obesity herbal plants, and review the scientific data, including experimental methodologies, active components, and mechanisms of action against obesity in human.

Methods

PubMed, Scopus, Google Scholar, Web of Science, and IranMedex databases were searched for studies reported between 30th December 2008 to 23rd April 2012 on human or animals investigating the benefits and harms of herbal medicines to treat obesity. The search terms were “obesity” and (“herbal medicine” or “plant”, “plant medicinal” or “medicine traditional”) without narrowing or limiting search items. Publications with available abstracts were reviewed. The main outcome measures were defined as body weight, body fat, including fat mass/fat weight or fat percentage/visceral adipose tissue weight, waist or hip circumference, triceps thickness and appetite, and the amount of food/energy intake.

Abstracts of publications on human studies with the main outcome as mentioned above were included. In vitro studies, review articles and letters to the editor were excluded. The articles were reviewed for abstracts and title by two reviewers. Due to our inclusion and exclusion criteria, the duplicate articles were eliminated.

Results

Body weight

Significant decrease in body weight was seen by Nigella Sativa, Camellia sinensis, Crocus sativus L, seaweed laminaria digitata, Xantigen, virgin olive oil, Catechin enriched green tea, Monoselect Camellia, Oolong tea, Yacon syrup, Irvingia Gabonensi, Weighlevel, RCM-104 compound of Camellia, Sinensis, Pistachio, Psyllium fibre, black Chinese tea, sea buckthorn and bilberries.

Body fat

Significant decrease in body fat was seen by Xantigen [16], Catechin-enriched green tea [18], Irvingia gabonensis a West African plant [22], RCM-104 a compound of Camellia Sinensis, Semen Cassiae and Flos Sophorae [23], Psyllium Fibre [25], and black Chinese tea [Pu-Erh tea] [26]. Oolong tea showed a decrease in subcutaneous fat content not total body fat [20]. Debese showed a reduction in triceps skin folds in a trial [28].

Waist and hip circumference

There was a significant decrease in waist and hip circumferences with Nigella Sativa [12], Xantigen [16], Catechin enriched green tea [18], Yacon Syrup [21], Irvingia gabonensis [22], Debese [28], Whole grain [29], Lycium barbarum [30], black Chinese Tea [26], Sea buckthorn, and bilberries [27]. Monoselect Camellia from green tea extract reduced the waistline only in men [19]. Pu’er tea [black Chinese tea] decreased the waist-hip ratio significantly [31].

Food intake

A significant decrease in appetite was shown in trials by Trigonella Foenum-graecum L. [32], Fungreek fiber [33]. An extract of Blueberry Bioactives [34], Epigallocatechin of green tea [35], Northern Berries [36], alginate-based brown seaweed Laminaria Difitata [15], and RCM-104 compound of Camellia Sinensis [23] did not show any relevant decrease in appetite.

Other effects

Anti-hyperglycemic, anti-hyperlipidemic, and anti-oxidant effects were detected in these trials [see Table 1].

Table 1 Human studies considering herbal medicines for treatment of obesity
Full size table

Adverse effects

Only alginate-based brown seaweed Laminaria digitata caused an abdominal bloating and upper respiratory tract infection as a side effect in the trial group [15]. There were no other significant adverse effects reported in all 33 trials included in this article.

Discussion

Many studies reported the anti-obesity effects of different herbal plants containing minerals or chemical extracts of plants. All herbal plants with anti-obesity effects are summarized in Table 1 with information of their active components and effects on the body. Anti-obesity effects such as decreasing body weight, body mass index or waist circumference in humans was seen in most of these studies. Some of them showed an anti-obesity effect by decreasing total body fat [16, 18, 20, 22, 23, 25, 26, 28].

A study showed a significant decrease in body weight by Cissus Quadrangularis (CQ), Sambucus Nigra, Asparagus Officinalis, Garcinia Atroviridis, Ephedra and Caffeine, Slimax (extract of several plants, including Zingiber officinale and Bofutsushosan) [11]. In this study, the effect of Epigallocatechin-3-gallate in combination with caffeine was evaluated, with no important changes in body weight or energy expenditure. However, anti-obesity effects of green tea components were reported in many trials.

Anti-obesity mechanisms for herbal plants included reduction in lipid absorption, reduced energy intake, increased energy expenditure, decreased pre-adipocyte differentiation and proliferation, or decreased lipogenesis and increased lipolysis [37]. Decreased energy intake from the gastrointestinal tract is caused by distinct types of tea [e.g. green, oolong, and black tea] acting on pancreatic lipase. In this review, weight loss by different tea components containing catechin and epigallocatechin-3-gallate polyphenols isolated from unlike kinds of teas was observed [18, 26, 35]. Polyphenols of different types obtained from tea extracts (e.g. L-epicatechin, epicatechin-3-gallate, epigallocatechin, epigallocatechin-3-gallate), showed strong inhibitory activity against pancreatic lipase, which led to weight loss [38, 39].

Nigella Sativa showed a significant weight loss and reduced waist circumference with a mild reduction in fasting blood sugar, triglycerides and low-density lipoprotein levels [12]. Pistachio [24], Psyllium Fibre [25], black Chinese Tea [26], Camellia Sinensis [23], Yacon Syrup [21], Oolong Tea [20], Xantigen [16] and olive oil [17] showed the same effects on the body. A systematic review on medicinal plants useful in diabetes mellitus showed that some herbal plants possess anti-hyperlipidemic effects, and this property is statistically significant in the treatment of obesity [40].

Some components affect body weight by changes in body-fat metabolism and oxidation or increasing metabolic rate, which was shown in trials by Epigallocatechin-3-gallate of green tea [35], virgin olive oil [17], Capsinoids [41] and Lycium Brbarum [30] causing a higher fat oxidation in human. These compounds act by activating lipid metabolism, acceleration of oxidation, suppression of fatty acid synthesis and PPARc agonistic activity [37].

A systematic review done on potential herbal sources effective in oxidant-related diseases showed some potential of some plants like Nigella sativa and green tea to decrease lipid peroxidation in plasma or liver, which seem a mechanism of anti-obesity effect. Higher anti-oxidant and anti-obesity activity was shown by green tea due to its high concentration of catechins, including epicatechin (EC), epicatechin-3-gallate (ECG) and epigallocatechin-3-gallate (EGCG) [13, 18, 23]. The anti-oxidative role of herbal plants in different kinds of human diseases, such as diabetes mellitus, obesity and hyperlipidemia has been already reported in literature [40, 42–44]. Those articles focused on herbal plants effective on obesity while lifestyle changes or dietary regimens were not included. However whole grain, pistachio, virgin olive oil and nuts were investigated solely and found efficient in reduction of obesity [17, 24, 29].

The alginate-based brown seaweed Laminaria Digitata [15] caused abdominal bloating and upper respiratory tract infections as a side effect but no other studies reported the same adverse effect.

In the included studies, only few has reported adverse effects, but it is notable that some kind of adverse effects may only happen when drugs used in higher sample size or when approved for marketing widely. Therefore, we cannot conclude that use of these herbals is without adverse effects. We believe that safety of these plants remains to be elucidated by further long-term studies.

Conclusion

Different methods have been used to reduce body weight and its complications for many years. Disappointing results after cessation the lifestyle modification or pharmacotherapy compelled the researchers and physicians to rethink to find a new, safe, and striking therapeutic alternative for this global health concern. Herbal medicines have been in attention as an effective option to reduce body weight and body fat. Taking all results collectively, Nigella sativa, Camellia synensis, green tea, and black Chinese tea were found to have acceptable anti-obesity effects. Furthermore, there have been some reports on anti-oxidative stress effects of some of these plants which may be important in the management of other diseases accompanying with obesity like cardiovascular diseases and diabetes [9, 45]. By now, only one anti-obesity drug called orlistat have been approved by the US food and drug administration for long-term treatment in obese patients. Recent researches show different medications having anti-obesity effects by several mechanisms, including exenatide a glucagon-like peptide [GLP] acting as an incretin hormone [46], Lorcaserin a novel selective serotonin 2C (5-HT2C) receptor agonist that modulates food intake in hypothalamus [47] and PYY 3–36 and oxyntomodulin, a glucagon-like peptide 1(GLP-1) receptor agonist that regulate food intake [48]. The need to discover anti-obesity drugs having better efficacy and lower adverse effect is still felt. The results of this kind of studies can be helpful for pharmaceutical industries to study on the components of these herbs and investigate further to find a mixture of those components with higher efficacy. Furthermore, further well-designed clinical trials are still needed to focus on both safety and efficacy of these herbal medicines.

Author contributions

Mohammad Abdollahi and Shirin Hasani Ranjbar gave the idea and designed the study, reviewed data, and edited the article. Zahra Jouyandeh did the search and drafted the article. All authors have read and approved content of the article.