Olive Leaf Extract Supplementation Combined with Calorie-Restricted Diet on Reducing Body Weight and Fat Mass in Obese Women: Result of a Randomized Control Trial

Obesity is defined as the excessive accumulation of fat in the body (body mass index [BMI] ≥ 30 kg/m²) and is associated with dysregulation of glucose and lipoprotein metabolism [1]. Obesity raises the risk of chronic disorders such as some cancers, cardiovascular disease, and type 2 diabetes mellitus [2]. According to the report of the World Health Organization (WHO) in 2016, more than 1.9 billion adults were overweight and 650 million were obese (13% of the world's adult population) [3]. Existing approaches for handling and treatment of obesity consist of weight loss diet, sport, behavioral changes, pharmacotherapy, and surgery [4]. The efficacy of each of these approaches is often inconclusive [5]. Anti-obesity drugs have adverse side effects such as headaches, vomiting, and myocardial infarctions [6]. However, natural products as the traditional source of medicinal compounds could be developed as complimentary safe and effective approaches for obesity [5, 7, 8].

The leaves of the olive plant (Olea europaea L.), family: Oleaceae, have been applied for centuries in traditional medicine to prevent and cure many illnesses such as wounds [9], fever [10], diabetes [11], gout [12], atherosclerosis [13], and hypertension [14]. Moreover, anti-inflammatory, antioxidant, anti-tumor, antiviral, and antimicrobial properties of olive leaf extract (OLE) were also reported [15, 16] in the European and Mediterranean countries [17, 18]. Its bioactive compounds such as oleuropein (most abundant biophenol), verbascoside, luteolin, rutin, catechin, and hydroxytyrosol (in lower quantities) may be responsible for the biologic activities [19, 20].

Several OLE components have been revealed to represent beneficial effects against obesity both in vitro and in vivo. An animal study demonstrated that the oleuropein-supplemented high fat diet (HFD) for 10 weeks exerted protective effects against hepatic steatosis by decreasing the expression of some hepatic genes involved in the oxidative stress, detoxification of lipid peroxidation products, and inflammation [21]. Moreover, Hsu et al. [22] studied the anti-obesity effect of rutin for 8 weeks in HFD fed mice. They observed rutin as a supplement at a dose of 50 mg/kg significantly decreased body and adipose tissue weights, oxidative stress, serum insulin, lipid profiles, and leptin, as well as hepatic triacylglycerol and cholesterol levels [22]. Caffeic acid supplementation at 0.02% weight dose in HFD-fed mice displayed anti-obesity properties by improving body weight and visceral fat mass, blood lipid profile, and obesity-related hormones such as insulin and leptin [23]. Recently, Fki et al. [7] investigated and compared the hypolipidemic and hepatoprotective protective effects of oleuropein and hydroxytyrosol-rich OLE at 16 mg/kg in HFD fed rats. Oleuropein and hydroxytyrosol similarly decreased body weight and white adipose tissue accumulation by inhibiting the excess increase in the number and/or size of adipocytes and improved the lipid metabolism by elevating the antioxidant system capacity and preventing the expression of the proteins implicated in inflammation and liver injury [7]. In a study by Shen et al. [20], the addition of OLE to HFD in mice significantly reduced body weight gain, visceral fat-pad weights, and plasma lipid levels by downregulating the expression of genes involved in adipogenesis and increasing the mRNA expression of genes involved in mitochondrial biogenesis in the visceral adipose tissue.

The effect of supplementation with olive leaf extract on glucose homeostasis was investigated[24]. Forty-six overweight individuals underwent olive leaf extract supplementation (51 mg oleuropein and 66.9 mg hydroxytyrosol) for 12 weeks. The results of this study showed that supplementation with olive leaf extract improved insulin sensitivity by 15%. However, in this study, food intake was not properly controlled, and a restricted calorie diet was not applied. In a clinical trial examined the effect of consuming 3 cups of olive leaf tea a day on lipid metabolism in 110 non-diabetic and non-obese individuals for 12 weeks. The results showed that after the intervention, low-density lipoprotein cholesterol (LDL-C) and triglycerides (TGs) and fasting glucose levels decreased while body weight, waist circumference (WC) and insulin levels did not change. In this study, not only the exact dose of polyphenols received by individuals is not known, but also the main weight loss intervention, i.e., calorie-restricted diet was not considered and individuals were in the range of normal BMI or overweight [25].

Although several animal studies have evaluated the OLE and its bioactive constituents' effects against diet-induced obesity, the effect of OLE supplementation in obesity along with a weight loss diet has not been reported in the clinical trial. Since the present study aims to investigate a supplementation of the 250 mg OLE in adjunct with a weight loss diet for 8 weeks on weight loss, body composition, glycemic indices, lipid profile, as well as serum level of adiponectin, leptin, and free fatty acid in obese women.

Four participants in the intervention group (three subjects due to noncompliance and one subject due to location change) and three participants in the placebo group (2 subjects due to travel and one subject due to noncompliance), were lost to follow. But, all the statistical analyses were done using the intention-to-treat principle (placebo group, n = 35 and OLE group, n = 35). The study groups were homogenous regarding the baseline age, height, weight, BMI, WC, married status, and physical activity levels (p > 0.05) (Table 1).

Table 1
Baseline characteristics of study participants

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Dietary intake

At the baseline, there were no significant differences in dietary intake variables between the study groups except for vitamin E (p = 0.025) and vitamin A intake (p = 0.011) (Table 2).

Table 2
Dietary intakes of the study participants at baseline and end of the intervention

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In both groups, compared to the baseline, energy, protein, carbohydrates, total fat, polyunsaturated fatty acid (PUFA), monounsaturated fatty acid (MUFA), saturated fatty acid (SFA), and fiber intakes reduced significantly (p < 0.05). In the OLE group, vitamin E intake decreased significantly (p = 0.017). In the placebo group, compared to the baseline, vitamin A intake increased (p = 0.028). However, there were no significant differences in dietary intake variables between the study groups at the end of the study (p > 0.05).

Anthropometric indices

The mean scores of subjects' anthropometric indices are illustrated in Table 3. At the baseline, there were no significant differences in anthropometric indices between the study groups (p > 0.05). In the OLE group, compared to baseline all the anthropometric indices were significantly reduced (p < 0.05). In the placebo group, compared to baseline, body weight, BMI, fat mass, and WC decreased significantly (p < 0.05). The results of ANCOVA presented significant decreases in BMI, body weight, and fat mass in the OLE group compared to those in the placebo group (p < 0.05), but regarding muscle mass, WC and VAI no significant differences was observed between the study groups (p > 0.05).

Table 3
Anthropometric indices of participants at baseline and end of the intervention

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Biochemical measurements

As shown in Table 4, at the baseline, no significant difference was observed between the OLE and placebo groups regarding the biochemical measurements (p > 0.05).

Table 4
Biochemical parameter values of the study groups at baseline and at the end of the intervention

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At the end of the study, the blood concentration of FBS, TC, LDL-C, leptin, insulin, FFA, and HOMA-IR significantly decreased and serum levels of adiponectin and HDL-C elevated in the OLE group (p < 0.05). In the placebo group, at the end of the study, serum levels of TC, leptin, HOMA-IR, and FFA significantly decreased, and HDL-C elevated compared to baseline (p < 0.05). The mean difference in the FBS (p = 0.02), TC (p = 0.01), LDL (p = 0.004), leptin (p = 0.014), adiponectin (p = 0.041), and FFA (p < 0.001) between the 2 OLE and placebo groups after OLE supplementation was significant (tested by ANCOVA after adjusting for covariates).

The findings of this 8-weeks clinical trial showed that oral supplementation of OLE with a weight-loss diet in obese women resulted in significant reduction in body weight, BMI, fat mass, and serum levels of FBS, TC, LDL-C, leptin, and FFA, and significant increase in adiponectin serum level.

Positive energy balance under constant conditions increases the accumulation of fat and body weight and thus induces obesity [25]. Studies have shown that weight loss interventions (diet, supplements, etc.) can improve metabolic parameters such as glycemic status and lipid profile and reduce the mortality rate in obese adults by reducing only 5% to 10% of body weight [31, 32]. Agreeing to the findings of this clinical trial, a cohort study showed the incidence of obesity was lower in individuals who consumed olive oil [33]. Various animal studies have shown that the use of OLE or oleuropein supplement has reduced body weight, fat mass, and expression of genes involved in adipogenesis and lipogenesis in obese mice on a high-fat diet [34, 35, 36]. Contrary to the results of the present study, a clinical trial by Araki et al. showed that consumption of 330 mL of olive leaf tea three times per day for 12 weeks in pre-diabetic individuals with BMI range between 23 to 29 kg/m² had no significant effect on weight loss and WC. Nevertheless, a significant reduction was seen in WC when the analysis was limited to individuals with abdominal obesity [37]. According to the results, it can be said that the difference in BMI and intervention on obese people in this study (BMI ≥ 30 kg/m²) is one of the important reasons for the difference in findings. On the other hand, the type of intervention and the use of diet along with supplementation can also be another possible reason for better results in this study. In another crossover trial by de Bock et al. [38], it was found that the use of supplement containing OLE for 12 weeks had no a significant effect on body composition in middle-aged overweight men. Here again, it can be said that intervention on overweight people and lack of high BMI in these people, the use of supplement without diet and difference in the composition of the 2 supplements are important possible reasons for the lack of conclusion. In this study, there was also a small reduction in fat-free mass. However, this reduction in fat-free mass was not clinically significant compared to weight loss. Physiologically, 75% of the weight loss that occurs during a low-calorie diet is related to fat mass, and only 25% is related to fat-free mass. Therefore, it is possible to prevent excessive degradation of fat-free mass by using olive oil along with a weight-loss diet. A similar study by Rezaei et al. [39] reported that the use of olive oil (20 g/day) in combination with a weight loss diet (−500 kcal/day) for 12 weeks could prevent from significant decrease in fat free mass.

According to recent evidence, one possible mechanism is that olive leaf compounds improve obesity by reducing the accumulation of intracellular fat in the cell line of 3T3-L1 pre-adipocyte [23, 40]. Another effect of OLE on obesity is to inhibit the adipogenesis process by inhibiting the conversion of pre-adipocyte to mature adipocyte (differentiation) [41]. In this process of differentiation, CCAAT-enhancer-binding proteins and peroxisome proliferator-activated receptor gamma (PPAR-γ) play a key role as transcription factors [42]. Therefore, OLE shows its anti-obesity effect by reducing the gene expression of these two transcription factors [34]. It has been shown that changes in the composition of the gut microbiota such as reduction of bacterial diversity and upsetting the balance between beneficial and harmful bacteria (decrease in Bacteroidetes and increase in Firmicutes) occur in obese individuals and the number of harmful bacteria increases in them [43]. Also, an animal study showed that germ-free mice extracted more energy from food and their body fat mass was significantly higher when were colonized with a microbiota from obese mice compared to when they were colonized with a microbiota from lean mice [44]. So, OLE maintains the balance of microbiota, reduces intestinal permeability, and in this way can also affect obesity [8, 45].

Obesity is associated with hyperglycemia, disturbance in lipid metabolism, and increase of insulin resistance [8]. Olive leaves are rich in many phenolic compounds, one of the most abundant of these compounds is oleuropein) approximately 80%) [46]. Studies have shown that OLE improves glucose metabolism [37, 47]. In line with the findings of the present study, a clinical trial by Wainstein et al. [48] described that consumption of OLE (one tablet of 500 mg/day) for 14 weeks resulted in improvement in fasting serum levels of insulin and HbA1c in type 2 diabetes mellitus patients. In the study of Wainstein, the supplementation dose and duration of intervention were higher. But in the present study, it seems that the weight loss diet could help supplementation in improving metabolic parameters. In another study, supplementation with OLE demonstrated a 28% improvement in pancreatic B-cell responsiveness and a 15% improvement in insulin sensitivity in middle-aged overweight men [38]. On the contrary, Rezaei et al. [39] showed that the supplementation with olive oil (20 g/day) in combination with a weight loss diet (−500 kcal/day) for 12 weeks could not induce a significant reduction in the serum levels of FBG, insulin, and HOMA-IR in patients with non-alcoholic fatty liver disease. Initial normal levels of data, different type of intervention (use of olive oil instead of extract) and different target population can be the reasons for different results in the two studies.

One of the mechanisms proposed for the hypoglycemic effects of OLE is the prevention of glucose uptake and starch digestion and also stimulation of the synthesis of hepatic glycogen. Another anti-glycemic effect is inhibition of pancreatic α-amylase activity [49]. Additionally, plasma levels of insulin-like growth factor-binding protein 1 (IGFBP-1) and insulin-like growth factor-binding protein 2 (IGFBP-2) are increased by OLE [49]. There is an inverse relationship between serum levels of insulin and IGFBP-1 concentrations. Also, increase of IGFBP-2 concentration leads to improve insulin sensitivity and prevent obesity [50].

Hypolipidemic effects of OLE have been reported in mice were fed a high-fat diet [8, 35] and in subjects with hypercholesterolemia [51]. In line with the findings of this study, Lockyer et al. [24] found 6 weeks receiving phenolic-rich OLE containing 136 mg of oleuropein significantly decreased the serum levels of TG, TC, and LDL-C in prehypertensive male. In another study, daily supplementation with 100 mg oleuropein for 12 months resulted in a significant decrease in serum levels of LDL-C and TC in postmenopausal women, but TG and HDL-C were not significantly changed compared to the control group [52 ]. Normal baseline levels of TG and HDL-C and different design study could be the possible reasons for the difference in results. Contrary to that observed in this clinical trial, de Bock et al. [38] reported supplementation with OLE had no significant effect on lipid profile in middle-aged overweight men. Relatively normal levels of lipid profile, different target population, different type of intervention and different dose of supplementation are possible causes of insignificance. One of the mechanisms of effect on the lipid profile is that OLE reduces the activity of the hydroxymethylglutaryl-CoA reductase enzyme (rate-limiting enzyme of cholesterol synthesis) and inhibits the production of cholesterol. OLE also reduces the synthesis of Sterol regulatory element-binding transcription factor 1 and PPAR-γ and through it can prevent the increase of serum fatty acids and the synthesis of triglycerides [25].

Obesity has been shown to be associated with high levels of serum free fatty acids and impaired secretion of various adipokines such as leptin and adiponectin from adipose tissue [53]. Leptin is involved in the regulation of food intake and energy expenditure [54]. Studies have shown that serum levels of leptin increase in obese individuals and leading to the induction of insulin resistance. On the other hand, serum levels of adiponectin (role in inducing insulin sensitivity and improving lipid profile) reduce in these individuals [55, 56 ]. Therefore, considering the relationship between leptin and adiponectin adipokines with metabolic factors, it can probably be said that the improvement of obesity through various interventions with an effect on these two adipokines can play a positive role in improving metabolic parameters. In confirmation of this subject, the study by Hotta et al. [57] showed that weight loss increased plasma levels of adiponectin in diabetic patients. Also, an inverse relationship was seen between adiponectin levels with glucose levels and fasting plasma insulin [57]. Similar to the beneficial effects of the OLE in this study, it was shown that supplementation with OLE significantly decreased serum levels of free fatty acids and leptin and increased serum levels of adiponectin in obese mice fed a high-fat diet [34]. Also, Mediterranean dietary patterns (rich in virgin olive oil) have been independently and positively associated with plasma levels of adiponectin and the oleic acid (monounsaturated fatty acid) in olive oil can affect the gene expression of leptin and adiponectin [58].

Therefore, it can be suggested one possible mechanism is that OLE improves glycemic status, lipid profile, and serum levels of free fatty acids by reduction of body weight, increasing serum levels of adiponectin, and decrease of serum leptin. Also, the use of a weight loss diet along with supplementation may help to enhance these effects.

To the best of the authors' knowledge, this is the first study evaluating the effects of OLE supplementation with a weight loss diet in obese women. Also, reporting the mean changes and presenting the results in the two forms of the crude model (without adjustment for confounding factors) and adjusted model (with eliminating the effect of confounding factors) increased the accuracy of the results. One of the limitations of this trial was the selection of only 2 groups. Therefore, it is suggested to design other studies with 4 study groups in the future (group 1, weight loss diet with placebo; group 2, weight loss diet with the supplement; group 3, placebo; group 4, supplement), also trials with more intervention time could be suggested for future studies.

In conclusion, based on results it seems that 8-week OLE supplementation along with a weight loss diet compared with a weight-loss diet alone may be more effective in improving obesity and metabolic risk factors.