# A Critical Review on Obesity: Herbal Approach, Bioactive Compounds, and Their Mechanism
## 1 Introduction  
Obesity is defined as an elevation of the fat tissue mass in the body, which is not limited to any age group. It is characterized by increased body and fat mass, hormonal disturbances, food intake (eating pattern), and genetic factors [1]. Obesity is a significant contributor to the global burden of several chronic diseases such as diabetes mellitus type 2, cardiovascular diseases, asthma, etc., which affect the human body. It has been proclaimed a global pandemic with a death rate of approximately 2.8 million people annually [2, 3]. Since fat is present all over the body, it is not possible to measure it directly. Therefore, body mass index (BMI) is used to observe the relationship between weight and height. Whereas waist circumference, waist/hip ratio, skinfold thickness, and bioimpedance help to assess obesity and overweight [4]. Suppose the BMI of the person lies in the range of overweight, the chances of getting affected by other diseases such as diabetes mellitus type 2, hypertension, cardiovascular diseases, gallstones, etc., increase. For obesity class 1, the chances are moderate, while in the case of obesity class 2, the chances are severe. In the case of extreme obesity, the chances are very high, particularly if the person is affected by other obesity-related diseases [5, 6]. Figure 1 summarizes the BMI classification recommended by WHO and the National Institute of Health in the United States. There are two types of fats, visceral and subcutaneous. The fat deposited over visceral organs such as the kidney, abdomen, liver, intestines, heart, and pancreas is known as visceral fat. The other name for visceral fat is active fat because research has proved that it significantly affects our hormonal activity, resulting in metabolic syndrome and insulin resistance. The hormonal activity influences metabolism, body fat distribution, and appetite [7]. Moreover, the fat underneath the skin is known as subcutaneous fat and can be felt underarm and legs [8]. The body attains two types of shapes, apple, and pear, due to fat deposition in certain body areas [9]. In the case of an apple shape (Android), fat accumulation occurs in the upper area of the waist and abdomen. Some deposition also appears on the neck, arms, and shoulders. The main reason behind this body shape is visceral fat, which is associated with the health risk of type 2 diabetes [10]. In the pear shape (Gynoid), fat deposition occurs on the thighs and buttocks. The amount of visceral fat is low in this type of body shape, resulting in low chances of weight-related diseases [11]. Obesity also causes several complications such as reproductive (women and men), respiratory, cardiovascular diseases, pancreas, and gastrointestinal, as shown in Figure 2.  
According to the survey performed by the Organisation for Economic Co-operation and Development (OECD) in 2017, the United States of America is ranked first, followed by China and India in obesity. From 1999 to 2000 and 2015 to 2016, there has been a remarkable increase in obesity [12]. In 2016, the World Health Organization (WHO) reported that over 1.9 billion adults who were either 18 years old or above were overweight, and among these 1.9 billion, 650 million were suffering from obesity [2]. According to a survey, 25 million people die annually because of obesity or being overweight [12]. According to another study by WHO (World Health Organization) in 2019, 38.2 million children under the age of 5 were either suffering from obesity or overweight [13]. Figure 1: Classification of weight according to body mass index (BMI). Even in India, the prevalence of obesity varies from urban to rural and state-wise. It was reported that the prevalence of obesity in males was higher in urban areas (37.5%) compared to rural areas (20.78%) [14]. Some of the states of India affected by obesity are Jharkhand, Kerala, Pondicherry, Chhattisgarh, Madhya Pradesh, Bihar, and Andhra Pradesh [14; 15].