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Obesity is simply defined as the excess of body weight with respect to the height of the person. In adults, obesity and overweight are calculated by a simple index of weight‐for‐height that is the body mass index (BMI). It is calculated using a person's weight in kilogrammes divided by the square of his height in metres (kg m⁻²). Obesity, a complex disease characterised by abnormal or excessive adiposity, fat accumulation or body fat (BF) leads to impairment of health as it affects not only in terms of the body size but also metabolically. Obesity greatly increases the risk of chronic disease morbidity including cardiometabolic syndrome, disability, depression, type 2 diabetes, CVD, and certain cancers leading to mortality. The challenge in this is that obesity and overweight initially were largely considered as a preventable disease but has evolved into a global burden. Since the 1960s, worldwide obesity has nearly tripled and reached epidemic proportions. Recently published data show that there is a gradual increase in the global prevalence of overweight adults (Figure 2.6) and that overweight and obesity, and its associated comorbidities cause over four million deaths yearly, worldwide (Grundy 2016; G. B. D. Obesity Collaborators et al. 2017).

Figure 2.6 Graphical representations showing how ecology and urbanisation have impacted obesity and cardiometabolic syndrome with years (d) cardio‐metabolic disease (CMD) per 100 000 population (WHO) is congruent with the data for (a) global temperature abnormalities (NOAA 2013), (b) global prevalence of overweight adults (‘Our World in Data: Obesity’ 2017), and (c) percentage of global urban population (‘The World Bank’ 2013) through a span of almost last 50 years.

Obesity is fundamentally caused by an imbalance in dietary and nutritional intake (Hruby and Hu 2015). The disproportion of the calories consumed and calories expended, along with higher intake of energy‐dense foods which are high in fat and sugars poses a major burden to chronic diseases and health complications such as insulin resistance, type 2 diabetes, inflammation, cardiometabolic syndrome, coronary heart disease (CHD), CVD, liver disease, cancer, and neurodegeneration (Figure 2.7) (Saltiel and Olefsky 2017).

Figure 2.7 Individual suffering from obesity leads to insulin resistance, glucose intolerance, visceral adiposity, diabetes, hyperinsulinemia which are the major causes for cardiometabolic syndrome.

Source: Based on Saltiel and Olefsky (2017).

It is also known that a high BMI is a major risk factor for non‐communicable diseases such as CVD (mainly heart disease and stroke), which were the leading cause of death in 2012, diabetes, musculoskeletal disorders, especially osteoarthritis, and cancers including endometrial, breast, ovarian, prostate, liver, gallbladder, kidney, and colon. As the BMI increases, the risk of the cardiometabolic syndrome greatly increases. There is huge evidence of the sharp increase in the obesity‐associated cardiometabolic risks and morbidities. These findings have led to an increase in studies that link and relate the fundamental reasons and risk factors in obesity and their mode of action which lead to these diseases (Table 2.1).

Table 2.1 Obesity‐induced risk factors and associated comorbidities.

Risk factors due to obesity	Mode of action	Associated morbidities	References
Chronic low‐level inflammation and oxidative stress	Causes DNA damage, affects growth‐promoting cytokines, and immune modulation	Chronic local inflammation which is a result of gastroesophageal reflux disease is a probable cause of oesophageal adenocarcinoma	Berger (2014); Fruh (2017); Gregor and Hotamisligil (2011); Bishayee (2014); Randi et al. (2006)
Gallbladder inflammation is a strong risk factor for gallbladder cancer
Chronic ulcerative colitis, hepatitis are risk factors for different types of liver cancer
Atherosclerosis formation, impaired fibrinolysis, increased risk for CVD, including stroke and venous thromboembolism
Increased blood levels of insulin and insulin‐like growth factor‐1 (IGF‐1) (hyperinsulinemia or insulin resistance)	Cause cells to divide more than usual which increases the chances of cancer cells being made	Promotes the development of colon, kidney, prostate, and endometrial cancers, type 2 diabetes	Roberts et al. (2010); Gallagher and LeRoith (2015); Dey and Senapati (2021b)
Adipokines, hormones produced by fat cells	Stimulate or inhibit cell growth, induce cell proliferation	Example: Leptin, an adipokine is linked to the pathophysiology of breast cancer, obesity‐induced hypertension, neurodegenerative diseases including Alzheimer's disease	Modzelewska et al. (2019); Ray (2018)
Fat cells may also have direct and indirect effects on other cell growth regulators	Dysregulation of rapamycin, mTOR pathway, and AMP‐activated protein kinase	Cancer, diabetes, cardiovascular disease, neurodevelopmental, and neurodegenerative disorders	Saxton and Sabatini (2017); Takei and Nawa (2014)