Introduction

Obesity is a complex multi‐factorial disease, which constitutes a major health problem for the United Kingdom and for the wider global community in terms of prevalence, incidence and economic burden. This chapter describes some of the epidemiological features including the prevalence, risk factors and related co‐morbidities of obesity along with its economic impact.

Definition of Overweight and Obesity

The World Health Organization (WHO) defines overweight and obesity as abnormal or excessive fat accumulation that presents a risk to health. Body mass index (BMI) is the most commonly used measurement in large‐scale epidemiological studies or population surveys. BMI is calculated by dividing the body weight in kilograms by the square of height in metres, which is a simple metric used to indicate overall body fatness in adult population. For adults, current guidelines from the United State Centres for Disease Control and Prevention (CDC) and the WHO define a normal BMI range as 18.5–24.9, whereas a BMI ≥ 25 kgm^‐2 is overweight, and a BMI ≥ 30 kgm^‐2 is classified as obese, with severe obesity defined as a BMI ≥ 30 kgm^‐2. In children, overweight is defined at or above the age‐specific 95% BMI percentile. At risk for overweight is defined as having a BMI between 85th and 95th percentiles of the BMI‐for‐age growth charts.

The distribution of excess adipose mass may be more important than total fat in increasing metabolic and cardiovascular risk. Furthermore, there is increasing evidence that abdominal obesity, rather than total body fat, is also a useful, independent predictor of several cardiovascular and cancer‐related outcomes. Therefore, some of other commonly used measures of abdominal obesity are waist circumference, hip circumference and waist‐to‐hip ratio.

Obesity – The Size of the Problem

Obesity (BMI ≥ 30 kgm^‐2) comprises an important threat to national and global public health in terms of prevalence, incidence and economic burden. The prevalence of obesity has increased worldwide in the past 50 years, reaching pandemic levels. Globally, the age‐standardised prevalence of overweight increased from 26.5% in 1980 to 39% in 2015, representing an almost 50% increase over 35 years. Likewise, the prevalence of obesity rose from 7% in 1980 to 12.5% in 2015, representing an almost 80% increase. Presently, it is estimated that nearly 30% of the world population are overweight (BMI ≥ 25 kgm^‐2) or obese (Figure 4.1), and 5% of the deaths worldwide were attributable to obesity. If the incidence continues at this rate, almost half of the world’s adult population will be overweight or obese by 2030.

The prevalence of obesity in the USA was almost 40% and affected nearly 100 million of adult population between 2015 and 2016. However, there was no significant difference in the prevalence of obesity between men and women overall or by age group. In the UK, on the other hand, obesity prevalence continues to rise, with prevalence for both men and women at its highest recorded level in the Health Survey for England in 2017. It was estimated that 36% of the UK adult population were overweight and 29% were obese. Comparing men and women in the UK, 30% of all adult women were obese and 31% were overweight compared with 27% of men who were obese and 40% who were overweight in 2017 (Figures 4.2 and 4.3). The prevalence of severe obesity (BMI ≥ 25 kgm^‐2) has more than tripled since 1993 to reach at 2% in men and 4% in women.

The prevalence of obesity seems to have shown slow increase during the past 10 years in several developed countries, such as USA and UK. However, the global obesity rate is still increasing in other regions of the world, mostly those known to be densely populated. The increase in the prevalence of obesity in developing countries is mainly due to rapid changes in socioeconomic status and demographic data, and the adoption of an energy and fat‐rich diet and sedentary lifestyle.

Pathogenesis of Obesity

Obesity is a highly complex, multi‐factorial disease: genetic, behavioural, social and environmental factors can influence its development and in different ways.

Genetic Factors

Certain human gene mutations and its association with the development of severe obesity have highlighted the importance of genetic factors in the pathogenesis of obesity. Nevertheless, genetic changes in human populations occur too slowly to be merely responsible for the obesity epidemic.