Definition of Obesity


Obesity is excessive body fat accumulation caused by overeating and the loss of the body’s control ability. Obesity is no longer a cosmetic issue, but a serious disease. Along with national and international medical and scientific societies, The World Health Organization (WHO) defined obesity as a chronic, progressive disease caused by many environmental and genetic factors (Table 1).

Obesity is a serious disease that threatens both individual and public health, and its prevalence is rapidly increasing worldwide. Because of its nature (as an epidemic disease), obesity requires a life-long struggle and control.


Table 1: Associations or organizations, which have declared obesity as a disease

  • National Institutes of Health (NIH)
  • US Food and Drug Administration (FDA)
  • Federal Trade Commission
  • American Medical Association
  • World Health Organization (WHO)
  • American College of Physicians
  • American Association of Clinical Endocrinologists
  • American College of Cardiology
  • The Endocrine Society
  • American Academy of Family Physicians
  • Institute of Medicine
  • The Obesity Society
  • World Obesity Federation
  • American Heart Association (AHA)
  • American Diabetes Association
  • American Society for Reproductive Medicine
  • American Urologic Association
  • American College of Surgeons


Measuring Obesity

Obesity is characterized by excessive accumulation of fat in the body. There are tools intended for assessing the amount of fat; however, they are expensive and time consuming methods. Obesity is clinically defined on the basis of the body weight, body structure, and height.


Body mass index (BMI)

This concept is the most common criterion used to define obesity. Large-scale community studies have shown that BMI successfully reflects the amount of body fat in adults, except for certain cases (pregnant women, elderly people, bodybuilders, wrestler, etc.).

However, it does not take into consideration the fat distribution of a person (abdominal or peripheral/abdominal fat or fat in the hipline, arms and legs), and it is not a good criterion for the assessment of the metabolic activity in the fat tissues of the person.

If the height and weight of the person are known, BMI can be calculated with an easy formula. According to this formula, the person’s weight in kilogram (kg) is divided by the square of the height in meters (m2). The value obtained is that person’s BMI. BMI=kg/m2

The BMI categories are shown in Table 2. You can easily calculate your BMI and determined your category, as well.


Table 2: Classification According to Body Mass Index (BMI)

Category BMI Range
Normal Weight 18.9 – 24.9
Overweight 25 – 29.9
Class I, Obese 30 – 34.9
Class II, Severely Obese 35 – 39.9
Class III, Morbidly Obese 40 and over


Prevalence of obesity and its rate of occurrence

According to the data from the World Health Organization (WHO) data, there are 500 million obese adults in the world. In addition, 1 billion overweight people live in the world. 65% of the world’s population lives in countries, where obesity is a problem, and where the mortality rate of obese people is much higher than that of none-obese people. In recent studies, 250 million children worldwide were found to be at risk of obesity.

According to the epidemiological data for 2008 in the United States, 34% of adults over 20 years of age are obese, and 68% are overweight.

The prevalence of obesity has been increasing at an alarming rate in the last 20-25 years. Since 1985, the Center for Disease Control (CDC) has started an ongoing study to examine the changes in obesity prevalence, and has found the following results:


  • In 1990, the obesity prevalence in most of the American states was 10% or less.
  • By 1995, more than half of the states had a prevalence of 15%
  • By 2000, almost half of the states had a prevalence of 20% or higher.
  • Five years later (2005), all but three of the states had a prevalence higher than 20%, and about one-third of them had a prevalence of 25% or higher.
  • By 2010, the data showed that most of the American states had a prevalence of 25%, and many of them had a prevalence of 30% or higher.


During the past two decades, the obesity epidemic has increased both in number and severity. Data in the United States found that the prevalence of severe obesity increased by 500% and super obesity (BMI greater than 50) increased by about 1,000% during the period from 1987 to 2005. According to the 2007-2008 NHANES findings, 5.7% of American adults, i.e. approximately 14 million people, are super obese.


Progressive nature of the disease of obesity

Obesity is a very complex disease with a strong genetic component. On a genetic background, many factors such as hormonal, metabolic, psychological, cultural and behavioral factors trigger obesity.


Excessive energy intake causes weight gain. It occurs when the amount of caloric intake exceeds the amount of calories burned (the amount of the calories used by the body for basic biological functions, daily activities, and exercise performance). Overeating or not getting adequate physical activity may cause energy surplus. However, there are some other conditions affecting energy balance and accumulation of body fat. These include:


  1. Chronic sleep disorder
  2. Consumption of foods that cause metabolic/hormonal changes likely to increase body fat, independently of their caloric content. These include sugar or high fructose corn syrup, and processed foods.
  3. Low intake of foods that contain less amount of fat, such as fruits, vegetables, legumes, hazelnut, seeds, high-quality protein sources.
  4. Stress factor
  5. Many types of medications
  6. Various environmental pollutants



Weight gain

Weight gain leads to many hormonal, metabolic and molecular changes in the body; and as a result, decreases fat burning while increasing conversion of glucose (carbohydrates) to fat as well as fat storage capacity.

This means that more of the calories consumed will be stored as body fat. What’s worse is that the hormonal factors regulating the appetite and hunger act in such a way as to cause an increase in meal sizes and the frequency of eating. Therefore, as the weight gain process continues, the biology of the body changes in a way allowing for further weight gain and obesity.


Problems associated with obesity

Some other conditions associated with obesity contribute to the progression of the obesity disease. Obesity limits mobility, preventing the person from burning calories that he/she would normally burn with physical activity. Weight gain may also lead to psychological or emotional problems. Such a condition, along with hormonal changes, causes further weight gain by stimulating the appetite.

Sleep apnea and other respiratory problems resulting from weight gain may stimulate the appetite, by impairing the sleep quality and altering the sleep duration. This means more caloric intake and fat.

Weight gain also contributes to the development of some other diseases such as hypertension, diabetes, heart disease, osteoarthritis (calcification of joints) and depression. Patients need to use many medications for the treatment of these conditions, and this contributes to weight gain, as well. Along with many coexisting problems that it causes, obesity traps the individual in a vicious weight gain cycle.



The primary treatment for obesity is a low-calorie diet; however, diets contribute to the development of obesity in certain cases. Dietary weight-loss leads to biological responses that contribute to weight regain in the long-term. One of these responses affects the energy balance (metabolism). When a person loses weight a low-calorie diet, he/she reduces energy consumption, thinking that the body is starving. Such a case requires less caloric intake for maintaining weight loss. This is not quite sustainable. A low calorie diet also increases appetite regulators (such as the desire to eat) in the long term. At a point, this resistance is broken and then the person regains not only the ponds he lost but also exceeds his/her pre-diet weight. This is called ‘yo-yo syndrome’.

Another biological response resulting from dieting is that the body‘s ability to burn fat decreases in consequence of the activation of various mechanisms of the body. Dieting reduces the amount of fat the body burns during low-grade activities such as slow walking and house cleaning. This causes the fat depots to store more fat than before the diet. It causes a progressive increase in fat accumulation, even if the person does not overeat.