Defining Obesity

Obesity is no longer considered a cosmetic issue that is caused by overeating and a lack of self-control. The World Health Organization (W.H.O.), along with National and International medical and scientific societies, now recognize obesity as a chronic progressive disease resulting from multiple environmental and genetic factors.

The disease of obesity is extremely costly not only in terms of economics, but also in terms of individual and societal health, longevity, and psychological well-being. Due to its progressive nature, obesity requires life-long treatment and control.

Measuring Obesity

The disease of obesity is characterized by an excessive accumulation of body fat. A variety of instruments are available for assessing the amount of fat versus lean tissue in the body. However, due to the expense of these instruments and the time required for body fat assessments, obesity is clinically defined by measures that ‘estimate’ adiposity from body weight, body build and height.

Ideal Body Weight (IBW)

One of these estimates for body size is ideal body weight (IBW), a number that is obtained from the Metropolitan Life Insurance Company Table. The IBW table is based upon mortality data and requires knowledge of body weight as well as body frame, a measure that is considered by many experts to be arbitrary. Using this table, overweight and obesity are defined by percentage of weight in excess of IBW.

Body Mass Index (BMI)

The body mass index (BMI) is another measure used to define overweight and obesity and is considered a more accurate estimate of body fatness than IBW. Large population studies find that the BMI generally reflects the amount of excessive body fat an adult has, although there are certain exceptions, such as the BMI of a woman who is pregnant, an athlete, a body builder or the elderly. BMI does not necessarily take into account a persons distribution of fat (abdominal vs. peripheral), however, and is not a good measurement of the metabolic activity of a person’s fat tissue.

BMI takes into consideration an individual’s height and weight can be determined by using a BMI chart or can be calculated according to one of the following formulas:

​(1) Weight in kilograms divided by Height in meters squared (BMI = kg/m2)


​(2) Weight in pounds divided by Height in inches squared and this value multiplied by 705 (BMI= [(lbs/in2) x 705])

Body size categories using BMI have been based upon the ranges of BMI associated with a certain risk for mortality1-2. The categories and respective BMI categories are:

Category BMI Range
Normal Size 18.9 to 24.9
Overweight 25 to 29.9
Class I, Obesity 30 to 34.9
Class II, Serious Obesity 35 to 39.9
Class III, Severe Obesity 40 and greater

Calculate your BMI Here

Obesity Prevalence and Rate of Occurrence

According to the W.H.O., 65 percent of the world’s population lives in countries where overweight and obesity kills more people than underweight. Approximately 500 million adults in the world are affected by obesity and one billion are affected by overweight, along with 48 million children.

In the United States, epidemiological data from an ongoing study that measures the actual body size of thousands of Americans, show that 34 percent of adults more than 20 years old are affected by obesity and 68 percent are overweight (2007-2008 data). Obesity affects 10 percent of children between two and five years of age, 2 percent of those between 6 to 11 years old, and 18 percent of adolescents.

Throughout the last 20 to 25 years, the prevalence of obesity has been increasing at an alarming rate. Since 1985, the Center for Disease Control (CDC) has supported an ongoing study, conducted on a yearly basis by state health departments, to examine changes in obesity prevalence state-to-state, and has found the following:

  • In 1990, the obesity prevalence for most of the states was 10 percent or less.
  • By 1995, more than half the states had a prevalence of 15 percent.
  • By 2000, nearly half the states had a prevalence of 20 percent or higher.
  • Five years later (2005), all but three states had a prevalence greater than 20 percent and nearly a third had a prevalence of 25 percent or more.
  • By 2010, the data show that most of U.S. states had a prevalence of 25 percent and many had a prevalence of 30 percent or higher.

For view of yearly changes in prevalence, view the CDC’s online data.

Not only has the obesity epidemic increased in number throughout the past two decades, but also in severity. Data obtained from the yearly ongoing CDC-supported U.S. study found that between the years 1987 to 2005 the prevalence of severe obesity increased by 500 percent and super severe obesity (BMI greater than 50) increased by nearly 1,000 percent. According to the 2007-2008 NHANES findings, 5.7 percent of American adults, or nearly 14 million people, are affected by severe obesity.

Progressive Nature of the Disease of Obesity

Obesity is considered a multifactorial disease with a strong genetic component. Acting upon a genetic background are a number of hormonal, metabolic, psychological, cultural and behavioral factors that promote fat accumulation and weight gain.

Positive Energy Balance

A positive energy balance causes weight gain and occurs when the amount of calories consumed (energy intake) exceeds the amount of calories the body uses (energy expenditure) in the performance of basic biological functions, daily activities, and exercise. A positive energy balance may be caused by overeating or by not getting enough physical activity. However, there are other conditions that affect energy balance and fat accumulation that do not involve excessive eating or sedentary behavior. These include:

  1. Chronic sleep loss
  2. Consumption of foods that, independent of caloric content, cause metabolic/hormonal changes that may increase body fat. These include foods high in sugar or high fructose corn syrup, processed grains, fat, and processed meats
  3. Low intake of fat-fighting foods such as fruits, vegetables, legumes, nuts, seeds, quality protein
  4. Stress and psychological distress)
  5. Many types of medications
  6. Various pollutants

Weight Gain

Weight gain is yet another contributor to weight gain or, in other words, obesity ‘begets’ obesity, which is one of the reasons the disease is considered ‘progressive’. Weight gain causes a number of hormonal, metabolic and molecular changes in the body that increase the risk for even greater fat accumulation. Such obesity-associated biological changes reduce the body’s ability to oxidize (burn) fat for energy, increase the conversion of glucose (carbohydrate) to fat, and increase the body’s capacity to store fat in fat storage depots (adipose tissue). This means that more of the calories consumed will be stored as fat. To make matters worse, obesity affects certain regulators of appetite and hunger in a manner that can lead to an increase in meal size and the frequency of eating. Weight gain, therefore, changes the biology of the body in a manner that favors further weight gain and obesity.

Obesity-related Conditions

A number of other conditions associated with obesity contribute to the progression of the disease. Obesity reduces mobility and the number of calories that would be burned in the performance of activity. Weight gain may also cause psychological or emotional distress which, in turn, produces hormonal changes that may cause further weight gain by stimulating appetite and by increasing fat uptake into fat storage depots.

Sleep duration is reduced by weight gain due to a number of conditions that impair sleep quality such as pain, sleep apnea and other breathing problems, a need to urinate more frequently, use of certain medications, and altered regulation of body temperature. Shortened sleep duration, in turn, produces certain hormones that both stimulate appetite and increase the uptake of fat into fat storage depots.

Weight gain also contributes to the development of other diseases such as hypertension, diabetes, heart disease, osteoarthritis and depression, and these conditions are often treated with medications that contribute to even further weight gain. In all of these ways and more, obesity ’begets’ obesity, trapping the individual in a vicious weight gain cycle.


A low calorie diet is the primary treatment for overweight and obesity, but, dieting is also a contributor to obesity progression. Dietary weight-loss causes biological responses that persist long-term and contribute to weight regain. One of these responses affects energy balance. When a person loses weight, the body ‘thinks’ it is starving and energy expenditure is reduced in order to conserve calories. The reduction in energy expenditure with dietary weight-loss requires that, in order to maintain weight-loss, the dieter eat even fewer calories than someone of equal body size who has never been on a diet. However, eating less is difficult following a diet because there are long-term changes in regulators of appetite that increase the desire to eat and the amount of food that can be consumed. Such diet-induced changes favor a positive energy balance and weight regain and, because the conditions responsible for the reduction in energy expenditure and increased drive to eat persist long-term, an individual will often not only regain all of their lost weight, but even more.

Another biological response that occurs with dieting involves changes in fat metabolism that reduce the body’s ability to burn fat and increase the capacity for fat to be stored in adipose depots (fat storage depots). With dietary weight-loss, the amount of dietary fat the body burns is reduced by approximately 50 percent. In addition, dieting reduces the amount of fat the body burns for fuel during low-grade activity such as walking, cleaning the house, fixing dinner, or working on a computer. The reduction in the amount of fat that is burned for fuel following a dietary weight-loss makes more fat available to be taken up by fat storage depots, and dieting increases the capacity for fat depots to store even more fat than before a diet. Altogether dietary weight-loss reduces the use of fat for fuel and increases the capacity for the fat that is not utilized to be stored. These changes lead to a progressive increase in fat accumulation even if the individual is not overeating.


Multiple factors acting upon a genetic background cause weight gain and obesity. Conditions associated with weight gain and biological changes in the body that occur as a result of weight gain contribute to progression of the disease, often trapping the individual in a vicious weight gain cycle. If you are concerned with your weight, please speak to your primary care physician to learn more about how to improve your weight and health.