Obesity is defined as excess adipose tissue. There are several different methods for determining excess adipose (fat) tissue; the most common being the Body Mass Index (BMI) (see below). A fat cell is an endocrine cell and adipose tissue is an endocrine organ. As such, adipose tissue secretes a number of products, including metabolites, cytokines, lipids, and coagulation factors among others. Significantly, excess adiposity or obesity causes insulin secretion, which can cause insulin resistance that leads to type 2 diabetes.

The biology of food intake is very complex, involving olfaction (smell), taste, texture, temperature, cognitive and emotional responses and metabolic/autonomic information, which signal the brain to initiate or cease eating. Recent scientific studies have identified several substances that act on the brain to signal a need for an increase in food intake. Likewise, several substances have been identified that signal the brain to decrease food intake.

Obesity is a disease that affects over one-third of the adult American population (approximately 72 million Americans¹). The number of overweight and obese Americans has increased since 1960, a trend that shows no sign of slowing down. Today, 66.3 percent of adult Americans (about 200 million) are categorized as being overweight or obese. Since 1960, adult Americans have increased average heights by 1 inch and average weight by 25 pounds. In 1963 a ten year old boy weighed, on average, 74.2 pounds. Now the average boy weighs 85 pounds.

Obesity is increasing around the world. High body mass index now ranks with major global health problems such as childhood and maternal under-nutrition, high blood pressure, high cholesterol, unsafe sex, iron deficiency, smoking, alcohol and unsafe water in total global burden of disease.

Each year, obesity causes at least 112,000 excess deaths in the US.² A study published in the August 24, 2006 New England Journal of Medicine showed increase risk of death among both men and women who were overweight but not obese. Obesity has been associated with numerous, adverse health effects. They include: type 2 diabetes, high cholesterol, hypertension, gallstones, fatty liver disease, sleep apnea, GERD, stress incontinence, heart failure, degenerative joint disease, birth defects, miscarriages, asthma, cancers in men (esophageal, colorectal, liver, pancreatic, lung, prostate, kidney, non-Hodgkin’s lymphoma, multiple myeloma and leukemia), and cancers in women (breast, colorectal, gallbladder, pancreatic, lung, uterine, cervical, ovarian, kidney, non-Hodgkin’s lymphoma and multiple myeloma).

Healthcare costs of American adults with obesity amount to approximately $147 billion.³  Discrimination and mistreatment of persons with obesity is widespread and often considered socially acceptable. 
                
What Is BMI?
 
Body Mass Index (BMI) is a mathematical calculation involving height and weight, irrespective of family history, gender, age or race. BMI is calculated by dividing a person's body weight in kilograms by their height in meters squared (weight [kg] height [m]2) or by using the conversion with pounds (lbs) and inches (in) squared as shown below, This number can be  misleading, however, for very muscular people, or for pregnant or lactating women.

                  [Weight (lbs) ÷ height (in)2  ] x 704.5 =BMI
              
BMI calculator

The BMI cutoffs are:

Below 18.5                    Underweight
18.5-24.9                      Normal weight
24.5-29.9                      Overweight
30 and greater               Obese
40 and greater               Morbid or extreme obesity    

BMI is frequently used in population studies because of its ease of determination and well-supported association with mortality and health effects. However, other measures of excess adipose tissue, such as waist circumference, waist-to-hip ratio and others are also used. Individuals may need to use additional factors to assess their individual risk including family history, level of physical activity, smoking and dietary habits.

Waist Circumference

Waist circumference is another widely used measurement to determine abdominal fat content. An excess of abdominal fat, when out of proportion to total body fat, is considered a predictor of risk factors related to obesity. Men with a waist measurement exceeding 40 inches are considered at risk. Women are at risk with a waist measurement of 35 inches or greater.

References:

1. Ogden CL, Carroll MD, McDowell MA, Flegal KM. Obesity among adults in the United States— no change since 2003–2004. NCHS data brief no 1. Hyattsville, MD: National Center for Health Statistics. 2007 http://www.cdc.gov/obesity/data/index.html

2. Katherine M. Flegal; Barry I. Graubard; David F. Williamson; Mitchell H. Gail.  Excess Deaths Associated With Underweight, Overweight, and ObesityJAMA, April 20, 2005; 293: 1861 - 1867.

3. Eric A. Finkelstein, Justin G. Trogdon, Joel W. Cohen, William Dietz, [Health Affairs 28, no. 5 (2009): w822-w831 (published online 27 July 2009; 10.1377/hlthaff.28.5.w822)]