Maps showing the prevalence of obesity across all 50 US states in 1998 and 2008. Source: "Battle of the Bulge," www.economist.com, July 13, 2009
The United States is the second most obese industrialized country in the world.  39.6% of American adults in 2016 were obese, compared to 14% in the mid-1970s.  Obesity accounts for 19.8% of deaths and 21% of healthcare spending in the United States. 
Proponents contend that obesity is a disease because it meets the definition of disease; it decreases life expectancy and impairs the normal functioning of the body; and it can be caused by genetic factors.
Opponents contend that obesity is not a disease because it is a preventable risk factor for other diseases; is the result of eating too much; and is caused by exercising too little.
How Obesity Is Measured
People are usually identified as obese, which is defined as "having an excessive amount of body fat,” based on their Body Mass Index (BMI).  BMI is calculated by taking a person's weight divided by the square of his or her height (kg/m2). The "normal" BMI range is between 18.5 and 24.9, while "overweight” is 25-29.9 and "obese” is 30 and above. For example, a person who is 5'5" tall and 180 lbs. (BMI of 30) and a person who is 6'0" tall and 221 lbs. (BMI of 30) are considered obese by the BMI measurement. Some people, such as muscle-heavy athletes, may have "overweight” or "obese” BMIs but may not have excess body fat. 
Obesity as an "Epidemic”
The Centers for Disease Control and Prevention (CDC) reported an "epidemic" of 72 million obese people in the United States in 2005-2006  According to a 2011-2012 study published in JAMA, 34.9% of American adults and 17% of youth are obese.  The study indicated there were no significant changes in obesity rates from 2003-2004 to 2011-2012.  According to the CDC, "Non-Hispanic blacks have the highest age-adjusted rates of obesity (47.8%) followed by Hispanics (42.5%), non-Hispanic whites (32.6%), and non-Hispanic Asians (10.8%).” Obesity is highest in adults who are 40-59 years old (39.5%). 
In the mid-1970s the obesity rate for the United States was about 14%.  1994 is the first year with CDC data for all states and shows all states at obesity rates of 19% or lower.  By 1997, three states were in the 20-24% range: Indiana, Kentucky, and Missouri. In 2001, Missouri became the first state to have an obesity rate between 25-29%.  By 2005, 14 states were in the 25-29% range while three states breached the 30% and higher rate (Louisiana, Missouri, and West Virginia).  By 2010, no state reported an obesity rate under 19%. 
(click to enlarge image)
Infographic with the lowest and highest 2013 US state obesity rates.
Obesity rates continued to increase in the United States each year since 2008, growing from 25.5% in 2008 to 27.1% in 2013.  11 states had obesity rates over 30% in 2013: Alaska (30.1%), Arkansas (32.3%), Delaware (34.3%), Kentucky (30.6%), Louisiana (32.7%), Mississippi (35.4%), Ohio (30.9%), Oklahoma (30.5%), South Carolina (31.4%), Tennessee (31.3%), and West Virginia (34.4%).  Montana had the lowest obesity rate at 19.6%.  The poll data show that the 10 most obese states had the highest rates of high blood pressure (35.8%), high cholesterol (28.2%), depression (20.7%), diabetes (14.3%), cancer (7.8%), and heart attacks (5.0%).  The data also show that the 10 least obese states reported higher rates of healthy eating, eating five or more servings of fruits and vegetables at least four days a week, and exercising for 30 minutes or more on at least three days a week. 
A 2009 study published in PLOS Medicine found obesity was the cause of 1 in 10 deaths in the United States.  Obesity was the third-leading cause of death in men and women, after high blood pressure and smoking.  Obesity and obesity-related health conditions cost an estimated 10% of annual medical spending in the United States, totaling $147 billion in 2008. 
The United Nations’ "The State of Food and Agriculture 2013” report found that Mexico was the most obese populous country with 32.8% of Mexicans falling into the obese category compared to 31.8% of Americans in the second-place United States.  The Republic of Nauru had the highest obesity percentage (71.1%) but, with a population of 9,488 in July 2014, was not considered a "populous country” by the study. 
According to the World Health Organization (WHO) 2013 Fact Sheet "Obesity and Overweight,” obesity rates worldwide have almost doubled since 1980, accounting for over 200 million men and almost 300 million women.  Combined, overweight and obesity cause at least 2.8 million deaths annually and are the fifth leading risk for death globally.  65% of the global population lives in countries where being overweight or obese causes more deaths than being underweight, making 2012 the first time overweight and obesity caused more deaths than malnutrition. 
Obesity and obesity-related health conditions cost almost ten percent of annual medical spending in the United States, totaling $147 billion in 2008.  In Apr. 2014 Duke Global Health Institute and Duke-NUS Graduate Medical School researchers found the lifetime medical costs for obese 10-year-olds alone will be $14 billion. 
(click to enlarge image)
Illustration of the health risks associated with obesity. Source: "Obesity and Health," www.nlm.nih.gov, July 25, 2007
Public Opinion and Medical Perspectives on Obesity
The FDA, the American Medical Association (AMA), the National Institutes of Health (NIH), American Heart Association, American College of Cardiology, and the Obesity Society have stated that obesity is a disease.  The WHO called obesity a "chronic disease" in 2000.  The IRS announced a policy in 2002 stating that "obesity is medically accepted to be a disease in its own right" and allows Americans who are medically diagnosed as obese to claim tax deductions for doctor prescribed treatments. In Sep. 1998 the National Institutes of Health's National Heart, Lung, and Blood Institute wrote, "Obesity is a complex multifactorial chronic disease."  The American College of Gastroenterology, in 2008, stated that obesity is a "chronic, debilitating and potentially fatal disease."  In 2013 the American Academy of Family Physicians (AAFP) stated, "Overweight and obesity are chronic diseases with behavioral origins that can be traced back to childhood.” 
In the health care bill H.R. 3962, "Affordable Health Care for America Act” approved by the House of Representatives on Oct. 29, 2009 (but later abandoned in favor of the Patient Protection and Affordable Care Act; also called the PPACA or Obamacare), being overweight or obese was not classified as a disease but as a "behavioral risk factor" along with alcohol and drug use, tobacco, poor nutrition, physical inactivity, untreated mental health problems, and risky sexual behavior. 
The American Academy of Pediatrics (AAP), American College of Physicians (ACP), Centers for Medicare and Medicaid Services (CMS), and American Nurses Association (ANA) do not have policy statements stating whether or not obesity is a disease, as of Apr. 18, 2014.