Obesity meets the definition of disease. The American Medical Association's 2013 "Council on Science and Public Health Report" identified three criteria to define disease: 1. "an impairment of the normal functioning of some aspect of the body;" 2. "characteristic signs and symptoms;" and 3. "harm or morbidity." Obesity causes impairment, has characteristic signs and symptoms, and increases harm and morbidity. Scott Kahan, MD, MPH, Director of the National Center for Weight and Wellness and Preventative Medicine Physician at Johns Hopkins University, stated obesity "satisfies all the definitions and criteria of what a disease and medical condition is... The one difference is that people who have obesity have to wear their disease on the outside."
Obesity, like other diseases, impairs the normal functioning of the body. People who are obese have excess adipose (or fat) tissue that causes the overproduction of leptin (a molecule that regulates food intake and energy expenditure) and other food intake and energy mediators in the body, which leads to abnormal regulation of food intake and energy expenditure. Obesity can impair normal mobility and range of motion in knees and hips, and obese patients make up 33% of all joint replacement operations. Obesity is also linked to reproductive impairment, contributing to sexual dysfunction in both sexes, infertility and risk of miscarriage in women, and lower sperm counts in men.
Obesity decreases a person's life expectancy and can cause death, like other diseases. Obesity in adults can lead to three years' loss of life. Extreme obesity can shorten a person's life span by 10 years. A July 13, 2016 Lancet meta-study found that even moderate obesity led to an increased chance of early death: 29.5% for men (compared to 19% for men of normal BMI weight) and 14.6% for women (compared to 11%). The authors calculated that one in five premature deaths in North America could be avoided if obese people were normal BMI weights. The Surgeon General reports an estimated 300,000 deaths per year may be attributed to obesity. People who are obese have a 50-100% increased risk of death from all causes.
Obesity can be a genetic disorder such as sickle-cell anemia and Tay-Sachs, which are classified as diseases. Researchers have identified 79 genetic syndromes associated with obesity. Studies have shown that obesity can be inherited like height. Genetic disposition plays a large role in determining if a person will be obese. A May 2014 study published in the Journal of Molecular Psychology linked higher rates of obesity to the "fat mass and obesity association" (FTO) gene. The FTO gene reportedly lowers activity in the part of the brain that "controls impulses and the response to the taste and texture of food," so people with the gene are more likely to eat more fatty foods and eat more impulsively as they age. A 1990 study published in the New England Journal of Medicine on body mass in twins not raised in the same home concluded, "genetic influences on body-mass index are substantial, whereas the childhood environment has little or no influence." In addition to genetic predisposition, almost 10% of morbidly obese people have defects in the genes that regulate food intake, metabolism, and weight.
Government entities and major medical groups recognize obesity as a disease. The Food and Drug Administration (FDA), the American Heart Association, the American College of Cardiology, and the Obesity Society, the National Heart, Lung, and Blood Institute (NHLBI), and the American College of Gastroenterology (ACG) have identified obesity as a disease. Even the Internal Revenue Service (IRS) recognizes obesity as a disease so weight loss expenses may be counted as a medical deduction from taxes. On June 18, 2013, the American Medical Association recognized obesity as "a disease requiring a range of medical interventions to advance obesity treatment and prevention."
Physicians from as early as the 17th century have referenced obesity as a disease. English physician Thomas Sydenham (1624-1689) wrote, "Corpulency [obesity] may be ranked amongst the diseases arising from original imperfections in the functions of some of the organs." William Wadd, a 19th century British surgeon and medical author, wrote, "when in excess--amounting to what may be termed OBESITY--[fat] is not only in itself a disease, but may be the cause of many fatal effects, particularly in acute disorders." In the Feb. 12, 1825 issue of The Medical Advisor and Guide to Health and Long Life, Robert Thomas, a 19th century doctor, wrote "Corpulence, when it arrives at a certain height, becomes an absolute disease."
Obesity is a preventable risk factor for other diseases and conditions, and is not a disease itself. Like smoking is a preventable risk factor for lung cancer and drinking is a preventable risk factor for alcoholism, obesity is a preventable risk factor for coronary heart disease, stroke, high blood pressure, type 2 diabetes, cancers (like endometrial, breast, and colon), high cholesterol, high levels of triglycerides, liver and gallbladder disease, incontinence, increased surgical risk, sleep apnea, respiratory problems (like asthma), osteoarthritis, infertility and other reproductive complications, complications during pregnancy and birth defects, and mental health conditions. Women who gain 20 pounds or more between age 20 and midlife double their risk of postmenopausal breast cancer. For every 2 pound weight increase, the risk of developing arthritis rises 9-13%.
Obesity is a side effect, not a disease. Obesity can be caused by hypothyroidism, Cushing's syndrome, Prader-Willi syndrome, polycystic ovary syndrome, arthritis, increased insulin levels (from carbohydrate-heavy diets or diabetes treatments) and depression. Certain drugs like antidepressants, anti-seizure medications, diabetes medications, anti-psychotic medications, antihistamines, anticonvulsants, steroids, beta blockers, and contraceptives can cause obesity. Obesity can also be caused by lack of sleep (or sleep debt), ingesting endocrine disrupters (such as BPA, DDT, and phthalates), consuming high-fructose corn syrup, a lack of temperature variation (due to air conditioners and heaters), and quitting smoking.
Many obese people live long, healthy lives. A 2013 Lancet article noted that as many as one third of obese people are "healthy obese," meaning that despite being significantly overweight they have normal cholesterol and blood pressure levels, and no sign of diabetes. Obese people tend to go to the doctor more and have regular checks for other risk factors and diseases. Many people with a BMI (Body Mass Index; a measure of body fat based on height and weight) in the obesity range are not physically impaired and live normal lives. BMI does not take into account the overall health of the individual and can identify fit, muscular people as obese because muscle weighs more than fat.
Obesity is the result of eating too much. The suggested daily caloric intake for 31-50-year-olds is 1,800 calories for women and 2,200 calories for men. In 2009-2010, 30-39-year-old women consumed an average 1,831 calories (which is 1.7% over the recommendation) while men of the same age consumed an average 2,736 calories per day (which is 22% over the recommendation). The average American restaurant meal portion size is four times as large as portions in the 1950s and 96% of entrees at chain restaurants exceed dietary guidelines for fat, sodium, and saturated fat, with some almost exceeding daily intakes in one meal. The body is doing what it has evolved to do by converting excess calories into fat cells. The CDC recommends reducing consumption of sugar drinks (like sodas) and high-energy-dense foods to prevent and reduce obesity. The Mayo Clinic states, "Having a diet that's high in calories, eating fast food, skipping breakfast, eating most of your calories at night, drinking high-calorie beverages and eating oversized portions all contribute to weight gain."
Obesity is the result of sedentary lifestyles. Compared to 40 years ago, people today spend more time commuting, sitting in front of a computer, watching television, playing video games, and generally exercising less. In 1960 50% of jobs required moderate physical activity compared to just 20% of jobs in 2011; the other 80% in 2011 required little or no physical activity. This shift represents 120-140 fewer calories burned per day, which closely aligns with the nation's overall weight gain trends. Men expended 142 fewer calories daily and weighed 32.8 pounds more in 2003-2006 than in 1960-1962, while women expended 124 fewer calories daily and weighed 25.13 pounds more in 2003-2006 than in 1960-1962.