Obesity is a health condition defined as having significantly more weight than is considered healthy or normal for one’s height. In obesity, the extra weight is mainly caused by extra body fat rather than muscle or bone.
Obesity is most commonly defined by calculating body mass index (BMI). BMI is calculated by dividing weight (in kilograms) by height (in meters) squared. According to the World Health Organization (WHO), weight ranges defined by BMI are:
BMI is an easy way to estimate the amount of fat someone has, but it is not always accurate. For instance, many professional athletes in the peak of fitness have a BMI associated with overweight or obesity. Likewise, recent research has identified different ratios of fat, muscle, and bone among different ethnic populations. However, calculating BMI requires no equipment, can be done at home, and is useful for providing statistics to help scientists understand excess weight as a risk factor for disease.
Other methods for more accurately measuring obesity include waist circumference, underwater weighing, measuring skinfold thickness with calipers, and bioelectrical impedance – using an electrical current to measure the water content of the body.
Hereditary and environmental factors influence the risk of developing obesity, and some health conditions and medications directly cause weight gain or make it harder to lose weight. Lifestyle factors such as eating habits and exercise are the determining factor for obesity in most people.
Read more about causes of obesity.
The idea of obesity as a medical issue is only about 50 years old. Problems like starvation and malnutrition have plagued cultures around the world for thousands of years. Obesity was not a concern.
The word obese comes from the Latin “obesus,” meaning “having eaten until fat.” Excess weight had long been a symbol of wealth and power. The obese female body was venerated in art. As early as 25,000 B.C., figurines like the Venus of Willendorf depicted round bodies. Religious art in the Middle Ages and the Renaissance featured the Virgin Mary and angels with extra fat, for example, the paintings in Michelangelo's Sistine Chapel in Vatican City. Female obesity was uplifted as a sign of power, sexuality, and fertility. Male obesity was not celebrated the same way, and often pegged as a result of poor discipline.
In the 19th century, Belgian astronomer and statistician Alphonse Quetelet created the body mass index to define the “average man” using data from the heights and weights of the French and Scottish armies. A BMI score is calculated by dividing a person’s weight in kilograms by the square of the person’s height in meters. It is still used today to diagnose obesity.
Well into the 20th century, standard medical advice held that carrying an extra 20 to 50 pounds of “excess flesh” was healthy. However, straight, slender female frames became the fashion in the 1920s and 1930s, and fad diets were all the rage during this period. As middle-class life emerged after World War II, food and gasoline were no longer rationed, and sweets and cars became popular purchases. Caloric intake has increased while walking has decreased.
By the 1960s, the study of obesity and nutrition had launched. Body fat was understood as an organ, influenced by metabolism, hormones, receptors, genetics, and cellular biology – all things scientists are still trying to piece together today. The media began to feature slim bodies, such as Twiggy in 1966 and Farrah Fawcett in 1977. Scales became a fixture in home bathrooms and “slimming” became a billion-dollar industry.
Scientists and doctors are still trying to understand obesity, its causes and solutions, and even questioning whether obesity is as harmful as it is made out to be. For example, researchers are confounded by what is known as the “obesity paradox:” Many studies have found that patients — especially the elderly — with several chronic diseases and elevated BMI have lower mortality rates than those with BMI scores in the “normal” range. Obesity has also been shown to lower the risk for osteoporosis.
Today we see obesity at its highest levels in human history, while dieting is a multi-billion-dollar industry. According to the June 2021 American Psychological Association's "Stress in America" poll, 42 percent of U.S. adults reported undesired weight gain after the start of the COVID-19 pandemic — with an average reported weight gain of 29 pounds. People are now being bombarded more than ever with well-meaning ads for weight-loss programs. More research is needed to better understand the relationship between weight and health.
Obesity is extremely common, and rates are increasing. According to the Centers for Disease Control and Prevention, one in three adults and one in six children between the ages of 2 and 19 have obesity in the U.S. Worldwide, about 13 percent of adults are obese. According to WHO, global obesity has tripled since 1975.
Women are more likely to be obese than men. In the U.S., 48 percent of Black adults, 42 percent of adults of Hispanic background, and 36 percent of white adults are considered obese, according to the National Institute of Diabetes and Digestive Kidney Diseases.
Analysis of statistics related to disease and obesity have found increased risk for developing certain serious health conditions — including type 2 diabetes, hypertension (high blood pressure), atherosclerosis (narrowing of the arteries), heart disease, stroke, some autoimmune diseases, and cancer – in people with BMIs in the overweight and obese range. These elevated risks have led researchers to look for links between higher body mass and disease. There are several theories about how higher BMI might influence the development of these conditions.
Losing weight lowers the risk for developing these and other serious health conditions.
More research is needed to better understand whether obesity does indeed contribute to disease — and how.
There is evidence that extreme obesity can reduce life expectancy. The National Institutes of Health (NIH) completed a study on longevity and obesity that involved analyzing the medical records of more than 300,000 people in Australia, Sweden, and the United States. The NIH found that people with a BMI between 40 and 44.9 died on average 6.5 years earlier than nonsmokers with a normal BMI, while those with a BMI between 55 and 59.9 died on average 13.7 years earlier.