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Obesity - An Overview

Updated on September 28, 2021
Medically reviewed by
Robert Hurd, M.D.
Article written by
Kelly Crumrin

Obesity is a health condition defined as having significantly more weight than is considered healthy or normal for ones height. In obesity, the extra weight is mainly caused by extra body fat rather than muscle or bone.

What Is Obesity?

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:

  • Below 18.5 — Underweight
  • 18.5 to 24.9 — Normal
  • 25 to 29.9 — Overweight
  • 30 to 39.9 — Obesity
  • 40 or above — Extreme obesity

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.

What Causes Obesity?

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 History 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.

How Common Is Obesity?

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.

Why Is Obesity Considered a Health Problem?

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.

  • Having a high body mass may force the heart to work harder to pump blood, leading to heart disease.
  • When the heart pumps harder to circulate blood, it may lead to hypertension.
  • Hypertension contributes to the development of atherosclerosis, heart disease, and stroke.
  • Fat cells may release hormones and chemicals that raise the body’s level of inflammation, aggravating cardiovascular, and autoimmune problems.
  • Excess fat may make the body’s cells resistant to insulin, raising blood glucose and leading to type 2 diabetes.
  • Higher levels of inflammation, blood glucose, and hormones may contribute to the development of cancer.

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.

Can Obesity Shorten the Life Span?

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.

Condition Guide

References

  1. Obesity — Mayo Clinic
  2. Defining Adult Overweight and Obesity — Centers for Disease Control and Prevention
  3. Overweight & Obesity Statistics — National Institute of Diabetes and Digestive and Kidney Diseases
  4. Definition and Facts for Adult Overweight and Obesity Statistics — National Institute of Diabetes and Digestive and Kidney Diseases
  5. A History of Obesity, or How What Was Good Became Ugly and Then Bad — Advances in Chronic Kidney Disease
  6. Obesity and the Modern Lifestyle: What Can We Learn From History? — BBC History Magazine
  7. Obesity and Cancer — National Cancer Institute
  8. Adult Obesity Causes and Consequences — Centers for Disease Control and Prevention
  9. About Adult BMI — Centers for Disease Control and Prevention
  10. BMI Classification — World Health Organization
  11. Obesity and Overweight — World Health Organization
  12. Health Risks of Being Overweight Statistics — National Institute of Diabetes and Digestive and Kidney Diseases
  13. NIH Study Finds Extreme Obesity May Shorten Life Expectancy Up to 14 years — National Institutes of Health
  14. Obesity Information — National Heart Association
  15. The Extra Weight of COVID-19 — Monitor on Psychology

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Robert Hurd, M.D. is a professor of endocrinology and health care ethics at Xavier University. Review provided by VeriMed Healthcare Network. Learn more about him here.
Kelly Crumrin is a senior editor at MyHealthTeams and leads the creation of content that educates and empowers people with chronic illnesses. Learn more about her here.

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