Obesity is a health condition defined by having more body weight or fat than what is considered healthy for your height. Obesity has several causes, including genetic and environmental factors.

Obesity is most commonly diagnosed with a person’s body mass index (BMI) score. A BMI score is calculated by dividing the body’s weight in kilograms by its height in meters squared. According to the Centers for Disease Control and Prevention (CDC), weight ranges defined by BMI are:

• Below 18.5 — Underweight
• 18.5 to 24.9 — Healthy weight
• 25 to 29.9 — Overweight
• 30 to 34.9 — Class 1 obesity
• 35 to 39.9 — Class 2 obesity
• 40 or higher — Class 3 obesity

BMI is a simple way to estimate body fat, but it’s not always accurate. For instance, many professional athletes in the peak of fitness have a BMI score in the overweight or obese range. Likewise, recent research has identified a difference in the normal ratios of fat, muscle, and bone for various ethnic populations. These natural discrepancies are not accounted for in a BMI score.

Other methods for measuring body fat include measuring waist circumference, underwater weighing, measuring skinfold thickness with calipers, and using bioelectrical impedance (using an electrical current to measure the water content of the body).

A person’s weight is not a direct indication of how healthy they are. Some people with higher body weights do not have any serious health issues. Other people with healthy or underweight BMI scores can also develop the same health conditions commonly associated with obesity, such as high blood pressure or type 2 diabetes. It’s important to see your doctor for any health symptoms you have, regardless of your body size or BMI score.

What Causes Obesity?

The causes of obesity are complex. Genetic and environmental factors both influence the risk of developing obesity. Lifestyle factors like high stress, lack of sleep, food choices, pregnancy, and having the time and physical ability to exercise can influence body size. Some health conditions and medications can directly cause weight gain or make it harder to lose weight. Read more about the causes of obesity.

The History of Obesity

The idea of obesity as a medical issue only took hold in the last half of the 20th century. While problems like starvation and malnutrition have plagued cultures around the world for thousands of years, having a higher body weight was historically not a concern.

The word obese comes from the Latin “obesus,” meaning “having eaten until fat.” However, we now know the causes of obesity are more complicated than simply eating food.

Throughout history, higher body weights have long been symbolic of wealth and power. Heavier female bodies were admired 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 visible body fat — such as the figures in the Michelangelo paintings in the Sistine Chapel in Vatican City.

In the 19th century, Belgian astronomer and statistician Adolphe Quetelet created the body mass index (BMI) to define the “average man” using data from the heights and weights of white men in the French and Scottish armies.

Well into the 20th century, standard medical advice held that being 20 to 50 pounds overweight was healthy. However, slender female frames became fashionable 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. Calories increased while walking decreased.

By the 1960s, researchers were actively studying obesity and nutrition. 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 the ’60s and Farrah Fawcett in the ’70s. Scales became a fixture in home bathrooms, and over time, dieting 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 have identified what is known as the “obesity paradox”: Many studies have found that patients — especially the elderly — with BMI scores in the overweight and obese range may have lower mortality rates from several chronic diseases than those with BMI scores in the healthy range. Obesity has also been shown to lower the risk of osteoporosis.

Today, obesity rates are at their highest levels in human history, while dieting is a multi-billion-dollar industry. Still, 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 data from the National Health and Nutrition Examination Survey in 2018, approximately 2 out of every 5 adults in the U.S. has a BMI in the obese range. One in 5 children in the U.S. between the ages of 2 and 19 has a BMI in the obese range.

Worldwide, about 13 percent of adults are considered obese. Global obesity rates have tripled since 1975.

In the U.S., 49 percent of Black adults, 44 percent of Hispanic adults, 42 percent of white adults, and 17 percent of Asian adults have obesity.

Why Is Obesity Considered a Health Problem?

Obesity has been associated with a risk of having other serious health conditions, including type 2 diabetes, high blood pressure, narrowing of the arteries, heart disease, stroke, sleep apnea, some autoimmune diseases, and some types of cancer.

These elevated risks have led researchers to look for links between higher body weights and disease. However, more research is needed to better understand whether obesity does indeed contribute to disease — and how.

If you have been diagnosed with obesity, it is important to have regular health checkups to make sure you minimize your risk for other health issues. Be sure to tell your doctor if you notice any symptoms that could signal another health condition. Your health care team can evaluate your blood pressure, blood sugar levels, and sleep quality and order screenings for other conditions as needed. Your doctor can help you decide on a treatment plan for obesity or any health issues you have, which may include medication, lifestyle changes, or weight loss.

Regardless of your body size, eating a variety of fresh, whole foods and incorporating movement into your day can improve your overall health and help you feel your best.

Can Obesity Shorten the Life Span?

Some evidence suggests that having a BMI score over 40 (class 3 obesity) can reduce life expectancy. The National Institutes of Health completed a study in 2014 on longevity and obesity that involved analyzing the medical records of more than 300,000 people in Australia, Sweden, and the United States. The study found that people with a BMI score between 40 and 44.9 died on average 6.5 years earlier than those with a healthy BMI score, while those with a BMI between 55 and 59.9 died on average 13.7 years earlier.

Find Your Team

On MyObesityTeam, the social network and online support group for people with obesity and their loved ones, more than 54,000 members come together to share experiences, seek advice, and offer support.

Have you been diagnosed with obesity? What steps do you take to manage your health? Join the conversation today to share your experiences and connect with others on MyObesityTeam.