What Is Obesity?

<p>JGI/Jamie Grill / Getty Images</p>

JGI/Jamie Grill / Getty Images

Medically reviewed by Roxana Ehsani, MS

When a person is diagnosed with obesity, they have excessive fat and are above the weight considered “healthy” for their height. Healthcare providers often use body mass index (BMI) to distinguish between adults who are overweight (BMI between 25 and 30) and those with obesity (BMI of 30 or above). Having these conditions is associated with several health risks, including:

Obesity is common. A wide-ranging survey of data from 2017 to 2018 found over 40% of American adults had it, with the numbers increasing.

This article will discuss the definition of “obesity,” its causes and effects, and how it’s treated and managed.

<p>JGI/Jamie Grill / Getty Images</p>

JGI/Jamie Grill / Getty Images

Obesity: Medical Definition and BMI Status

Clinically, obesity is often defined in terms of the BMI. It’s determined by measuring a person's weight in kilograms (kg) and dividing it by their height in meters squared (m2). After determining a person's BMI, a healthcare provider may refer to their weight in one of the following four categories:

  • Underweight: BMI of 18.5 or less

  • Healthy weight: BMI of 18.5 to 24.9

  • Overweight: BMI of 25 to 29.9

  • Obesity: 30 or more

Is BMI Accurate?

Assessing weight status with BMI is flawed and dated. This measure excludes relevant factors, such as body composition, ethnicity, sex, race, and age. Its widespread use continues predominantly because it’s of no cost and can quickly determine a person’s potential health status and outcomes.

Healthcare providers may use other tests alongside BMI to screen for obesity, including:

  • Waist circumference: A provider uses a measuring tape around the waist just above the hips. A waist circumference over 35 inches in people assigned female at birth, or 40 in those assigned male at birth may indicate potential health problems.

  • Waist-to-hip ratio: A provider measures waist and hip circumference using a measuring tape.

  • Skinfold thickness: A healthcare provider uses a caliper tool to pinch your body in several places; with these measurements, healthcare providers estimate total body fat composition.

  • Ultrasound: This type of imaging relies on sound waves to visually represent body fat content.

Research has shown these methods to be as or more effective in determining obesity-related risks. A recent review comparing BMI to other measures found all forms comparable but noted that ultrasound might miss some instances of obesity, especially among older adults.

The review also noted that, while some studies have found few differences among the screening tools, others reported weight circumference and weight-to-hip ratio were more reliable than BMI at predicting obesity and related disease risk.

Obesity in Children and Teenagers

Obesity rates have been rising among American children and teenagers. In a health data survey from 2017 to 2018, nearly 1 in 5 (19.3%) of those between ages 2 and 19 had obesity. Childhood obesity increases the lifetime risk of developing type 2 diabetes and heart disease. Moreover, children and teens with obesity are more prone to anxiety, depression, and problems with self-esteem.

What Causes Obesity?

The human body requires calories (energy) from food and drink to function. Obesity and weight gain occur due to energy imbalances, when a person consumes more calories than their body uses over a period of time.

If a person doesn't immediately use the calories they consume, the body stores them for long-term use as a type of fat known as triglycerides. Body fat increases as triglycerides accumulate from excess calories.

Risk Factors

While excess calories directly cause obesity, various health issues and behaviors can increase a person's risk of developing obesity, including:

  • Dietary factors: High-calorie diets, or those in which 10% of calories come from sugars or saturated fats, increase obesity risk.

  • Insufficient physical activity: Not getting enough exercise is another common risk factors. The exercise recommendation for adults is at least 150 minutes of moderate to light physical activity weekly.

  • Insufficient or poor sleep: Irregular sleep can affect hormones and lead to overeating; adults should aim for seven to eight uninterrupted hours of rest a night.

  • Health conditions: Certain diseases, including metabolic disorders, polycystic ovary syndrome (PCOS), and hypothyroidism (underactive thyroid), increase a person's risk for developing obesity.

  • Stress: Living with stress can trigger cortisol and other hormones, increasing hunger and leading to overeating.

  • Genetics: Genetics can predispose you to store excess fat, leading to obesity; at least 15 genes have been identified as influencing body weight.

  • Certain medications: Some people experience weight gain as a side effect of some medications, such as antidepressants, antipsychotics, beta-blockers (used for blood pressure), birth control, glucocorticoids (for autoimmune diseases), and insulin (a diabetes treatment).

  • Environmental factors: People in areas lacking access to healthy, fresh foods and green spaces are at higher risk.

Obesity: Physical Symptoms

Obesity causes a range of symptoms and can lead to dangerous conditions, so in 2013, the American Medical Association recommended that obesity be considered a disease that should be medically managed.

Obesity Symptoms in Adults

Notably, some people who develop obesity don’t have symptoms. However, in adults, this condition is often characterized by:

  • Excess body fat, often around the waist (known as visceral fat)

  • Difficulty breathing or shortness of breath

  • Excess sweating

  • Sleep disorders

  • Skin symptoms, sensitivity, or rashes due to moisture collecting in the folds

  • Difficulty or inability to perform physical tasks, such as standing up

  • Fatigue

  • Pain in the back and joints

  • Depression, anxiety, negative self-esteem, shame, and social stigmatization

Obesity Symptoms in Children and Teenagers

In children and teenagers, obesity commonly causes symptoms, including:

  • Fatty tissue, especially in the breasts

  • Acanthosis nigricans (a darker, smooth skin appearing around the neck or other parts of the body)

  • Stretch marks, often on the back or hips

  • Loss of breath during physical activity

  • Constipation (difficulty passing stool)

  • Gastroesophageal reflux disease (GERD) (a type of chronic acid reflux)

  • Sleep apnea

  • Low self-esteem

  • Earlier onset of puberty in people assigned female at birth

  • Delayed puberty in people assigned male at birth

  • Flat feet, dislocated hips, or other orthopedic issues

Complications of Obesity

Obesity increases the risk of developing additional complications and health conditions, including the following:

  • Type 2 diabetes

  • Hypertension (high blood pressure)

  • High cholesterol

  • Heart failure

  • Osteoarthritis

  • Obstructive sleep apnea

  • Gastroesophageal reflux disease (GERD)

  • Barrett’s esophagus (tissue similar to the lining of the intestines replaces the esophagus's lining)

  • Chronic kidney disease

  • Nonalcoholic fatty liver disease (fat buildup in the liver)

  • Metabolic syndrome (a group of diseases that together can lead to heart disease, stroke, or type 2 diabetes)

  • Certain cancers, especially that of the bowel, breast, and the womb

  • Gallstones

  • Reduced fertility

  • Liver disease

  • Chronic kidney disease

  • Pregnancy complications like preeclampsia (a spike in blood pressure) and gestational diabetes (diabetes that develops during pregnancy)

Obesity Treatment Approaches

Managing obesity involves various approaches, from lifestyle changes to medications to surgery.

Healthy Diet

The cornerstone of any weight management program is dietary changes. Talk to a healthcare provider before making any significant changes.

Your medical provider may refer you to a registered dietitian (RD) or registered dietitian nutritionist (RDN), who will work with you to create a personalized healthy eating plan. Strategies include:

  • Reducing meat consumption

  • Increasing intake of fresh fruit, vegetables, whole grains, and cooked dry beans

  • Substituting snacks like chips or crackers with healthy, lower-calorie alternatives

  • Drinking water or seltzer instead of soda or juice

  • Using salt-free spices and avoiding high-salt foods

  • Choosing fat-free or low-fat dairy products

When incorporating a new eating plan, it’s essential to make the changes gradually. A healthy diet should be paired with other lifestyle changes to ensure results.


Increasing physical activity is essential for weight loss. How much exercise do you need? Every week Adults should aim for 150 to 300 minutes of moderate-intensity activity weekly. Activities could include:

  • Walking

  • Jogging

  • Swimming

  • Cycling

Those who prefer high-intensity aerobic exercise should aim for 75 to 150 minutes weekly. Examples of high-intensity exercise include:

  • Running

  • Speed walking

  • Jumping rope

  • Stair-climbing

As with diet, talk to a healthcare provider before making changes. Start small and scale up if you don’t currently focus on fitness.


Research has shown that not getting enough sleep can increase the chances of developing obesity. Adults should aim for seven to eight hours of uninterrupted rest every night; children and adolescents require more. Follow a consistent sleep schedule to improve sleep quality.

Behavioral Programs

Behavioral weight loss programs in individual or group settings are an excellent obesity management tool. Trained professionals, such as weight loss counselors, RDs and RDNs, exercise specialists, or psychiatrists, assist people with customized diet and exercise plans to manage the condition. They may also provide behavioral strategies to ensure consistent and long-lasting results.


A healthcare provider may prescribe medications if diet, exercise, or behavioral programs are not yielding results. These medications reduce appetite, block food absorption in the intestines, or stimulate the pancreas to release insulin.

Examples of medications prescribed to treat obesity include:

  • Victoza, Saxenda (liraglutide)

  • Contrave (naltrexone bupropion)

  • Amfepramone (diethylpropion)

  • Phendiet, Melfiat, Anorex-SR (phendimetrazine)

  • Xenical (orlistat)

  • Wegovy, Ozempic (semaglutide) injectable medications

These are not meant as standalone treatments; they work alongside other methods to promote results.


Specialized devices to reduce the amount of food an individual can digest can promote weight loss. Two such devices approved by the Food and Drug Administration (FDA) are:

  • Gastric balloon: Gastric balloons are positioned in the stomach using an endoscope (an adjustable tube with a camera). Once in place, they are filled with air, gas, or liquid, reducing stomach capacity. After six to nine months, gastric balloons are removed.

  • Gastric emptying systems: A surgeon implants a specialized tube in the stomach, which drains some of the contents from the stomach 20 to 30 minutes after you eat. Once a person reaches their target weight, the system is removed.


A healthcare provider may recommend weight loss surgery for difficult-to-manage cases of obesity that pose significant health risks. There are three primary approaches:

  • Gastrectomy: This surgery involves removing a large portion of the stomach, making it smaller, and limiting how much food and drink a person can digest at one time.

  • Gastric band: During gastric band surgery, a specialized band is implanted and tightened around the upper stomach, just below the esophagus. Gastric band surgery aims to reduce how much food the stomach can digest at one time and assist in weight loss.

  • Gastric bypass: This surgery involves connecting a small portion of the stomach directly to the middle of the intestine, bypassing the first section. As a result, the body absorbs less food and has a reduced capacity for consumption.


If left unmanaged, obesity can significantly impact a person’s health, reduce their quality of life, and lead to complications. Research has shown that all-cause mortality—death due to associated conditions, such as cancer, type 2 diabetes, and heart diseases—is significantly elevated in people with this condition.

In addition to the physical health impact, there’s a mental health burden to living with obesity. According to one cross-sectional analysis, people with obesity are 25% more likely to have depression, anxiety, or other mental health conditions. Additionally, those with this condition are more likely to face social stigmatization and discrimination in employment and education.

Resources for People With Obesity

Many resources are available to manage obesity and reduce the risk of complications. If you are trying to manage weight and make changes to your lifestyle, there are several resources for support, including:

  • Healthcare providers: If you’ve been diagnosed with obesity or are concerned about your weight, talk to a healthcare provider about treatment options and additional support.

  • Governmental programs: Initiatives and campaigns led by governmental agencies, such as the Centers for Disease Control and Prevention (CDC), Institute for Health Metrics and Evaluation, and the World Health Organization (WHO), provide helpful information and data.

  • Programs and campaigns: Nongovernmental organizations and nonprofits organize and lead campaigns to increase obesity awareness and provide helpful information. Current programs include Eat Well & Keep Moving, aimed at kids in schools, and Partnership for a Healthier America, among others.

  • Technology: Using apps or specialized devices, activity monitors, and meal planners can help keep you on track with exercise and dietary goals.

  • Counseling: Individual or group-based counseling with a mental health or registered dietitian nutritionist can help you cope with the condition and associated challenges.

  • Social media: Facebook groups, message groups, and other online communities can also be helpful information and social support resources.