What Is Obesity?

Obesity is defined by the World Health Organization (WHO) as “abnormal or excessive fat accumulation that presents a risk to health.” The disease of obesity affects more than one-third of U.S. adults, according to the Centers for Disease Control and Prevention (CDC). The CDC uses body mass index (BMI), or a person’s weight in kilograms divided by the square of their height in meters, to measure obesity. Individuals with a BMI of 30 or higher are considered to have obese. While BMI can be used as an initial tool to screen for obesity, it does have limitations. The WHO recognizes these limitations, stating that BMI is only “a crude population measure of obesity.”

Another definition of obesity comes from the Obesity Medicine Association (OMA) and is stated in OMA’s Obesity Algorithm®. Obesity is defined by OMA as a “chronic, relapsing, multi-factorial, neurobehavioral disease, wherein an increase in body fat promotes adipose tissue dysfunction and abnormal fat mass physical forces, resulting in adverse metabolic, biomechanical, and psychosocial health consequences.”

Why Is Obesity a Disease?

Obesity was first recognized as a disease in 1948 by the World Health Organization (WHO).1

In the United States, the American Medical Association (AMA) recognized obesity as a disease in 2013.2 Why is this recognition of obesity as a disease important? Obesity has long been a topic of bias and stigma. While public perception is slowly shifting, many people (including nearly half of healthcare professionals) still assume that obesity is the individual’s fault, solely the result of them eating too much and moving too little. When the AMA and other professional medical and health organizations recognize obesity as a disease, it educates healthcare professionals and the public that it is far more complex than that. Obesity is not a personal choice. Obesity is a disease caused by a complex interplay among genetics, the environment, and many other factors. Further, recent studies have demonstrated that there are 236 other diseases impacted by obesity, 13 of which are cancers.

The recognition of obesity as a disease allows us to research interventions to prevent obesity and implement evidence-based treatments with those who are affected by obesity. These treatments include behavioral and nutritional interventions, medical treatment, and even surgery. The recognition of obesity as a disease also distinguishes obesity as something to be taken seriously; for most people suffering with obesity, simply eating less and moving more will not result in sustainable long-term weight loss.3

A person is no more at fault for developing obesity than they are at fault for developing heart disease or high blood pressure. A person with obesity deserves the same respect and compassionate healthcare. The United States Preventive Services Task Force (USPSTF) recommends screening all adults for obesity and offering or referring those with obesity to intensive, multi-component behavioral intervention programs.4

What Causes Obesity?

Obesity is a medical disease that occurs when a person’s body accumulates and stores excessive amounts of body fat. There are many factors that contribute to obesity, including:

  • Genetics: Studies suggest genetics contribute to 40-70% of obesity. There are thousands of different genes associated with obesity, and some genetic mutations are known to directly cause obesity. However, possessing genes that predispose you to obesity does not mean developing the disease is inevitable.5
  • Nutrition: Growing evidence shows that not all calories are created equal. Certain foods, such as refined carbohydrates and unhealthy saturated fats, contribute in greater proportion to obesity.
  • Hormones: Abnormalities in several biological pathways are involved in obesity. Insulin resistance, which causes the body not to process sugar properly, has been heavily implicated in obesity.
  • Medication: Many prescription medications are associated with weight gain, including most antidepressants, steroids, contraceptives, and medications for the treatment of diabetes, hypertension, migraines, seizures, bi-polar disorder, allergies, insomnia and more.
  • Environment: Family, culture, and even where you live can have a significant impact on developing obesity.
  • Sleep: Disrupted sleep and sleeping for less than 7 hours or more than 9 hours can cause obesity. Obesity is also associated with sleep apnea, which is a breathing condition that can hinder an individual from getting quality sleep.

Diseases Associated with Obesity

Current studies show that obesity decreases the lifespan by up to 8 years7 and is associated with at least 236 other medical problems including 13 types of cancer,8 type 2 diabetes, sleep apnea, high blood pressure, high cholesterol, and depression.9


Both biological and lifestyle factors associated with obesity can contribute to the higher risk of cancer. Fat cells produce chemicals harmful to the body, causing inflammation that can damage healthy cells over time. Fat cells can also increase hormone levels that can trigger genes in people with higher risks for certain cancers. Lifestyle factors also play a part in the connection between obesity and cancer. Proper nutrition and physical activity are key components in lowering the risk for both obesity and cancer.10

These 13 types of cancer have shown to have the strongest association to obesity:

  • Breast (post-menopausal women)
  • Colon and rectum
  • Esophageal (adenocarcinoma)
  • Gallbladder
  • Kidney
  • Liver
  • Meningioma (cancer of tissues that covers the brain and spinal cord)
  • Multiple Myeloma (cancer of the blood cells)
  • Ovaries
  • Pancreas
  • Thyroid
  • Upper stomach
  • Uterus

Type 2 Diabetes

More than 85% of patients with type 2 diabetes also suffer from overweight or obesity. The relationship between obesity and type 2 diabetes begins long before the type 2 diabetes is often recognized, when insulin resistance begins. Insulin is a hormone that regulates blood sugar levels by determining how the body stores glucose and fat. When the body develops insulin resistance, it requires higher levels of insulin to allow the body to use glucose as an energy source. This resistance is a precursor to prediabetes and type 2 diabetes.11

Sleep Apnea

One in five Americans are estimated to be affected by sleep apnea. People with sleep apnea experience momentary cessations of breathing while they sleep. Sleep apnea is most common in men, older individuals, and those with higher BMIs. In addition, women with a neck circumference of 16 inches or more and men with a neck circumference of 17 inches or more are at high risk.

Obesity contributes to the risk of sleep apnea because excess weight puts increased pressure on upper airways, which leads to collapse and decreased neuromuscular control from the fatty deposits. This contributes to decreased lung volume and increased difficulty breathing. Sleep apnea symptoms include excessive daytime sleepiness, snoring, morning headaches, and elevated blood pressure. Sleep apnea can lead to many other serious medical disorders.12

Hypertension (High Blood Pressure)

Hypertension refers to the pressure applied to the inner walls of the arteries by blood. In the 44-year Framingham Heart Study, overweight and obesity were estimated to account for 26-28% of cases of hypertension.13 Obesity and fat distribution are both determinants of hypertension risk. For those suffering from hypertension, weight loss can lead to a significant drop in blood pressure, lessening the need for medications as well as lowering the risk for hypertension-related diseases.


Depression is common in patients with overweight and obesity. In a recent study, researchers found individuals with obesity have a 20% higher risk of developing depression, and in some cases the risk is elevated to 44% higher than those at a healthy weight. Symptoms of depression include fatigue, low-energy affected sleep, and over-eating.14

Obesity Facts

  • In 2014, obesity was estimated to cost the United States $1.42 trillion, using up 14.3% of healthcare spending dollars and resulting in 320,000 deaths.
  • It is estimated that losing just 5% body weight can save an individual with a BMI over 40 $2,137 per year.15
  • Obesity is the most common chronic disease in the United States. According to the CDC, about 93.3 million U.S. adults were affected by obesity from 2015-2016. Within 15 years, it is estimated that 50% of the U.S. adult population with suffer from obesity.
  • Higher education levels are associated with a lower risk of obesity.
  • Lifespan is estimated to be shorted by up to 8 years for those with a BMI greater than 40, which is comparable to the effects smoking.16

Frequently Asked Questions

What treatments are available for obesity?

Medical professionals can use a comprehensive, evidence-based approach to manage and treat obesity. This approach is comprised of four pillars: nutrition, physical activity, behavior, and medication. Treatment is tailored to the patient and their needs. Patients may be treated with combinations of any or all of the four pillars. Nutritional treatment includes implementing prepackaged meals, decreasing portion sizes, and decreasing intake of certain foods or types of foods. Physical activity treatment may include starting a walking program, or progressing to a regular workout pattern based on the patient’s current levels of activity. Behavioral goals can include modifications like eating slowly; eating smaller portions; paying attention to when, how, and what you eat; and identifying reasons or triggers for eating when not hungry. Medication is the last pillar; there are quite a few medications available that are approved for obesity and should be administered under the supervision of an obesity medicine specialist.

What are the classifications of obesity?

An easy way to classify obesity is on the basis of BMI. Class 1 being BMI 30 -34.9, Class 2 being BMI 35-39.9, and Class 3 being BMI more than 40. Although this is a commonly accepted classification of obesity it has its limitations as it does not counter in abdominal distribution of unhealthy weight which may signify a more serious case of obesity or racial disparities.

How fast should your body lose weight?

Active weight loss is loss of approximately 1 to 2 pounds per week, however, it can be very frustrating to lose 1-2 pounds at a time with all of the hard work it takes to lose weight. Seeing significant weight loss when initially starting the process can help people stick to their lifestyle changes. Fad diets which promise quick weight loss are rarely sustainable but there are healthy supervised weight loss programs to help with faster weight loss. The key is to find lifestyle changes that lead to weight loss that can also be maintained over time. Working with your provider to adopt a healthy weight loss plan is always the best solution.

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