What It Means To Be Morbidly Obese, And How Your BMI Is Classified

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BMI explained and how it is used to classify morbid obesity

Before diving into the medical definition of morbid obesity and what it means, it's essential to initially understand the concept of Body Mass Index (BMI) and its role in classifying whether you are at a healthy weight, obese, or morbidly obese. Your level of obesity (i.e., Overweight, Class I Obesity, Class II Obesity, Class III Obesity) can be measured by how much you exceed your ideal body weight, with Class III Obesity/Morbid Obesity ranging from 80 to 100 pounds over¹.

However, BMI is the best way to define morbid obesity and is most commonly calculated by the following formula:

BMI = weight (kg)/[height(m)]²

BMI is divided into the following categories for both men and women³:

  • Underweight — BMI below 18.5

  • Healthy Weight — BMI between 18.5 – 24.9

  • Overweight — BMI between 25.0 – 29.9

  • Obesity — BMI of 30.0 and higher

  • Class I Obesity — BMI between 30.0 and 34.9

  • Class II Obesity — BMI between 35 and 39.9

  • Class III Obesity/Morbid Obesity — BMI of 40.0 and higher 

The same formula is used for children and teens, but the interpretation is different. BMI for teens and children is very age and sex-specific because body fat differs for boys and girls and changes with age³. Instead, BMI is calculated in percentiles, with severe obesity being within the 99th percentile and higher. According to a 2019 study⁴, around 17% of U.S. youth are considered obese, in addition to a 2013 study⁵ that found between 4% and 6% of children in the United States are severely obese.

What is morbid obesity?

The CDC⁶ reports that the prevalence of obesity among adults in the U.S. is about 40.3% for individuals between 20 and 39 years old, 46.4% for those between 40 and 59 years old, and 42.2% for 65 and older. A 2019 study⁷ projects that nearly one in two adults will be obese by 2030, and almost one in four adults will have severe obesity. Unlike many other health conditions, the diagnosis of obesity and morbid obesity did not have any specific link to age or sex⁶.

Morbid obesity, however, is linked to overeating and lack of exercise.

The medical definition of morbid obesity is "a serious health condition that results from an abnormally high body mass that is diagnosed by having a body mass index (BMI) greater than 40 kg/m², a BMI of greater than 35 kg/m² with at least one serious obesity-related condition, or being more than 100 pounds over ideal body weight (IBW).⁸

Serious obesity-related conditions include:

  • Hypertension

  • Type II diabetes mellitus

  • Cardiovascular disease

  • Osteoarthritis

  • Reproductive disorders (infertility)

  • Respiratory disorders (sleep apnea)

  • Cancers (endometrial/ovarian)

This medical condition can cause many serious problems to your health as your BMI and IBW increase. In many cases, morbid obesity can even become fatal. In order to get across to the patients the seriousness of morbid obesity without sounding mean and judgemental, medical professionals have moved away from using the term 'morbid obesity' and instead use either 'severe obesity' or 'class 3 obesity'⁸. This is because medical providers and patients often have a different view of the definition of 'morbid,' so similar vocabulary can make patients more comfortable and receptive to seeking help.

The most significant difference between morbid obesity and obesity is the severity of the condition regarding your weight and BMI. While both obesity and morbid obesity can harm your body and its normal functions, morbid obesity is the most severe case of obesity.

The causes

Morbid obesity is commonly caused by two contributing factors⁹: environmental and societal (external) and genetic (internal).

Environmental and societal influences

The environmental and societal influences that can contribute to obesity and severe obesity include industrial influences like food production, cultural influences like sedentary lifestyles and body image attitudes, socioeconomic influences (higher social status is linked to obesity in developed countries), and societal changes like those that cause increased stress⁹.

Availability also plays a big role:

  • Healthy food availability — the availability of reasonably priced and healthy foods in an area can play a big role in the risk of obesity. This is because availability can influence purchasing decisions. For instance, an individual will be more likely to purchase pizza rolls and frozen dinners that are much cheaper than whole foods and fruits and vegetables with a short shelf-life.

  • Recreational sport availability — people are more likely to participate in sports and physical activity if they have easy access to it. This means having access to a nearby park can influence more people to get active compared with having a park that is a half-hour drive away. 


Moreover, genetics plays a role in severe obesity. According to a 2018 study¹⁰, research supports the presence of at least ten different gene variants that could cause severe obesity. The report found that the frequency of the variants also contributed to common obesity.

Eating behaviors

A diet high in simple carbohydrates like soft drinks, sugar, desserts, beer, wine, and fructose can play a large role in weight gain due to their rapid absorption into the bloodstream compared to complex carbs (i.e., grains, pasta, vegetables, raw fruit, etc.)¹¹. Overeating and frequency of consumption are other eating behaviors that can lead to being overweight and eventually severely obese if not controlled.


According to a 2015 study¹² that explored the link between weight change and medications, the following drugs were linked to weight gain:

  • Amitriptyline (1.8 kg)

  • Mirtazapine (1.5 kg)

  • Olanzapine (2.4 kg)

  • Quetiapine (1.1 kg)

  • Risperidone (0.8 kg)

  • Gabapentin (2.2 kg)

  • Tolbutamide (2.8 kg)

  • Pioglitazone (2.6 kg)

  • Glimepiride (2.1 kg)

  • Gliclazide (1.8 kg)

  • Glyburide (2.6 kg)

  • Glipizide (2.2 kg)

  • Sitagliptin (0.55 kg)

  • Nateglinide (0.3 kg)

These drugs include antidepressants, anticonvulsants (for controlling seizures), some diabetes medications, oral contraceptives, and some high blood pressure medications¹¹.


According to MedicineNet¹¹, diseases such as hypothyroidism, insulin resistance, and polycystic ovary syndrome have been linked to obesity.

Associated Complications

The most severe complication associated with morbid obesity is death. According to a 2017 study¹³, the life expectancy of a severely obese person is reduced by an estimated 5–20 years. Another study in the same report found that the overall mortality was 30.7% lower among the bariatric group compared with the control group. Additional research¹ found that the obesity epidemic has led to approximately 300,000 deaths per year.

Other common complications associated with obesity include¹⁴:

  • Pulmonary (lung) conditions 

  • Heart conditions like high blood pressure and heart failure

  • Diabetes

  • Reproductive conditions like infertility, menstrual irregularity, and frequent UTIs

  • Gallbladder disease

  • Chronic low back pain and degeneration of hips and knees

  • Cancer risks such as renal, colon, pancreatic, breast, gallbladder, prostate, and uterine.

A 2010 study¹⁵ confirmed these risks; the findings showed that body weight was a major risk factor for diabetes, and weight loss was positively associated with improved diabetes control. The report also found that cardiovascular disease was more common among individuals who are obese, and cancer mortality rates were 52% higher in men and 62% higher in women compared with those with a normal BMI.

Gallbladder disease was more common among individuals with higher BMI, resulting in more days spent in hospital with the disease. As for pulmonary complications, the prevalence of morbidly obese patients was 30% higher in 2-3% of middle-aged women and 4-5% of middle-aged men.


Many approaches can be taken to treat morbid obesity, and they often vary from person to person. You may notice that treatments are similar for most morbidly obese patients, since it's important to take more aggressive actions in these severe cases. For instance, morbidly obese patients often undergo bariatric surgery in addition to lifestyle changes.

Bariatric surgery, also known as weight loss surgery, is a surgery that changes the position of your stomach and small intestines, resulting in changes in your appetite, metabolism (how your body burns calories), and satiety (when you feel full). Research shows that this surgery is the most successful treatment for severe obesity, resulting in 65% to 85%¹⁶ of patients reaching their weight loss goals.

In addition to surgery, some common lifestyle changes must also be made:

  • Make a change in eating habits

  • Increase your physical activity

  • Get more familiar with your body and the nutrients it needs to stay healthy

  • Take part in a support group

  • Make realistic goals for weight management


According to the Health Statistics Center¹⁴, the combination of being overweight and obesity make this condition the second leading cause of preventable deaths in the United States. The report found that the lowest mortality rates were in BMIs between 23.5 and 24.9 for men and 22.0 and 23.4 for women. Risks increased substantially as age and BMI increased.

To prevent morbid obesity and the complications that can emerge, it is recommended that you make these three lifestyle changes⁹:

  • Healthier eating behaviors: some healthier eating habits include reducing the amount of sugary and processed food you eat, increasing your intake of fruits and vegetables, and eating a good amount of dietary fiber.

  • More physical activity: regular physical activity like walking, running, weight lifting, yoga, etc., are essential to the health of your body, both mentally and physically. It is also a tried and true way of preventing severe obesity. Small steps like walking around the block each day can make a significant difference if you're trying to live a healthier lifestyle.

  • Limit sedentary activities: as much as many of us love to watch TV and play video games, it is essential to limit the amount of time you spend doing these activities if you want to prevent obesity and severe obesity. 

The lowdown

Morbid obesity is a serious health condition that affects the majority of the United States population. However, it is treatable and preventable if proper measures are taken in time. This includes making healthier lifestyle changes like moving away from hours of TV and video games to walking around the block or at a park more often. It also includes making healthier dietary changes to supply more nutrients to your body and limit the amount of sugar, fructose, and processed foods you take in.

Although mortality rates are high among morbidly obese patients, these changes, in addition to bariatric surgery, can make a significant difference for most patients. The key is to stay educated on proper weight management and how to maintain a healthy BMI with adequate nutrition and exercise.

It is important to note that the same BMI formula is used for children and teens, but it is not categorized in the same way. There are more moving parts in the diagnosis of obesity and severe obesity among our youth, so a pediatrician can be extremely helpful in identifying dangerous percentages and how you can respond appropriately. It is best to avoid implementing an extreme diet or exercise routine on a child or teen which can become harmful to their mental, emotional, and physical health. The pediatrician can help you develop a more effective and safe game plan if one is needed.

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