Learn more about the definition of obesity and what it means for your health.
By Kelly Burch
Updated on February 14, 2024
Medically reviewed by Allison Herries, RDN
Obesity is a chronic disease that is diagnosed when a person’s body weight is about what’s considered healthy for their height.1 Obesity, defined as having a body mass index (BMI) above 30 for adults, is associated with health risks including cardiovascular disease, diabetes, and metabolic conditions.2
However, some people with obesity are metabolically healthy and don’t have this increased risk for disease, just as some people who are not overweight still suffer from these diseases.
Continue reading to learn more, including classes of BMI and why your BMI doesn’t tell you everything you need to know about your health.
A Note on Gender and Sex Terminology
Verywell Health acknowledges that sex and gender are related concepts, but they are not the same. To reflect our sources accurately, this article uses terms like “female,” “male,” “woman,” and “man” as the sources use them.
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What Is Obesity, and How Is It Calculated?
Obesity is calculated by comparing a person's weight to their height. This is a calculation known as the body mass index. For adults, obesity is diagnosed when BMI is 30 or higher.1 That means for a male who is 5 feet, nine inches tall (the average male height), weighing 203 pounds or more qualifies him as obese.
Although the use of BMI is widespread in modern medicine, it is a problematic and overly simplistic metric.3 BMI ranges were initially determined based on measurements of northern European white men in the 1800s. Thus, the data do not reflect modern life. It also ignores sex, gender, ethnicity, socioeconomic status, and other important factors.
In addition, the measurement does not consider a person’s body composition, including whether they have lots of muscle. For that reason, athletes often have a high, sometimes even obese, BMI.4
WHy BMI Is Flawed
One of the main reasons BMI is a flawed measurement is because it does not take into account the gender, ethnic, racial, and socioeconomic differences in people that may cause obesity. It also doesn't differentiate between weight from fat, bone, muscle mass,4
Obesity Classes
The Centers for Disease Control and Prevention (CDC) breaks down adult obesity into three subclasses. They are:1
Class 1: BMI of 30–35
Class 2: BMI of 35–40
Class 3: BMI of 40 or higher
Class 3 obesity is sometimes categorized as severe obesity, which used to be known as morbid obesity. In recent years that term has been dropped to combat stigma around obesity.
What Is Childhood Obesity?
Childhood obesity is diagnosed when a person’s BMI is at the 95th percentile or higher on growth charts for their age and sex.5 Just as with adults, the calculation of BMI is problematic for children, and athletes may have a high BMI due to their muscle mass compared with their peers.5
Obesity Rates
About 4 in 10 American adults are obese. Learn more about obesity rates and stats here.
What Are the Symptoms of Obesity?
There are no specific symptoms of obesity, because it is such a complex condition.6 About 7% people with obesity are metabolically healthy—that means their excess weight isn’t causing any health concerns.2 However, researchers have determined that obesity increases risks for certain health conditions.
What Are the Complications of Obesity?
Compared to people with a BMI in the healthy range, people with obesity are at increased risk of:7
Cardiovascular diseases, including coronary heart disease and stroke
Type 2 diabetes
High blood pressure
Sleep apnea
Breathing problem
Joint problems and pain
Certain cancers
Gallbladder disease
Fatty liver disease
High cholesterol
Mental health issues
Arthritis
Infertility and irregular menstrual cycles
While there are physical reasons for many of these complications, others are caused or worsened by how society treats people with obesity. Research shows that people with obesity get lower quality care from healthcare professionals when compared with people of lower weight.8 Stigma, judgment, and shame from family, colleagues, or self can also contribute to mental and emotional complications of obesity.
The Obesity Paradox
People with obesity are more likely than lower-weight people to die from all causes. However, researchers have seen that obese people are more likely to survive from cardiac events like heart attacks or heart surgery than people of a normal BMI. This is known as the obesity paradox.
What Causes Obesity?
The causes of obesity are incredibly varied and are about much more than simply the food you eat—although that can play a role. However, obesity is much more complex than the outdated idea of calories consumed vs. calories burned, and is not a personal failing or a reflection of your self-control. Many different factors contribute to the development of obesity, including:9
Food, activity, and sleep: A person’s eating patterns, movement, and sleep can all contribute to weight gain.
Genetic factors: Many different genes can contribute to obesity, and your genetics often impacts body size.
Social determinants of health: Where you live, work and learn can impact your risk for obesity. Socioeconomic factors, including access to quality fresh food, also play a role.
Illnesses and medications: Certain diseases and medications can lead to weight gain.
Health Conditions That May Lead to Weight Gain
Some health conditions can contribute to weight gain, including:9
Polycystic ovary syndrome (PCOS)
Metabolic syndrome
Cushing syndrome
Thyroid disease
Prader-Willi syndrome
Medications, including those that treat depression and other mental health conditions, can also lead to weight gain. Treating these illnesses, or adjusting medications, may help manage obesity.
How Is Obesity Diagnosed?
Obesity is most often diagnosed using a person’s BMI. Since the BMI defines obesity, this is the true diagnostic test.
However, there are other diagnostic criteria that a healthcare provider can use to determine whether the classification of obesity is impacting your health. These can give a better understanding of your body composition, and include:
Measuring your waist circumference
Calculating body fat, including by using a skinfold test
Looking at labs that measure your blood sugar, cholesterol, liver function, and more
Getting a physical exam and giving a thorough health history
How Is Obesity Treated?
Obesity is a complex disease, and treating it is difficult. While weight loss is often a goal for people with obesity, sustaining weight loss through dietary and lifestyle changes alone is challenging.
Fortunately, even a small amount of weight loss can make a difference. in fact, losing just 5% to 10% of your body weight can lead to more energy, better sleep, and lower blood sugars, even if you still have obesity.10
To many lasting changes to your weight, the CDC recommends:10
Eat whole foods: Add more fruits and vegetables, while reducing processed foods and sugary beverages.
Move more: Aim for 150 minutes of moderate exercise, like walking, each week. Add in one or two days of strength training.
Treatments for obesity can also include medical interventions, like weight loss surgery or weight loss drugs. While these options help many people, they’re not a magic pill. They come with their own serious side effects, which you should discuss with your healthcare provider if you are considering them.
How Can You Prevent Obesity?
Focusing on health throughout your life may help prevent obesity. More importantly, it will help contribute to your overall health. Try these tips.10
Eat whole foods.
Limit processed foods.
Get regular physical activity.
Limit screen time.
Find a healthcare provider or care provider who listens to your concerns and doesn’t stigmatize your weight.
Treat underlying health concerns.
Practice self care and body acceptance.
Build your self confidence.
Create supportive environments.
Summary
Obesity is a chronic condition that is diagnosed when a person’s BMI is 30 or higher. Having a BMI considered obese is associated with increased risk for adverse health effects, including cardiovascular disease and diabetes. However, some people with obesity are metabolically healthy.
Eating healthy, whole foods, exercising more and sleeping well may help you lose weight, but for most people treating obesity is much more complex than lifestyle changes. It can include surgical or medical interventions, and treating underlying health conditions. Finding a trusted healthcare provider who does not stigmatize your weight can help you develop a treatment plan that works for you.
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