What is obesity?
Body mass index (BMI) is a calculation that takes a person’s weight and height into account to measure body size.
Obesity is associated with a higher risk for serious diseases, such as type 2 diabetes, heart disease, and cancer.
Obesity is common. The CDC estimates that 42.4 percentTrusted Source of Americans 20 years old and older had obesity in 2017 to 2018.
But BMI isn’t everything. It has some limitations as a metric.
According to the CDCTrusted Source: “Factors such as age, sex, ethnicity, and muscle mass can influence the relationship between BMI and body fat. Also, BMI doesn’t distinguish between excess fat, muscle, or bone mass, nor does it provide any indication of the distribution of fat among individuals.”
Despite these limitations, BMI continues to be widely used as a way to measure body size.
How is obesity classified?
Weight that is higher than what is considered healthy for a given height is described as overweight or obesity. Body Mass Index (BMI) is a screening tool for overweight and obesity.
Adult Body Mass Index
BMI is a person’s weight in kilograms divided by the square of height in meters. A high BMI can indicate high body fatness.
- If your BMI is less than 18.5, it falls within the underweight range.
- If your BMI is 18.5 to <25, it falls within the healthy weight range.
- If your BMI is 25.0 to <30, it falls within the overweight range.
- If your BMI is 30.0 or higher, it falls within the obesity range.
Obesity is frequently subdivided into categories:
- Class 1: BMI of 30 to < 35
- Class 2: BMI of 35 to < 40
- Class 3: BMI of 40 or higher. Class 3 obesity is sometimes categorized as “severe” obesity.
Note: For individuals, BMI is screening tool, but it does not diagnose body fatness or health. A trained health care provider should perform appropriate assessments to evaluate an individual’s health status and risks. If you have questions about your BMI, talk with your health care provider.
See the following table for an example.
|5′ 9″||124 lbs or less||Below 18.5||Underweight|
|125 lbs to 168 lbs||18.5 to 24.9||Healthy weight|
|169 lbs to 202 lbs||25.0 to 29.9||Overweight|
|203 lbs or more||30 or higher||Obesity|
|271 lbs or more||40 or higher||Class 3 Obesity|
BMI does not measure body fat directly, but BMI is moderately correlated with more direct measures of body fat obtained from skinfold thickness measurements, bioelectrical impedance, underwater weighing, dual energy x-ray absorptiometry (DXA) and other methods 1,2,3. Furthermore, BMI appears to be strongly correlated with various adverse health outcomes consistent with these more direct measures of body fatness
What is childhood obesity?
From 2015 to 2016, 18.5 percentTrusted Source (or about 13.7 million) American youth between 2 and 19 years old were considered to have clinical obesity.
What causes obesity?
Eating more calories than you burn in daily activity and exercise — on a long-term basis — can lead to obesity. Over time, these extra calories add up and cause weight gain.
But it’s not always just about calories in and calories out, or having a sedentary lifestyle. While those are indeed causes of obesity, some causes you can’t control.
Common specific causes of obesity include:
- genetics, which can affect how your body processes food into energy and how fat is stored
- growing older, which can lead to less muscle mass and a slower metabolic rate, making it easier to gain weight
- not sleeping enough, which can lead to hormonal changes that make you feel hungrier and crave certain high-calorie foods
- pregnancy, as weight gained during pregnancy may be difficult to lose and might eventually lead to obesity
Certain health conditions can also lead to weight gain, which may lead to obesity. These include:
- polycystic ovary syndrome (PCOS), a condition that causes an imbalance of female reproductive hormones
- Prader-Willi syndrome, a rare condition present at birth that causes excessive hunger
- Cushing syndrome, a condition caused by having high cortisol levels (the stress hormone) in your system
- hypothyroidism (underactive thyroid), a condition in which the thyroid gland doesn’t produce enough of certain important hormones
- osteoarthritis (OA) and other conditions that cause pain that may lead to reduced activity
Who is at risk for obesity?
A complex mix of factors can increase a person’s risk for obesity.
Some people have genes that make it difficult for them to lose weight.
Environment and community
Your environment at home, at school, and in your community can all influence how and what you eat, and how active you are.
You may be at a higher risk for obesity if you:
- live in a neighborhood with limited healthy food options or with manyTrusted Source high-calorie food options, like fast-food restaurants
- haven’t yet learned to cook healthy meals
- don’t think you can afford healthier foods
- haven’t foundTrusted Source a good place to play, walk, or exercise in your neighborhood
Psychological and other factors
Quitting smoking is always a good thing, but quitting may lead to weight gain too. In some people, it may lead to excessiveTrusted Source weight gain. For that reason, it’s important to focus on diet and exercise while you’re quitting, at least after the initial withdrawal period.
How is obesity diagnosed?
BMI is a rough calculation of a person’s weight in relation to their height.
Other more accurate measures of body fat and body fat distribution include:
- skinfold thickness tests
- waist-to-hip comparisons
- screening tests, such as ultrasounds, CT scans, and MRI scans
Your doctor may also order certain tests to help diagnose obesity-related health risks. These may include:
- blood tests to examine cholesterol and glucose levels
- liver function tests
- a diabetes screening
- thyroid tests
- heart tests, such as an electrocardiogram (ECG or EKG)
A measurement of the fat around your waist is also a good predictor of your risk for obesity-related diseases.
What are the complications of obesity?
Obesity can lead to more than simple weight gain.
Having a high ratio of body fat to muscle puts strain on your bones as well as your internal organs. It also increases inflammation in the body, which is thought to be a risk factor for cancer. Obesity is also a major risk factor for type 2 diabetes.
Obesity has been linked to a number of health complications, some of which can be life threatening if not treated:
How is obesity treated?
If you have obesity and been unable to lose weight on your own, medical help is available. Start with your primary care physician, who may be able to refer you to a weight specialist in your area.
Your doctor will work with you on making needed lifestyle changes. Sometimes, they may recommend medications or weight loss surgery as well. Learn more about treatment for obesity.
Which lifestyle and behavioral changes can help with weight loss?
Your healthcare team can educate you on food choices and help develop a healthy eating plan that works for you.
Lifestyle and behavioral changes are the preferred weight loss methods for children, unless they’re extremely overweight.
Which medications are prescribed for weight loss?
Your doctor may also prescribe certain prescription weight loss medications in addition to eating and exercise plans.
Medications are usually prescribed only if other methods of weight loss haven’t worked and if you have a BMI of 27.0 or more in addition to obesity-related health issues.
Prescription weight loss medications either prevent the absorption of fat or suppress appetite. The following are approved for long-term use (at least 12 weeks) by the Food and Drug Administration (FDA):
- phentermine/topiramate (Qsymia)
- naltrexone/bupropion (Contrave)
- liraglutide (Saxenda)
- orlistat (Alli, Xenical), the only one that’s FDA-approved for use in children 12 years old and older
These drugs can have unpleasant side effects. For example, orlistat can lead to oily and frequent bowel movements, bowel urgency, and gas.
Your doctor will monitor you closely while you’re taking these medications.
What are the types of weight loss surgery?
Weight loss surgery is commonly called bariatric surgery.
This type of surgery works by limiting how much food you can comfortably eat or by preventing your body from absorbing food and calories. Sometimes it can do both.
Weight loss surgery isn’t a quick fix. It’s a major surgery and can have serious risks. Afterward, people who undergo surgery will need to change how they eat and how much they eat, or they risk getting sick.
However, nonsurgical options aren’t always effective at helping people with obesity lose weight and reduce their risk for comorbidities.
Types of weight loss surgery include:
- Gastric bypass surgery. In this procedure, your surgeon creates a small pouch at the top of your stomach that connects directly to your small intestine. Food and liquids go through the pouch and into the intestine, bypassing most of the stomach. It’s also known as Roux-en-Y gastric bypass (RYGB) surgery.
- Laparoscopic adjustable gastric banding (LAGB). LAGB separates your stomach into two pouches using a band.
- Gastric sleeve surgery. This procedure removes part of your stomach.
- Biliopancreatic diversion with duodenal switch. This procedure removes most of your stomach.
Candidates for surgery
For decades, experts recommended that adult candidates for weight loss surgery have a BMI of at least 35.0 (classes 2 and 3).
However, in 2018 guidelines, the American Society for Metabolic and Bariatric Surgery (ASMBS) endorsed weight loss surgery for adults with BMIs of 30.0 up to 35.0 (class 1) who:
- have related comorbidities, especially type 2 diabetes
- haven’t seen sustained results from nonsurgical treatments, such as eating and lifestyle modifications
For individuals with class 1 obesity, surgery is most effective for those between the ages of 18 and 65 years old.
People will often have to lose some weight before undergoing surgery. Additionally, they’ll normally undergo counseling to ensure that they’re both emotionally prepared for the surgery and willing to make the necessary lifestyle changes that it’ll require.
Only a few surgical centers in the United States perform these types of procedures on children under 18 years old.
How can you prevent obesity?
There’s been a dramatic increase in obesity and in obesity-related diseases in the last couple decades. This is the reason why communities, states, and the federal government are putting an emphasis on healthier food choices and activities to help turn the tide on obesity.
On a personal level, you can help prevent weight gain and obesity by making healthier lifestyle choices:
- Aim for moderate exercise like walking, swimming, or biking for 20 to 30 minutes every day.
- Eat well by choosing nutritious foods, like fruits, vegetables, whole grains, and lean protein.
- Eat high-fat, high-calorie foods in moderation.