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.
The following classesTrusted Source are used for adults who are at least 20 years old:
|18.5 or under||underweight|
|18.5 to <25.0||“normal” weight|
|25.0 to <30.0||overweight|
|30.0 to <35.0||class 1 obesity|
|35.0 to <40.0||class 2 obesity|
|40.0 or over||class 3 obesity (also known as morbid, extreme, or severe obesity)|
|Percentile range of BMI||Class|
|5% to <85%||“normal” weight|
|85% to <95%||overweight|
|95% or over||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.
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
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.
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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.
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:
- type 2 diabetes
- heart disease
- high blood pressure
- certain cancers (breast, colon, and endometrial)
- gallbladder disease
- fatty liver disease
- high cholesterol
- sleep apnea and other breathing problems
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.