What Is Obesity? BMI Ranges, Causes, Treatment, Prevention, and Statistics
Obesity is a term that, well, carries a lot of weight, because of varying cultural norms and stigmas about body size and shape.
But for the purposes of this guide, we’ll focus on the World Health Organization’s (WHO) definition of obesity: abnormal or excessive fat accumulation in the body that may pose a risk to your health.
Below, discover how doctors determine whether someone has obesity, as well as causes and risk factors of obesity, treatment options for obesity, and much more.
Obesity is a chronic condition that for many people can be progressive, and something that needs to be managed for life. Some people may be able to shift to lower weights, but many others will continue to struggle despite mainstream interventions like lifestyle changes, and medical and surgical approaches.
As mentioned, obesity is generally defined as having excess weight. But it’s not so simple.
BMI takes both a person's height and weight into account and is expressed in units of kilograms per square meter (kg/m2). You can also use this formula to get your BMI: Divide your weight in pounds by your height in inches squared. Then multiply the amount by 703.
You can also find BMI calculators for both adults and children online, including from the Centers for Disease Control and Prevention (CDC).
BMI Ranges
BMI levels for adults are categorized as follows:
Optimal weight: 18.5 to 24.9 kg/m2
Overweight: 25 to 29.9 kg/m2
Obesity: 30 kg/m2 or greater
But keep in mind that BMI is limited when used to measure health risk.
For instance, health experts say people of Asian descent may have increased health risks even if their BMI is in the optimal range. They suggest lowering the beginning point for overweight BMI to 23 and the beginning point for obesity to 27 among this group.
And some studies have found that Black people have lower body fat and higher lean muscle mass than white people at the same BMI; meaning that at the same BMI, they may have a lower risk of developing obesity-related diseases. Some researchers believe other ways to measure obesity may be more useful, such as percentage of body fat or waist-to-hip ratio.
All the same, BMI can be a good starting point as a screening tool, says Mir Ali, MD, a bariatric surgeon and the medical director of MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in Fountain Valley, California. BMI can be used together with other measures of health including body composition, weight-related complications, and functional status, to help guide a weight loss goal.
Obesity is further classified as:
Class 1 obesity: BMI of 30 to 34.9 kg/m2
Class 2 obesity: BMI of 35 to 39.9 kg/m2
Class 3 obesity: BMI of 40 kg/m2 or higher
Generally speaking, the higher the obesity class, the greater the risk for associated health problems. The obesity class also helps determine what treatment is most appropriate.
Obesity in Children
For children and teenagers, doctors use a percentile system rather than BMI alone to diagnose obesity. If your child’s BMI is higher than that of 95 percent of other children of the same sex and age, that’s considered obesity.
Parents of children who are developing obesity or its complications may not recognize the warning signs, says Sheila Perez-Colon, MD, a pediatric endocrinologist who practices in San Juan and Dorado, Puerto Rico. “You see a progressive increase in weight gain, and when plotted in the growth chart, the BMI is increased. You may see what we call central obesity, which is when fat accumulates around the abdominal area. You may also see darkening around the neck known as acanthosis nigricans, which is associated with insulin resistance and an increased risk for diabetes.”
The following conditions and factors can put you at higher risk of obesity:
Behavior Not getting regular physical activity and having unhealthy eating habits can contribute to obesity risk.
Community Environment Living under conditions that do not enable and encourage safe movement and access to healthy food can increase a person’s chances of developing obesity. Childhood adversity, such as abuse or the death of a parent, is also associated with a higher risk for obesity in adulthood.
Genetics Scientists have found many genes that are responsible for appetite and metabolism, and some rare genetic disorders can cause obesity directly.
Drugs Some medications can cause weight gain, such as steroids, certain antidepressants, and medications used for treatment of chronic pain and mental health conditions.
Race and Ethnicity Being Black or Hispanic is associated with higher risk for obesity compared with white or Asian people.
Socioeconomic Status People who don't have a college degree may have a higher risk of obesity than people who do. Among men, middle-income status puts you at higher risk for obesity compared with higher income; among women, middle or lower income puts you at higher risk compared with higher income.
For adults, obesity is diagnosed using the aforementioned BMI ranges. For kids, doctors use a BMI percentile scale instead.
Depending on the physician, they may also take other factors such as body composition into account before making a final diagnosis, says Dr. Ali.
Prognosis of Obesity
Being overweight (BMI of 25 to 29.9 kg/m2) will not necessarily take years off your life, but having class 3 obesity (BMI ≥40 kg/m2) may. Researchers at the National Cancer Institute analyzed 20 studies involving more than 300,000 participants for a report published in 2014. They found that class 3 obesity was associated with an estimated shorter life of about 6.5 to 13.7 years compared with people who have a healthy weight. People with class 3 obesity who had even higher BMIs had even shorter life spans.
Obesity increases the risk of health complications including high blood pressure, high cholesterol, type 2 diabetes, heart disease, stroke, gallbladder disease, osteoarthritis, sleep apnea, and serious COVID-19 disease. The shortened lifespan for people with class 3 obesity is likely due to the weight-related complications.
For people with any class of obesity, treatment starts with daily changes to how you eat and live. “We always encourage patients to try to diet and exercise first because that is the most safe method for weight loss,” says Ali. Be sure to speak with your doctor and potentially a registered dietitian-nutritionist (RDN) before embarking on any diet or exercise plan, to ensure you are making changes that are safe and appropriate for you.
Diet
There are a variety of diets that may help support your efforts to lose weight and maintain weight loss, but U.S. government guidelines say that whichever you choose should do the following:
Emphasize nutrient-dense foods and beverages, which include fruits, vegetables, whole grains, fat-free or low-fat milk and milk products, fortified soy beverages, lean meats, poultry, fish, beans, eggs, vegetable oils, nuts, and seeds (with a greater emphasis on beans and peas, soy products, nuts, and seeds if you are vegetarian).
Stay within your daily calorie needs.
Limit intake of added sugars, saturated fats, and sodium, specifically:
Added sugars: less than 10 percent of calories per day
Saturated fat: less than 10 percent of calories per day
Sodium: less than 2,300 milligrams per day
Limit the consumption of alcoholic beverages to two drinks or less per day for men and one drink or less per day for women. Any amount of alcohol consumption comes with health risks, so it’s best not to drink at all.
Calorie needs vary by age, sex, height, weight, activity level, and whether you are trying to lose, gain, or maintain weight. For adult women the recommended range is 1,600 to 2,400 calories per day, and for adult men the range is 2,000 to 3,000 calories per day. For children ages 2 to 8 the range is from 1,000 to 2,000, and for older children and adolescents the range is 1,400 to 3,200. Your primary care doctor and an RDN, among other specialists, can help provide more individualized advice, especially in cases where you are looking to manage other underlying health conditions.
Exercise
According to federal physical activity guidelines for Americans, aim for the following level of physical activity, even if you are not trying to lose weight:
For adults:
Do at least 150 minutes to 300 minutes a week of moderate-intensity exercise, like walking briskly, line dancing, yoga, or playing doubles tennis.
Alternatively, do 75 minutes to 150 minutes a week of vigorous-intensity aerobic physical activity, such as jogging, running, cycling more than 10 miles an hour, or high-intensity interval training (HIIT).
Older adults with chronic conditions should do as much activity as their health allows.
For youths:
Preschool-age children should remain active throughout the day and be encouraged to play or otherwise move their bodies.
Children 6 to 17 years old should do an hour or more of moderate to vigorous physical activity daily.
“In the National Weight Control Registry over 90 percent of those who maintained long-term weight loss engaged in at least 60 minutes per day of physical activity,” notes Jamie Kane, MD, director of the Center for Weight Management at Northwell Health in Long Island, New York.
Meanwhile, if you are at the beginning of your weight loss journey and have a lot of weight to lose, be patient with yourself, Dr. Kane advises. “Patients with obesity beginning an exercise program must think of the long term. In the end they will want to be engaged in regular physical activity, but it takes time to build up strength, endurance, and flexibility. Be persistent, but safe.”
If your doctor has recommended lowering your BMI to move out of the obesity category, there are additional treatment options to explore along with help from your healthcare team.
Medication Options
If diet and exercise alone aren’t enough to get you to a healthy weight, and your BMI is 30 kg/m2 or more (or your BMI is 27 to 29.9 kg/m2 and you have a complication such as type 2 diabetes or high blood pressure), your doctor may prescribe medication to add to your regimen.
Among the more common medications that are approved by the U.S. Food and Drug Administration (FDA) for long-term use are:
orlistat (Xenical), which is available at a lower dose without a prescription under the name Alli
These medications are approved for children age 12 and older:
orlistat (Xenical)
phentermine-topiramate (Qsymia)
liraglutide (Saxenda)
semaglutide (Wegovy)
setmelanotide (IMCIVREE) can be given to children as young as 6 who have rare genetic mutations that cause obesity.
Weight loss medications can be pricey, and they’re typically not covered by health insurance, which makes them inaccessible to many people.
And Kane cautions, “[Some] obesity medications at higher doses can cause more rapid weight loss, but one typically sees weight regain without continuing them indefinitely.”
Surgery Options
Another possible option for people with obesity who have not been able to lose weight through diet, exercise, and medication is weight loss surgery.
Qualifications for bariatric surgery include having a BMI of 40 kg/m2 or greater or having a BMI of 35 to 39.9 kg/m2 along with obesity-related complications such as type 2 diabetes, high blood pressure, heart disease, osteoarthritis, metabolic dysfunction–associated steatotic liver disease (formerly known as nonalcoholic fatty liver disease), sleep apnea, or other breathing problems. Candidates usually undergo thorough psychological counseling and a medically supervised diet before surgery.
Weight loss surgery leaves a patient with a stomach that can hold far less food than before, and most options also restrict the body’s ability to absorb calories and nutrients. The procedures are typically minimally invasive laparoscopic surgeries. Surgery types include:
Laparoscopic Sleeve Gastrectomy (Sleeve) About 80 percent of the stomach is surgically removed from the body, leaving a banana-shaped pouch.
Roux-en-Y Gastric Bypass (Gastric Bypass) The stomach is divided into two sections and only one small part of it holds the food you eat. The lower part of the small intestine is attached directly to that small stomach pouch, so food bypasses most of the stomach and the upper part of the small intestine.
Combination Procedures Biliopancreatic diversion with duodenal switch (BPD/DS) and single anastomosis duodeno-ileal bypass with sleeve gastrectomy (SADI-S) are less-common surgeries that combine a gastric sleeve procedure with a bypass of some portion of the small intestine.
In addition to reducing the amount of food you can eat, these surgeries also cause changes to gut hormones that suppress hunger, make you feel full when you eat, and reduce high blood sugar.
While results vary based on the procedure, people who have bariatric surgery lose on average 60 to 70 percent of their excess weight (20 to 30 percent of starting body weight), and are better able to maintain weight loss in the long term than people who only change their eating and exercise habits.
Medical Devices
For people who have a BMI of 30 to 39.9 kg/m2, with or without other chronic health conditions, a less-invasive procedure using a device called an intragastric balloon is an option. The balloon is placed into the stomach endoscopically or through a capsule that is attached to a catheter and swallowed. It is then filled with gas or saline and sealed to take up space in the stomach and delay the exit of food from the stomach into the small intestine. The balloon is removed at a later date. Typical weight loss is about 10 to 15 percent of your body weight during the six months after balloon placement.
Obesity is associated with various other health conditions.
Complications of Obesity in Adults
In adults, excessive body fat is linked to the following:
