What Is Obesity, Exactly?

Obesity is one of the biggest public health issues facing the world. According to the World Health Organization, the rate of obesity across the globe has tripled since 1975.

In the United States, 40 percent of adults over the age of 20 now have obesity, and that number climbs to 72 percent when you include people who have overweight. That's almost three-quarters of the entire population.

The American Medical Association officially recognized obesity as a disease in 2013. "Obesity is truly a disease state which involves an excess amount of body fat," says Natasha Bhuyan, MD, a practicing family physician in Phoenix. "It [also] increases your risk of other diseases."

Obesity is usually defined by numbers, namely body mass index (BMI) and waist circumference. BMI is your weight in kilograms divided by the square of your height in meters. It's an imperfect measure of obesity, says Dr. Bhuyan, but it's the best we have right now. And it's easy to calculate (you can calculate your BMI here).

Doctors also sometimes measure your waist size to determine if you're at risk for any medical conditions.

"A waist circumference over 35 inches in women or 40 inches in men is abnormal," says Caroline M. Apovian, MD, professor of medicine and pediatrics at Boston University School of Medicine. "It means there's too much visceral body fat [fat around your belly] and it's a pretty good measure of fat where it's not supposed to be.

Many different factors affect whether a person has obesity. It's not just how much you eat and how much you exercise.

"We have moved away from the old adage of calories in and calories out," says Dr. Bhuyan. "You also have to think of hormones and genetics and a range of environmental and social factors."

Here are some of the known risk factors for and causes of obesity:

About one-third of a person's BMI is due completely to genetics, says Dr. Apovian, who is also director of nutrition and weight management at Boston Medical Center.

"If both of your parents have obesity, there's an 80 percent chance you will have obesity," she adds.

Certain genetic syndromes — when genes are somehow altered in a person — are also linked with obesity, per the National Heart, Lung, and Blood Institute (NHLBI). These include Prader-Willi syndrome, Bardet-Biedl syndrome, Alstrom syndrome and Cohen syndrome.

Problems with hormones involved in eating, metabolism and satiety (feeling full) can also contribute to weight gain.

One example is hypothyroidism (low levels of thyroid hormone), which slows down your metabolism. Another is Cushing's syndrome, which is when you have too many circulating stress hormones, per the NHLBI. People with polycystic ovary syndrome (PCOS) are also more likely to have overweight or obesity.

Certain drugs can cause weight gain, including some antidepressants, antipsychotics and medications to treat epilepsy, according to Endocrine Society practice guidelines published in the ​Journal of Clinical Endocrinology & Metabolism​.

If you think this is happening to you, check with your doctor to see if there are alternatives, but don't stop taking your medications on your own.

You're more likely to gain weight as you get older, notes the NHLBI. That said, many young people now have obesity. According to the CDC, 18.5% of kids ages 2 to 19 have the condition.

For many people, unhealthy eating, being sedentary, not getting enough sleep and stress (which can affect hormones) can contribute to weight gain, says the NHLBI.

The NHLBI says this encompasses a variety of factors, such as being poor or being exposed to chemicals. It could also mean living in a "food desert," where healthy food just isn't easily available, says Dr. Bhuyan, or in neighborhoods where it's not safe to take walks.

Black people have the highest obesity rates, followed by Latinx people, then white people and, finally, Asian people. More Black or Latinx people assigned female at birth have obesity than Black or Latinx people assigned male at birth in the U.S.

Some people eat to bury feelings (aka emotional eating), perhaps because of childhood trauma, says Dr. Apovian. Other emotional issues could also come into play.

The main symptom of obesity is extra body fat, verified by BMI and waist circumference measurements.

Obesity can contribute to several health complications, some of them life-threatening. Between 300,000 and 350,000 deaths a year are from obesity and obesity-related issues, with higher mortality rates for people with higher BMIs, says Dr. Apovian, who also chaired the task force crafting obesity guidelines for the Endocrine Society in 2015. That makes it the leading cause of preventable death after tobacco, according to the University of Southern California.

This condition is closely related to obesity. According to a November 2017 article in ​Diabetes Spectrum​, 90 percent of people with type 2 diabetes have a BMI of 25 or higher.

Extra fat also leads to insulin resistance, which is a major risk factor for diabetes and heart disease.

"The prevalence of type 2 diabetes has increased over the past 50 years along the lines of obesity," says Dr. Apovian. "It's clear that we could do a lot of eradication of type 2 diabetes if we cured obesity."

Uncontrolled diabetes can lead to heart disease and stroke. Extra fat can also raise blood pressure and LDL ("bad") cholesterol levels while lowering HDL ("good") cholesterol levels, per the University of Pennsylvania Medicine. Cholesterol and blood pressure are key risk factors for heart disease.

About half of people with sleep apnea have overweight, per the National Sleep Foundation. Sleep apnea is when your airways become blocked during the night, causing you to repeatedly start and stop breathing.

Having overweight or obesity causes fat to build up in your neck area, which obstructs your airways.

Experts believe extra fat around your belly increases the pressure on your stomach, making it easier for acid to splash up into the esophagus. Not only does this cause burning and irritation in the moment, it can also raise the risk for ulcers and for Barrett's Esophagus, which is the leading risk factor for esophageal cancer, according to the Obesity Action Network.

More weight means your joints have to work harder to keep you up and around. According to the Arthritis Foundation, 10 pounds of extra weight actually adds 15 to 50 pounds of pressure to your knees. Rates of osteoarthritis in the U.S. have soared in tandem with higher levels of obesity.

Having obesity raises the risk of certain types of cancer, including kidney, ovarian, endometrial, pancreatic, colorectal, liver and breast cancer, per the National Cancer Institute. This may be because obesity increases inflammation, which can contribute to cancer or because fat tissue produces extra estrogen. Estrogen has been linked to various cancers.

In experiments in her own lab, Dr. Apovian has discovered that 65 percent or more of people with obesity have fat in their liver. "That is never good," she says.

Twenty percent of those people will end up with steatosis and about 20 percent of those will develop cirrhosis, possibly requiring a liver transplant, she says.

"It used to be that the most prevalent [reason] for liver transplants was alcoholism," says Dr. Apovian. "Now it's obesity."

Metabolic syndrome is a collection of conditions (including hypertension, high blood sugar, abdominal fat and abnormal cholesterol and triglyceride levels) that increase the risk of heart disease, stroke and type 2 diabetes, according to the Mayo Clinic.

This includes low self-esteem and moodiness as well as body image issues, according to a September 2016 paper in ​Endocrinology and Metabolism Clinics of North America​.

"There's no cookie-cutter formula to treat obesity," says Dr. Bhuyan. "There are a lot of actors we should talk about. It really depends on the person."

"Lifestyle is the core of everything," says Dr. Apovian.

Even if you're receiving other treatments for obesity, such as medications or surgery (more on those below), the Endocrine Society recommends eating healthful foods, exercising more, getting good sleep, cutting stress and watching how much alcohol you drink.

Therapies that address mental health can also help, Dr. Bhuyan says.

Some medications help you shed pounds by hitting the satiety centers of your brain and making you feel full with less food, says Dr. Apovian. These include the diabetes drug semaglutide (Trulicity, Ozempic).

"This drug is pushing the envelope of weight loss," says Dr. Apovian. "It's an injection once a week."

Another drug, orlistat, reduces fat absorption by the intestines, according to the Endocrine Society.

Bariatric surgery can facilitate weight loss through several different pathways, says Dr. Apovian.

"After surgery, your hunger hormones go down and satiety goes up, and that's why it works so well."

With an average weight loss of 33 percent, Roux-En-Y (gastric bypass) surgery seems to be the most effective.

"We have 20 years worth of data on Roux-En-Y and we know it works and it works long term," says Dr. Apovian.

Gastric sleeve surgery (gastrectomy) removes a portion of your stomach, while a gastric band (lap band) mechanically reduces the size of your stomach by placing a band around the top of the organ.

"Surgery is certainly an option in certain limited cases, for people who have failed multiple interventions and are already having complications from obesity and usually with a BMI over 40," Dr. Bhuyan says.