Let's Talk About the Causes of Obesity

Learn about the varied and complex factors that make a person vulnerable to this condition as we get to the roots of the matter.

by Leslie Goldman Health Writer

What triggers obesity in the first place? Is there anything we can do to prevent it? Here, we delve into the many factors that can predispose a person to this condition—from genetic codes to zip codes. You might be surprised to see what causes this incredibly common health challenge...and what decidedly does not. (Spoiler alert: It has nothing to do with a lack of willpower.)

Causes of Obesity

Our Pro Panel

We went to some of the nation's top experts on obesity to bring you the most up-to-date information possible.

Sharon Zarabi, RD headshot.

Sharon Zarabi, R.D.

Registered Dietitian and Bariatric Program Director

Lenox Hill Hospital

New York, NY

Robert F. Kushner, M.D. headshot.

Robert F. Kushner, M.D.

Medical Director

Center for Lifestyle Medicine at Northwestern Medicine; Author of "Six Factors to Fit: Weight Loss That Works for You!"

Chicago, IL

Sara N. Bleich, Ph.D.

Sara N. Bleich, Ph.D.

Professor of Public Health Policy

Harvard Chan School of Public Health

Boston, MA

Obesity Causes
Frequently Asked Questions
What are five causes of obesity?

The causes of obesity range from those steeped in nutrition, exercise and genetics to living in an obesogenic environment. Five common causes include a surplus of calories (taking in more calories a day than you burn); calorie quality (ultra-processed foods and beverages like chips, candy and soda promote overeating); having a family history of obesity; poor sleep (sleep helps regulate hunger and satiety hormones); and taking certain obesity-promoting medications (antidepressants, anti-anxiety drugs, and mood-stabilizing drugs, for example, activate brain receptors that increase appetite or decrease the number of calories burned).

What foods cause obesity?

No single food causes obesity, but certain foods are known to promote obesity when consumed in large quantities, particularly in people who are prone to obesity or who live in obesogenic environments. For instance, ultra-processed foods and beverages have been engineered using fat, sugar, salt, and other addictive ingredients to light up the reward centers of the brain, driving us to continue eating. But because there’s no protein or fiber to fill you up, your brain tells you to keep eating and eating.

How does fast food cause obesity?

Fat food restaurants are part of our obesogenic (or obesity-promoting) environment. Not only do the heavily processed foods served at fast food restaurants drive you to continue munching, but their ballooning serving sizes don’t exactly encourage moderation. In the 1950s, a typical fast food beverage was seven ounces. Today it’s 42 ounces. Hamburgers and fry portions are three times bigger than they ones were, as well. When we’re given a bigger portion of food, we tend to eat more of it.

How can sugar cause obesity?

While sugar doesn't necessarily cause obesity, especially not when consumed in the form of an occasional treat, it is a risk factor for obesity, and mounting evidence strongly suggests that consuming a diet high in added sugars,found in candy, baked goods, and sweetened sodas and juices, is known to promote weight gain and obesity. While these sweet crystals may taste great, they provide calories without nutrition, a.k.a. "empty calories." (This does not apply to foods that contain naturally occuring sugars, like fruit. Those calories are far from empty, as they also provide vitamins, fiber, antioxidants, and more.) Foods high in added sugar — like cookies and ice cream—tend to also be high in calories, and excess calories are tied with obesity. Because these foods taste delicious and aren’t terribly filling, they send a message to the brain that says, "Keep eating." That, in turn, can promote obesity. Additionally, frequent sugar intake raises blood sugar levels, which can lead to a condition called insulin resistance which promotes fat storage.

What Exactly Is Obesity?

Contrary to popular belief and comedy routines, obesity isn’t a sign of laziness or lack of willpower. Just like type 1 diabetes or multiple sclerosis, obesity is a diagnosable disease, and shouldn’t be stigmatized or joked about. This medical condition occurs when your body accumulates and stores excessive amounts of fat.

Obesity is determined by calculating Body Mass Index (BMI), a rough estimate of whether a person’s weight is healthy based on their height. While BMI isn’t a perfect measure—it doesn’t distinguish how much muscle versus fat a person is carrying, therefore, a fit person and a sedentary person could have the same BMI—but it is a good gauge of whether one’s body fat is in the healthy range.

How to measure BMI? Plug your numbers into this BMI calculator, or use this simple mathematical equation:

Divide your weight (in pounds) by your height (in centimeters) squared

Multiply that number by 703

= your BMI

A BMI of 25.0 to 29.9 is considered overweight by most doctors
A BMI of 30 or more is considered obese

Those who hit the number 30 and above are in plenty of company: 40% of the U.S. population is living with obesity and that number is slated to soar to nearly half of the American population by 2030, according to research in the New England Journal of Medicine. If that sounds alarming, it is: Obesity contributes to eight of the 10 leading causes of death in the United States.

Get the Full Scoop on Obesity

What Causes Obesity?

There is no single, isolated factor that determines whether a person will develop obesity. Rather, it’s a result of a complex interplay of biological, psychological and environmental triggers.

Despite the prevalence, society tends to depict people with obesity as being lazy or out-of-control eaters. Not only is that inaccurate, it does nothing to help those living with the condition, who are at risk for a host of other chronic diseases, including heart disease, cancer, and Type 2 diabetes.

When it comes to what causes obesity, poor nutrition and lack of exercise are huge factors, but researchers are constantly discovering evidence behind other key contributing factors—from poor sleep habits to often-prescribed medications to the ads that pop up on social media. Let’s take a look at some of these variables and how they may be fueling the obesity epidemic.

A Calorie Surplus

If you were explaining obesity to a roomful of schoolkids, you’d likely say that obesity is a result of eating and drinking more calories than you burn off through exercise and daily movement. Consuming more calories than you burn, over time, can lead to weight gain and possibly obesity.

Snacking is one culprit for the surplus as those calories tend to be in addition to meals, meaning we don’t compensate for that bag of chips by eating less at dinner.

Another contributor? Dining out. More than half of our calories come from outside the home. These dishes tend to be higher in fat and sugar than home-cooked food. If that restaurant meal involves a cocktail or glass of wine, it may also usher in more calories.

While alcoholic drinks have been billed as “social lubricants” (i.e. they help you open up and talk more), they’re also potent appetite lubricants, rendering you less mindful of what you’re eating. We also tend to eat more in general—appetizers, shared plates, bread baskets, desserts—when dining with friends.

The number of calories one needs to thrive without putting on pounds varies depending on age, gender, height, activity level, and more, but general guidelines for weight management suggest:

  • 1,600 and 2,400 calories per day is healthy for the average woman

  • 2,000 to 3,000 calories per day is healthy for the average man

The upper estimates typically apply to highly active folks in their 20s and 30s, while sedentary adults in their 60s and 70s should stick with the lower range to avoid gaining weight.

As we mentioned, these calories need to be burned off to avoid being stored in the body. Over the past 50 years, our society has become increasingly sedentary as we’ve moved away from a highly physical lifestyle to one which usually requires us to sit behind a desk all day.

The result: We burn at least 100 calories a day less now than we did in the 1960s. We also walk far less (thanks to cars and public transportation). Without cows to milk and miles to trek to the general store, carving out time to exercise is essential.

Empty Calories

Not all calories are created equal. A specific category of foods called ultra-processed foods may shoulder a large portion of the blame, research suggests. These include things like:

  • Chips

  • Candy

  • Cakes

  • Ice cream

  • White bread

Ultra-processed foods have been scientifically engineered using fat, salt, and other addictive ingredients to light up the reward centers of the brain, making us want to keep eating more. These foods have been processed so much that they’ve lost any vitamins and nutrients that were present in the initial form (potato chips versus a potato; white bread versus an actual wheat plant), so the body doesn’t recognize it as real food.

When you eat fruits, veggies, protein, and dairy, your brain recognizes the nutrients and signals you to stop eating after a reasonable amount. Ultra-processed foods, in contrast, cause something called hyperphagia, meaning they prompt you to keep eating more and more because there’s no protein or fiber to fill you up.

In a recent small but powerful study by the National Institutes of Health, 20 healthy adults were randomly assigned to eat meals made up of ultra-processed foods (sample breakfast: bagel with cream cheese; turkey bacon) or minimally processed foods (oatmeal with bananas, walnuts, and skim milk) for two weeks. All meals contained the same amounts of calories, fat, sugar, carbs, and fiber, and subjects could eat as much or as little as they wanted. Those in the ultra-processed group ate about 500 calories more per day, resulting in an average gain of two pounds. The other group lost about two pounds each, demonstrating that ultra-processed foods cause people to eat too many calories (even at later sittings) and gain weight.

Bodies also have to work harder to digest foods that are rich in fiber and protein compared to digesting refined carbohydrates. If you were to eat 200 calories of beans, for example, you would burn off a percent of those calories simply through the act of digestion, whereas barely any work is required to digest 200 calories worth of chips.


Genetics can contribute anywhere from 40 to 70 percent of obesity cases, according to the Obesity Medicine Association, and there are more than 200 genes known to be strongly associated with the disease. In most cases, a person is born with multiple genes that increase—but don’t necessarily guarantee—the likelihood of obesity.

Meaning, a person’s weight isn’t completely up to fate; genetics may contribute to them carrying extra pounds. Studies involving identical twins raised in different locations demonstrate this, as one twin may end up obese while the other doesn’t, despite sharing the same DNA.

This leads us to the next cause: Experts believe that the interaction of genes with the environment contributes to a person’s chances of carrying an unhealthy amount of weight.

Obesogenic Environments

We now know that the environment—the social, economic, cultural, and even physical factors that surround us every day—has a surprisingly bold influence on what foods we eat, how much we consume, and how quickly or inefficiently we burn it off. What environments are likely to be obesogenic, or obesity-promoting? Features of an obesogenic environment include:

  • Ballooning Portion Sizes: A typical fast food beverage in the 1950s was seven ounces. Today? It’s 42 ounces. Hamburgers and fry portions have tripled in size. Research shows that when we’re given a bigger portion of food, we tend to eat more of it.

  • Targeted Marketing: If you shop online for a certain less-than-healthy snack, targeted ads for similar products will start popping up on your social media feed. In another form of targeted marketing, companies that make fast food, soda, and other processed foods are known to concentrate their marketing efforts in Black and Hispanic communities. Similarly, ads for candy and sugar-sweetened beverages tend to cluster near public schools compared with private schools and these marketing efforts contribute to sociocultural differences.

  • Socioeconomic Factors: Latinx and Black communities are also more likely to be situated in food deserts, areas with reduced access to affordable fresh produce and other healthy foods. In food deserts, convenience stores reign, filled with ultra-processed foods and little in the way of fruits and veggies.

    Food deserts are most often “found in black and brown communities and low-income areas (where many people don’t have cars),” according to the Food Empowerment Project. If the nearest supermarket is miles away and you lack transportation, but the 24-hour convenience store is on your nearest corner, that’s likely where you will shop. Even if you do have access to grocery stores, better-for-you foods cost more to buy.

    Consider this: Between 1989 and 2005, the overall price of fruits and vegetables in the US increased by nearly 75%, while the price of fatty foods fell by more than 26%.


Several widely-prescribed prescription medications are known to cause weight gain and, in some cases, obesity. This type of obesity is called iatrogenic obesity. Here are some of the more common culprits:


You may recognize names of steroids (also called corticosteroids) like prednisone, cortisone, and dexamethasone. These powerful anti-inflammatory drugs are used to treat certain conditions in which the body’s own immune defense system isn’t working well, such as Crohn's disease, lupus, rheumatoid arthritis, or asthma.

Steroids are known for causing weight gain. They do this by ramping up appetite, promoting fluid retention, increasing the storage of body fat and even redistributing your body fat so it settles around the belly, which is one of the more dangerous places for it to accumulate, as belly fat is linked with diabetes and heart disease​.

Medications for Depression, Anxiety, and Bipolar Disorder

Antidepressants, anti-anxiety drugs, and mood-stabilizing drugs act in the brain either by increasing appetite or decreasing the number of calories burned. On the other hand, anti-obesity medications such as Qsymia and Saxenda, decrease appetite. Popular mental health meds that can have this effect include:

  • Selective serotonin reuptake inhibitors (SSRIs) like Prozac, Zoloft, Paxil, and Celexa

  • Anti-depressants such as Amitriptyline, Nortriptyline, Imipramine, and Trazodone

  • Monoamine oxidase inhibitors (MAOIs) like Lithium and various benzodiazepines

  • Antipsychotics such as Clozapine, Risperidone, Quetiapine, and Haloperidol

Medications for Diabetes

Diabetics are unable to properly utilize glucose, or sugar, our main source of energy. Glucose levels are regulated by a hormone called insulin.

In people with Type 2 diabetes, the body is resistant to insulin thanks in part to a diet consistently high in starches (bread, bagels, cereals, juice, pasta, sweets), leading to chronically elevated blood sugar levels.

The classic drug treatment for diabetes is supplemental insulin, which affects other hormones in a way that promotes fat storage. (It’s worth noting that a high percentage, though not all, of people with Type 2 diabetes are already overweight, so adding insulin therapy isn’t the sole culprit behind this type of obesity.)

That said, a recent paradigm shift in diabetes treatment has many people trying a new class of medications called GLP-1 receptor agonists, which actually promote weight loss. They keep you fuller, longer, helping to prevent overeating.

Other Common Medications

All medications come with side effects and for many, these include weight gain. Other drugs that may promote weight-gain include:

Stress Levels

Stress affects people in different ways and a growing body of literature suggests that chronic stress may impact the metabolism in toxic ways, by ramping up the production of a stress hormone called cortisol and promoting fat accumulation.

Cortisol is a steroid hormone, so this makes sense considering that weight gain is a common side effect in people taking steroid medications. According to a 2017 study published in the journal Obesity, elevated cortisol levels and being overweight are closely linked. It’s not always what you’re eating, but what’s eating you.

Poor Sleep

Getting enough zzz helps to ensure a humming metabolism and well-balanced hunger hormones, while getting too little sleep leads to the increase in ghrelin, a hormone that promotes hunger. Simply by virtue of staying up later, we have more time to eat, which can also contribute to weight gain.

In the Nurses’ Health Study, which followed 68,000 middle-age American women for up to 16 years, women who slept for five hours or less were 15% more likely to become obese compared with those who slept seven hours a night.

What Are the First Steps Toward Treatment?

While the causes of obesity are complex and interconnected, there is hope. Losing just 3% to 5% of weight—and maintaining the loss—can spur improvements in health. However, because obesity is a complicated disease, treating it is also complex.

While a very small percentage of people will hit their weight-loss goals through lifestyle tweaks like eating less and moving more, others will require nutritional interventions, medication or even surgery. The first step toward healing? Working with your doctor to assemble a weight-loss team of specialists you can trust for the journey ahead.

Learn More About Obesity Treatment Options
Leslie Goldman
Meet Our Writer
Leslie Goldman

Leslie Goldman is a health and wellness writer who regularly contributes feature stories and essays to O: The Oprah Magazine, Women’s Health, Parents, Better Homes and Gardens, and more. She has authored and contributed to several books, including Locker Room Diaries: The Naked Truth About Women, Body Image, and Re-Imagining the “Perfect” Body and Brave Girls: Raising Young Women with Passion and Purpose to Become Powerful Leaders. Leslie also speaks at colleges and universities on the topics of body image, media literacy and female empowerment.