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The #1 Reason People Get Obese, According to Science

There’s not one reason, per say, but there is one leading factor.
FACT CHECKED BY Emilia Paluszek
Overweight Asian woman wearing yellow orange dress relaxing in the park.

What is the number one reason people get obese, according to science?

Emily Rubin, RD, LDN, Director of Clinical Dietetics Celiac Center, Fatty Liver Center, Weight Management Center at Thomas Jefferson University, explains. "Two different patients come into my office," she says. "They live in different zip codes, are the same age and sex and eat a similar number of calories each day. Why does one patient weight 150 pounds and the other weigh 250 pounds? Obesity is a complex health issue—a disease resulting from a combination of factors, including behavioral, community environment, and genetics—all of which reflect your zip code. Where you live determines your risk for obesity. Behaviors are based on physical activity and dietary patterns in the community environment, like having access to grocery stores and safe areas to exercise. Tack on the genetics, the predisposition or family history for developing obesity and living in a specific zip code, will then determine how people respond to physical inactivity and intake of high-calorie foods."

 "There's not one simple reason people get obese," agrees Samantha Cassetty, MS, RD, NYC-based nutrition and wellness expert and co-author of Sugar Shock. As a weight management specialist, she's seen clients who struggle with obesity, which WHO defines "as abnormal or excessive fat accumulation that presents a risk to health. A body mass index (BMI) over 25 is considered overweight, and over 30 is obese." Adds WHO: "The issue has grown to epidemic proportions, with over 4 million people dying each year as a result of being overweight or obese."

"There are so many factors, including access to healthcare; availability of safe places to participate in physical activity; ability and time to prep and cook, and access to whole foods," says Cassetty. However, there is one leading factor, she says, that leads people to become obese.

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The Leading Factor is Our Food Environment

"A leading factor is that our food environment promotes heavily processed foods that get digested quickly and leave you hungry soon after eating," says Cassetty. "There's also evidence that heavily processed foods may disrupt signals to the brain that tell you when you're full, so in many ways, they contribute to overeating. On top of that, when you eat ultra-processed foods, your brain tells you they're really rewarding, so it reinforces your desire to want to continue to eat them. Fast food, chips, pizza, ice cream, and the like are all considered heavily processed foods. These foods contain refined grains and may be high in sugar, salt, or both. Added sugars and refined grains promote inflammation, which is an underlying factor in the development of many health problems, including obesity. So, the convenience and affordability of heavily processed foods are important factors in the development of obesity." (For her sources of this intel, see here, here, here and here.)

Melissa Mitri, MS, RDN, Owner of Melissa Mitri Nutrition, LLC, agrees. "Highly processed foods," she says when asked about the #1 cause of obesity. "Processed foods that come in a package are linked to excess weight gain. Research has shown that those who consume more processed foods tend to weigh more. This is because these foods are designed to be highly palatable and addicting. Processed foods are also not as filling, so they are easier to overeat." 

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Here's What to Eat Instead

So what to eat instead? "Choosing more whole foods that don't come in a package helps to support a healthy weight," says Mitri. Obese or not, challenge yourself to take one full day where you eat nothing processed—just whole grains, lean proteins and healthy fats, that come from the Earth. Then try it for two days, or three. For inspiration, try any of these 19 Weight Loss Foods That Really Work, Say Experts.

Alek Korab
Alek Korab is a Co-Founder and Managing Editor of the ETNT Health channel on Eat This, Not That! Read more