By Angela Ryan Lee, MD 

 Medically reviewed by Elizabeth Barnes, RDN

While historically attributed mainly to excess caloric intake, obesity is a complex condition with many contributing factors. About 3 in 4 adults in the United States have obesity.¹ Causes include hormones related to appetite and metabolism, which can be altered by environmental and medical conditions, genetics, and medications. Even the body's collection of microorganisms, known as the microbiome, plays a role.

Read on to learn more about obesity and its causes.

SDI Productions / Getty Images

Defining Obesity

The medical definition of obesity is a body mass index (BMI) of 30 kg/m2 or greater. BMI is a calculation of weight (in kilograms) divided by height (in meters squared) to gain a sense of weight per height. However, using BMI can be problematic, since body weight doesn't directly correlate with body fat.² It's important to understand this when calculating and interpreting BMI.

For example, a very fit and muscular person will have a higher BMI because muscle weighs more than fat, and they won't necessarily have obesity or be at a high risk of obesity-related complications.

There are other ways to measure body fat, such as dual-energy X-ray absorption (DEXA), underwater weighing, and bioelectrical impedance. However, these methods aren't as widely available as BMI measurements. Measuring skin fold thickness with calipers is an alternative method, but it can be difficult to standardize.² BMI is a simpler and easily measured way to screen for obesity, but it is still a problematic measurement that may need to be re-interpreted in the proper context for individuals.

Lifestyle Related Causes of Obesity

Obesity is usually blamed on behaviors of taking in too many calories while not burning enough calories. While there is truth that not burning calories you eat over time leads to weight gain, this simplistic explanation doesn't account for the role of biology and environment. Not everyone absorbs, stores, and expends energy the exact same way, and there are variations in hormones related to metabolism in individuals.

Researchers are still discovering the exact causes and contributors to obesity. However, there is no denying that certain lifestyle habits have a great impact on obesity. The imbalance of food intake and activity is believed to be one of the biggest contributors to weight gain.³

Diet

Diet is a major contributor to obesity. Eating high-calorie foods and a diet high in saturated fats and refined sugars, especially processed foods has been linked to weight gain and obesity. A study looking at worldwide populations found that increased soda consumption in adolescents was associated with a higher prevalence of obesity.⁴

It's not hard to see why since one 12-ounce can of soda contains about 40 grams (almost 10 teaspoons!) of sugar. Processed snack foods are loaded with a combination of salt and saturated fats or refined sugars, a high-calorie combination that is suggested to alter appetite signals and lead to addiction-like and withdrawal behaviors.⁵⁶

Activity

Another major contributor to obesity is a sedentary lifestyle. Spending more time sitting burns less calories and means less time is spent doing physical activity that would burn more calories. Asedentary lifestyle is linked to obesity and other chronic conditions like diabetes, heart disease, and high blood pressure.⁷

On the other hand, regular physical activity, such as moderate-intensity exercises like brisk walking, jogging, swimming, or aerobic sports has many health benefits, including helping maintain a target weight.⁸

Sleep

The quality and duration of sleep also appears to play a role in obesity. Sleeping less than 7 hours per night is linked to higher body weight and obesity, which researchers suggest may be related to the effect sleep deprivation has on appetite-related hormones.⁹ Furthermore, evidence suggests an association of weight gain with circadian misalignment, which is the body being awake during times when the body's sleep/wake cycle, such as in people doing night-shift work.¹⁰

Stress

Stress levels over time can also impact weight gain and contribute to obesity. Chronic stress, which may be due to any number of causes, is associated with obesity.¹¹ Stress increases the amount of stress hormone (cortisol) in the body, which, over time, can lead to weight gain.¹ Furthermore, the way people cope with stress can also affect their weight in some cases. For example, binge-eating (or "stress-eating") contributes to excess calories and potential weight gain.¹²

Genetic Causes of Obesity

If your parents are obese, you're more likely to have it, too. This is partly related to lifestyle factors that tend to be similar to family members. However, genetics also play a role in obesity. Researchers have discovered many genetic variations that can increase the susceptibility to obesity. These include mutations in appetite regulation and energy balance, such as proteins and hormones that help us feel hungry or full.¹³

Prader-Willi syndrome is a rare genetic condition that causes insatiable appetite in affected children. This leads to overeating and obesity.¹⁴ Bardet-Biedl syndrome is another rare genetic condition affecting the eyes that also causes obesity starting in childhood.¹⁵

Biologic and Medical Contributors to Obesity

Several medical conditions can contribute to excess body weight and obesity. Many of these are related to hormones involved in metabolism and appetite. And the gut microbiome is becoming increasingly recognized for its impact on obesity.

Hypothyroidism

Hypothyroidism is a condition where there are not enough thyroid hormones circulating in the blood. Thyroid hormones are involved in many body processes, and are important in weight, energy levels, growth of skin, hair and nails, and digestion, among other things.¹⁶

When your body doesn't have enough thyroid hormones, you can expect to have low energy levels and gain weight, which can lead to obesity. However, the American Thyroid Association notes that much of the weight gain in hypothyroidism is related to salt and water retention, with an expected weight gain of up to 10 pounds.¹⁷

Cushing's Syndrome

Cushing's syndrome is caused by too much stress hormone (cortisol) in the body. This can happen when taking steroid medications, or when the body produces too much of the hormone.¹⁸ Cortisol is involved in regulation of blood pressure, blood sugar. People with Cushing's syndrome often develop excess fat tissue in the abdomen and upper back.

Polycystic Ovarian Syndrome

Polycystic ovarian syndrome (PCOS) is a syndrome resulting in cysts on the ovaries as well as hormonal changes, such as insulin resistance and excess androgen hormones. There is a definite link between PCOS and obesity. Researchers think this relationship is due to genetic and hormonal factors.¹⁹

The Gut Microbiome

There are more bacterial cells than human cells in the human body, and the population of these microorganisms in the gastrointestinal system is called the gut microbiome.²⁰ Science has shown that the gut microbiome patterns are linked to obesity. The bacteria in the gut are affected by our own behaviors, medical conditions, and environment.²¹

Studies suggest that the typical Western diet with foods high in saturated fats and refined sugars may also promote obesity through the effects this diet has on the gut microbiome.²²

Social Drivers of Obesity

In addition to the biologic and behavioral causes of obesity discussed above, there are social drivers of obesity. While these social drivers can impact behavior, it's important to recognize that

The following are social factors related to increased risk of obesity:²³

• Income level

• Education levels

• Access to healthcare

• Living in certain geographic locations (South and Midwest) as well as specific urban and rural environments results in less access to fresh foods and recreational activities

• Food availability, such as higher number of fast food restaurants and food insecurity- not having the economic means to buy food. Food insecurity results in relying on lower-cost options, which are often more calorie-dense, with less access to fresh foods

• Neighborhood walkability

• Work environment, which can lead to increased sedentariness

Summary

Obesity is a common condition that has many causes spanning behavioral, biologic, and environmental. Diet and physical activity are major contributors, but stress and sleep also play a role. Biologic factors such as genetics and medical conditions affect appetite and satiety as well as energy balance and metabolism. Lastly, social and environmental factors play a role that can be beyond an individual's immediate control.