Lifestyle, genetics, medical and environmental

Lifestyle, genetics, medical and environmental

While historically primarily attributed to excess caloric intake, obesity is a complex condition with many contributing factors. About 3 in 4 adults in the United States are obese. 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.

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Definition of obesity

The medical definition of obesity is a body mass index (BMI) of 30 kg/m2 or more. BMI is a calculation of weight (in kilograms) divided by height (in meters squared) to get an idea of ​​weight for height. However, using BMI can be problematic since body weight does not directly correlate with body fat. It is 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 will not necessarily be obese or at high risk for 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 are not as widely available as BMI measurements. Measuring skinfold thickness with calipers is an alternative method, but can be difficult to standardize. BMI is a simpler and easier way to measure obesity, but it remains a problematic measurement that may need to be reinterpreted in the appropriate context for individuals.

Lifestyle-related causes of obesity

Obesity is generally attributed to behaviors of eating too many calories without burning enough calories. While it’s true that not burning the calories you consume over time leads to weight gain, this simplistic explanation doesn’t take into account the role of biology and the environment. Not everyone absorbs, stores and expends energy in exactly the 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, it cannot be denied that certain lifestyle habits have a great impact on obesity. An imbalance between food intake and activity is believed to be one of the biggest contributors to weight gain.


Diet is a major contributor to obesity. Consuming 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 that analyzed populations around the world found that higher soda consumption in adolescents was associated with a higher prevalence of obesity.

It’s not hard to see why, since a 12-ounce can of soda contains about 40 grams (almost 10 teaspoons!) of sugar. Processed snacks 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- and withdrawal-like behaviors.


Another important factor that contributes to obesity is a sedentary lifestyle. Spending more time sitting burns fewer calories and means less time is spent doing physical activity that would burn more calories. A sedentary lifestyle is linked to obesity and other chronic conditions such as diabetes, heart disease and high blood pressure.

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


The quality and duration of sleep also appear to influence obesity. Sleeping less than 7 hours a night is linked to higher body weight and obesity, which researchers suggest may be related to the effect that lack of sleep has on hormones related to appetite. Additionally, evidence suggests an association between weight gain and circadian misalignment, which is when the body is awake during times of the body’s sleep/wake cycle, such as in people who work the night shift.


Stress levels over time can also affect weight gain and contribute to obesity. Chronic stress, which can be due to various causes, is associated with obesity. Stress increases the amount of the stress hormone (cortisol) in the body, which, over time, can lead to weight gain. Additionally, 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 possible weight gain.

Genetic causes of obesity

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

Prader-Willi syndrome is a rare genetic disease that causes an insatiable appetite in affected children. This leads to overeating and obesity. Bardet-Biedl syndrome is another rare genetic disease that affects the eyes and also causes obesity since childhood.

Biological and medical contributors to obesity

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


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

When your body does not 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 expected weight gain of up to 10 pounds.

Cushing’s syndrome

Cushing’s syndrome is caused by too much of the stress hormone (cortisol) in the body. This can happen when steroid medications are taken or when the body produces too much of the hormone. Cortisol is involved in regulating blood pressure and blood sugar. People with Cushing’s syndrome often develop excess fatty tissue in the abdomen and upper back.

Polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a syndrome that causes cysts in 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 believe that this relationship is due to genetic and hormonal factors.

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. Science has shown that gut microbiome patterns are linked to obesity. Gut bacteria are affected by our own behaviors, medical conditions, and environment.

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.

Social drivers of obesity

In addition to the biological and behavioral causes of obesity discussed above, there are social factors that drive it. While these social drivers can affect behavior, it is important to recognize that

The following are social factors linked to an increased risk of obesity:

  • 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 food and recreational activities.
  • Food availability, such as an increased number of fast food restaurants, and food insecurity: not having the financial means to purchase food. Food insecurity results in reliance on lower-cost options, which are often higher in calories and with less access to fresh foods.
  • Neighborhood Pedestrian Accessibility
  • Work environment, which can lead to a greater sedentary lifestyle.


Obesity is a common condition that has many causes spanning behavior, biology, and the environment. Diet and physical activity are important contributors, but stress and sleep also play a role. Biological factors such as genetics and medical conditions affect appetite and satiety, as well as energy balance and metabolism. Finally, social and environmental factors play a role that may be beyond an individual’s immediate control.

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