Lifestyle, genetics, medicine and environmental

Lifestyle, genetics, medicine and environmental

Although historically attributed primarily to excess caloric intake, obesity is a complex condition with many contributing factors. About 3 out of 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 greater. BMI is a calculation of weight (in kilograms) divided by height (in meters squared) to get a sense of weight per height. However, using BMI can be problematic, as body weight does not directly correlate with body fat. It is important to understand this when calculating and interpreting BMI.

For example, a very healthy and muscular person will have a higher BMI because muscle weighs more than fat and will not necessarily be obese or at high risk of obesity-related complications.

There are other ways to measure body fat, such as dual-energy x-ray absorptiometry (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 easily measured way to screen for obesity, but it is still a problematic measurement that may need to be reinterpreted in the appropriate context for individuals.

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Lifestyle-related causes of obesity

Obesity is usually blamed on behaviors of taking in too many calories while not burning enough calories. While it is true that not burning the calories you eat over time leads to weight gain, this simplistic explanation does not take into account the role of biology and environment. Not everyone absorbs, stores and expends energy in exactly the same way, and there are variations in the hormones associated with metabolism in individuals.

Researchers are still discovering the exact causes and factors that contribute to obesity. However, there is no doubt that certain lifestyle habits have a large impact on obesity. An imbalance of food intake and activity is believed to be one of the biggest factors contributing to weight gain.


Diet is a major contributor to obesity. Eating high-calorie foods and diets high in saturated fat and refined sugars, especially processed foods, have been linked to weight gain and obesity. A study looking at global populations found that increased soda consumption among teenagers 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 packed with a combination of salt and saturated fat or refined sugars, a high-calorie combination that is suggested to alter appetite signals and lead to addictive and withdrawal behaviors.


Another major factor contributing to obesity is a sedentary lifestyle. Spending more time sitting burns fewer calories and means less time for physical activity that will 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 exercise such as brisk walking, jogging, swimming or aerobics has many health benefits, including helping to maintain a weight goal.


The quality and duration of sleep also appears to play a role in obesity. Sleeping less than 7 hours a night is associated with higher body weight and obesity, which the researchers suggest may be related to the effect of sleep deprivation on appetite-related hormones. Additionally, evidence suggests an association of weight gain with circadian misalignment, which is the body being awake during the body’s sleep/wake cycle, such as in night shift workers.


Stress levels over time can also affect weight gain and contribute to obesity. Chronic stress, which can have many 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. In addition, how people deal with stress can also affect their weight in some cases. For example, overeating (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. 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 variants that can increase susceptibility to obesity. These include mutations in appetite regulation and energy balance, such as proteins and hormones that help us feel hungry or full.

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

Biological and medical contributors to obesity

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


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 for weight, energy levels, skin, hair and nail growth, and digestion, among other things.

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.

Cushing’s syndrome

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

Polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a syndrome that results in cysts on the ovaries as well as hormonal changes such as insulin resistance and excess androgen hormones. There is a clear 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 tract is called the gut microbiome. Science has shown that gut microbiome patterns are linked to obesity. The bacteria in the gut are affected by our own behaviors, medical conditions and environment.

Studies suggest that the typical Western diet of foods high in saturated fat 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 determinants of obesity. While these social drivers can influence behavior, it is important to recognize this

The following are social factors associated with an increased risk of obesity:

  • The level of income
  • Education levels
  • Access to health care
  • Living in certain geographic locations (South and Midwest) as well as certain urban and rural environments results in less access to fresh food and recreational activities
  • Food availability, such as more fast food restaurants and food insecurity – lack of financial means to buy food. Food insecurity results in relying on lower cost options, which are often more calorie dense, with less access to fresh food
  • Walking the neighborhood
  • The work environment, which can lead to increased sedentary life


Obesity is a common condition that has many causes spanning behavioral, biological, and environmental causes. Diet and physical activity are major 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 direct control.

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