Causes of Obesity

Medically Reviewed by Brunilda Nazario, MD on January 06, 2023
6 min read

Many things are involved in causing obesity, a chronic and complicated medical condition that involves havinge too much body fat.

Along with eating and activity patterns, your genes, your environment, and your emotions play a role. So can your hormones, medical conditions you have, and medications you take.





Among the things that contribute to obesity are what and how much we eat, and how much we move. 

Food and diet

People can gain weight when they take in more calories from foods and drinks than their bodies burn over time. You burn calories for basic body functions like breathing, blinking, and digestion, as well as for movements like walking. Your body stores any extra calories as fat.

But obesity goes beyond simple calorie math. The type of food you regularly include in your diet matters, too. A diet rich in whole grains, fruits, and veggies not only helps protect against weight gain, but is good for your overall health. 

Physical activity

Lack of physical activity contributes to weight gain, too. Your activity level doesn't include just your exercise routine, but all the ways you move your body every day. Across the world, people drive more, use more labor-saving devices, and are less likely to have jobs that involve physical activity, compared to a few decades ago. In fact, 1 in 3 people worldwide get little or no exercise.



Several medical conditions can lead to excess weight, including: 

  • Hypothyroidism. This is a condition in which the thyroid gland in your neck produces too little thyroid hormone. Thyroid hormone regulates your metabolism. Too little of it slows your metabolism and often causes weight gain. If your doctor suspects thyroid disease, they may do blood tests to check your hormone levels.
  • Cushing's syndrome. This happens when your adrenal glands (which are on top of each kidney) make too much of a steroid hormone called cortisol. Among other things, this leads to a buildup of fat in areas like the face, upper back, and abdomen.
  • Polycystic ovary syndrome. This happens when your ovaries make too much of a male hormone called androgen, resulting in problems with your period. This hormonal imbalance can cause you to gain weight. 

Certain inherited conditions and other diseases of the brain can also cause excess weight gain.

Some medications can also cause increased body weight or hinder weight loss, such as:

If you gain weight after you start one of these medications, tell your doctor. They may be able to suggest another option. 

Aging is another factor in weight gain and obesity. As you grow older, you lose muscle. This slows down the rate at which you burn calories. Many people also become less physically active as they age. 

The hormonal changes of menopause can contribute to weight gain, too.


Sometimes, genes are directly linked to obesity, or at least a condition that leads to it. This is the case with Prader-Willi syndrome, a genetic condition in which you constantly to want to eat and never feel full. 

Your genes can also affect how much fat your body tends to store, and where you store it. They can influence your metabolism, how your body controls your appetite, and how well you burn calories when you exercise. 

But, in most cases, heredity alone doesn't cause obesity. It works together with things in your environment, such as unhealthy eating patterns, to influence your weight. Healthy lifestyle habits can help to offset its effects. 



Among the other things that experts believe are involved in obesity are various hormones, your emotions, and whether you get enough sleep.


Certain hormone problems like hypothyroidism are among the medical problems that can cause weight gain. Beyond that, several hormones your body makes are involved in hunger, calorie burning, and how you store fat. They include: 

  • Leptin, a hormone made by fat cells that affects appetite and fat storage 
  • Insulin, which is made by your pancreas and regulates blood sugar levels and fat burning
  • Ghrelin, also known as the “hunger hormone,” which is made by your stomach and regulates appetite
  • The sex hormones estrogen and androgen, which help determine where your body stores fat 

People with obesity tend to have differences in how much of these hormones their bodies make and how their brains and bodies respond to them. These differences may lead to weight gain. But obesity can also cause hormonal changes. 

Stress and other emotions

When you’re bored, stressed, angry, or sad, you may tend to eat more. You also may be more likely to choose high-calorie foods like sugary treats and drinks. If you have a mood issue like depression, you might not feel like exercising. 

Issues that often go along with obesity, like discrimination and physical pain, tend to affect your moods. This can create a cycle of emotional upheaval and weight gain. 


Research has found that people who don't get enough good sleep are at higher risk for obesity. Hormones your body releases during sleep help to control your appetite and metabolism. And when you're over-tired, you may be less likely to exercise and more likely to choose unhealthy foods.


You might not realize it, but your environment has a big effect on your weight.

For example, if your neighborhood has lots of fast-food restaurants and few markets that sell healthy foods, it may be hard to eat well. If you don't have easy access to parks in which to play sports, or if a lack of sidewalks makes walking dangerous, you may not get enough activity. 

Other things in the world around you that can contribute to obesity include:

  • Marketing of high-calorie foods
  • High prices for quality food
  • Harsh weather that limits outdoor exercise
  • Large restaurant portions
  • Easy access to unhealthy foods though delivery apps

The conditions in which you were born, work, and live influence your odds of developing obesity. Experts call these “social determinants of health.” Along with your environment, they include things like your income level, education, access to health care, and social relationships. 


Research shows that adults with lower incomes and lower levels of education are more likely to face obesity. Food insecurity – when you regularly don't have enough food to feed each person in your household – may play a role. Healthy, whole foods are often more expensive than highly processed ones. 

People with lower incomes may not be able to afford gym memberships, or have reliable transportation to places to exercise or buy healthy foods.

Your socioeconomic status can also be a source of stress or poor sleep, which, in turn, increase the risk of developing obesity.

Racial and ethnic differences

Researchers found that nearly half of Black people over 20 in the U.S. have obesity, as do about 45% of Latinos, 42% of White people, and 17% of Asians. Income levels, education, racism, and other social and cultural factors are among the things that could influence who’s at risk.

Social networks and support

Social connections affect obesity in several ways:

  • You're more likely to have obesity if your friends and relatives do.
  • Social isolation may put you at a higher risk for developing obesity as well as other chronic health problems.
  • A reliable support network can make a big difference when you’re trying to lose weight.