Causes of Obesity

Written by Alicia Racelis, PhD
Medically Reviewed by Michael Dansinger, MD and Zilpah Sheikh, MD on March 25, 2025
9 min read

Many things lead to the complex health condition called clinical obesity. Although the exact cause remains unknown, many factors can contribute to it. It's not just excess adiposity (too much body fat). In fact, you need your body fat for many reasons.

Your adipose organ stores your body fat. Adipose controls your hunger and satiety (feeling full). It also plays a metabolic role in insulin sensitivity and inflammation. You need body fat to keep your body warm and supply energy, too.

Just like any organ in your body, if your adipose stores too much fat, it can become out of control. And too much visceral fat (body fat around your organs) may cause health problems.

When this happens, i.e., when your excess fat leads to health problems or impairment, it is called clinical obesity. In other words, obesity is not just an elevated BMI — it impairs your health, too.

Along with your eating and activity patterns, your genes and the environment can impact your body's metabolism. Your emotions, hormones, and other medical conditions can also play a role. Even the medications you take can change how your body grows or metabolizes energy.

Researchers think that clinical obesity is more complex than simply taking in more calories than the body burns. What and how much you eat is controlled by signals in your brain (neuroendocrine signals). 

Attempts to lose weight can cause a shift in these brain signals. Your body will try to slow down, get hungry, or make other changes to gain weight again. Everyone's body is unique. But some studies may link weight gain with certain foods and lower physical activity.

Food and diet

Studies point out that certain foods may be linked to higher weight gain. These ultra-processed foods may contain corn or soy by-products. These foods or drinks are often calorie-dense. 

You burn calories for basic body functions such as breathing, blinking, and digestion. You also burn them for movements such as walking. Your body stores any extra calories for later, as fat. But obesity goes beyond simple calorie math.

The type of food you regularly include in your diet matters. A diet rich in whole grains, fruits, and veggies is good for your overall health. 

Physical activity

Lack of physical activity may contribute to weight gain. Some studies show it can also lower your metabolism. 

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 and use more labor-saving devices. 

You may be less likely to have a job that involves physical activity, compared to a few decades ago. In fact, 1 in 3 people worldwide get little or no exercise.

Several medical conditions can alter how your body stores fat. These conditions can cause your body to store extra calories. You may have a harder time reaching your weight goal if you have medical conditions like:

Hypothyroidism. This condition causes your thyroid gland in your neck to produce too little thyroid hormone. Thyroid hormone controls your metabolism. Too little of it slows your metabolism and can cause 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 (that sits on top of each kidney) make too much of a steroid hormone called cortisol. Among other things, this leads to higher insulin levels. These changes can make your body store fat in areas such as your face, upper back, and belly.

Polycystic ovary syndrome. If your ovaries make too many androgens (male sex hormones), you may have problems with your period. This hormonal imbalance can also cause you to gain weight.

Hypothalamic obesity. Certain brain problems can also cause excess weight gain. These are called hypothalamic obesity. They're a group of issues caused by brain injury, tumors, or infection. These conditions impact your hypothalamus in your brain. 

This part of your brain controls your appetite and how your body burns calories and stores fat. If your hypothalamus isn't working, you can gain weight. You can also be hungry all the time or not be able to stop eating. 

You can also be born with hypothalamic obesity, which can occur in conditions like Prader-Willi syndrome.

Medications. Some medications can also cause increased body weight. These medicines may hinder your attempts at weight loss, such as:

  • Steroids
  • Certain antidepressants
  • Antipsychotics
  • High blood pressure drugs called beta-blockers
  • Some seizure medications
  • Some medications for diabetes
  • Some hormonal birth control medications, such as pills

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

Getting older. 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. Hormonal changes, such as menopause and lowered testosterone, may add to weight gain, too.

Sometimes, genes are directly linked to obesity or at least a condition that leads to it. There are three genetic types you can have:

  • Monogenic, change in one gene
  • Polygenic, change in more than one gene
  • Syndromic, often linked with another medical condition

The rarest is monogenic obesity. It's often caused by a change in your leptin gene. Leptin is a hormone that controls your appetite. Certain gene changes can cause weight gain, uncontrolled hunger, and clinical obesity.

Polygenic gene changes involve more than one gene. They add up to alter how and where your body stores fat. The gene changes can influence your metabolism. It can also change how your body controls your appetite or how well you burn calories when you exercise.

Syndromic obesity is usually linked to other medical conditions. This is the case with Bardet-Biedl syndrome (BBS). It's a genetic condition in which your body stores fat around your trunk. But it often causes problems with your eyes and heart, too.

In most cases, heredity by itself doesn't cause clinical obesity. Many things, such as your environment, medications, and genetics, work together to influence your weight. Healthy lifestyle habits can sometimes help lower the effects. But talk to your doctor to find out what's best for your health.

Among the other things, experts think various hormones, stress, and sleep also affect your weight.

Hormones

Certain hormone problems, such as hypothyroidism, may cause weight gain. Beyond that, several hormones help your body control hunger, regulate calorie burning, and manage how you store fat. They include: 

  • Leptin, a hormone made by fat cells to control your appetite and fat storage.
  • Insulin, made by your pancreas to regulate blood sugar and fat burning.
  • Ghrelin, also known as the "hunger hormone," made by your stomach to control appetite.
  • The sex hormones, estrogen and androgens, help decide where your body stores fat.

Everyone's body is different. But clinical obesity can cause your body and brain to make different amounts of hormones. These differences may lead to weight gain and hormonal changes, too.

Stress and other emotions

When you're bored, stressed, angry, or sad, you may tend to eat more. It depends on how your body reacts to stress. 

You also may be more likely to choose high-calorie foods, such as 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 distress and may lead to weight gain.

Sleep 

Research has found if you don't get enough good sleep, you're at a higher risk for clinical obesity. The hormones your body releases during sleep can help control your appetite and metabolism. 

And when you're overtired, you may be less likely to exercise and more likely to choose unhealthy foods. So, getting enough exercise can help you sleep deeper. This will also release good, restful hormones at night to better control your appetite in the daytime.

You might not realize it, but your environment has a big effect on your weight. If your neighborhood has lots of fast-food restaurants and few markets selling healthy foods, it may be harder for you to eat well. 

If you don't have easy access to parks, it can be harder to play sports. It may also be tougher to exercise if you don't have sidewalks. Without them, perhaps walking isn't safe and 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 limiting outdoor exercise
  • Larger restaurant portions
  • Easy access to unhealthy foods by delivery apps

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

Socioeconomics

Research shows adults with lower incomes and lower levels of education may be more likely to face clinical 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 may also cost more than highly processed ones.

People with lower incomes may not be able to afford gym memberships. Reliable transport to go exercise or to buy healthy foods may also be a factor.

These things can be a source of stress, leading to poor sleep. And poor sleep can raise your risk for clinical obesity.

Racial and ethnic differences

Researchers found that nearly half of Black people over age 20 in the U.S. have clinical obesity, as do about 45% of Latinos, 42% of White people, and 17% of Asians. 

Of course, how BMI is defined, income levels, education, racism, and other social and cultural factors are some things that can impact who's at risk.

Social networks and support

Social connections may also affect clinical 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.
  • Having other chronic health problems can also raise your risk.

But having a reliable support network can make a big difference when you're trying to lose weight. You can look for support at the American Association of Clinical Endocrinology. Or ask your doctor about finding local support groups in your area.

Why am I gaining weight when I don't eat much?

There can be many reasons for weight gain, even if you aren't trying to gain it. Medications and medical conditions may cause weight gain or slow your metabolism. Or perhaps it's a natural part of life, such as aging or pregnancy. Even quitting smoking or menstruation (bloating) can cause it. 

But if you get sudden swelling from a condition such as a heart problem, it could be a serious sign of fluid retention. Get medical help right away if you quickly gain weight and you have:

  • Constipation
  • Hair loss
  • Cold more often
  • Swelling in your hands or feet
  • Shortness or breath
  • Hunger with fluttering heartbeat
  • Tremors or sweating
  • Changes in your vision

Can obesity cause shortness of breath?

Sometimes extra weight can press on your lungs. This may lead to feeling short of breath. But in rare cases, you may have obesity hypoventilation syndrome (OHS). If you're also tired, feel sluggish, lack energy, get headaches, or dizziness, talk to your doctor.

If you snore, choke, gasp, or pause breathing when you sleep, see your doctor. You may need help breathing at night, such as with a CPAP or BiPAP.

How does lack of sleep cause obesity?

Sleep helps you control glucose in your body. It also raises your body's sensitivity to insulin, and lowers cortisol and ghrelin (appetite). And if you exercise, you can get a longer, deeper sleep. These can work together to keep your body healthy.

What are the main causes of childhood obesity?

Clinical obesity in children is complex, like in adults. Many different things can play a role, such as:

  • Genetics
  • Environment
  • Stress (bullying, racism, and more)
  • Sleep
  • Social factors
  • Economic influences
  • Eating habits and access to healthy food
  • Physical exercise