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Causes - Obesity

There is no single cause of obesity. But there are many factors that can influence obesity.

These factors include:

  • your genetics
  • stress
  • your appetite and your appetite hormones (the hormones that control your hunger)
  • where you live, work and play, and how they affect your eating and activity
  • your sleep habits
  • some medicines
  • other medical reasons
  • certain times in your life


Your genetics impact your risk of developing obesity.

They influence:

  • how and where your body stores fat
  • how you respond to changes in what you eat
  • how full you feel
  • how soon you will feel hungry again after you eat


Stress affects your health and weight in lots of ways.

Cortisol is a stress hormone which has been linked to obesity. Higher levels of cortisol can impact the hormones that effect appetite.

This can:

  • make you feel hungrier
  • cause you to eat foods high in fat and sugar

Stress can also disrupt your sleep, which can make it harder to be active.

Trying to lose or maintain your weight can also be stressful. It can lead to negative thoughts and feelings about yourself.

Stress and weight stigma can make it harder to lose weight.

Appetite and appetite hormones

Obesity can develop when the hormone signals that control your appetite don’t work as they should. 

Stress, poor sleep, chronic pain and fatigue can all affect these hormones.

If you have obesity it is harder for your appetite hormones to communicate when you are hungry or full. You may not feel hungry or full the same way as someone who doesn't have obesity.

There are 2 different types of appetite. Understanding the difference between the 2 can be useful when managing obesity.

Body hunger

This is what makes you eat when you are physically hungry. This is caused by hormones working between your brain and your gut. Sometimes those feelings aren’t well balanced across the day if you have obesity.

Reward hunger

When you feel this hunger, you may eat even though you are not physically hungry. Reward hunger is often a sign that you are overwhelmed or stressed. It's controlled by the brain and includes eating while you are stressed or upset.

Where you live, work and play

Where you live, work and play can impact your risk of developing obesity.

For example: 

  • it's harder to be physically active if it's hard to get to parks, paths and gyms
  • bigger food portions in restaurants, canteens and takeaways can make it harder to manage how much you eat
  • it's harder to eat a balanced diet if you can't get to supermarkets that sell affordable, healthy foods
  • food advertising can encourage you to buy less healthy foods
  • desk jobs and sitting a lot can make it harder to be physically active


Not getting enough sleep can:

  • reduce how quickly your body turns food into energy (your metabolism)
  • make you feel more hungry
  • lower your mood
  • lower your energy levels

You may be more likely to eat higher energy foods and be less active.

Your body clock can also be affected by many factors, including:

These can have a bad affect on the hormones that control your appetite and make it more difficult to manage your weight.


Some prescription medicines can lead to you gaining weight.

These include some:

  • medicines for epilepsy, diabetes and heart disease
  • medicines used to treat mental illness – including anti-depressants
  • steroids

Do not stop taking your medicine without speaking with your GP first. They may be able to switch you to another medicine that helps your condition but does not cause weight gain.

Other medical reasons

Other medical conditions may contribute to obesity. 

These can include:

  • an underactive thyroid gland (hypothyroidism) – this is where your thyroid gland does not produce enough hormones
  • Cushing's syndrome – this is a rare disorder that causes you to produce too many steroid hormones
  • conditions that affect your mobility can make it harder to be active

Certain times in your life

There are times in our lives where we may be at risk of weight gain.

These include:

  • pregnancy - it's normal to gain weight during pregnancy due to the growth of the baby
  • after giving birth - caring for a newborn can affect how you sleep and eat, and your stress levels
  • stopping smoking can affect your appetite
  • being less active after an injury
  • moving from being a teenager to a young adult due to changes in your routine
  • menopause because of the changes in your hormones

Content supplied by the NHS and adapted for Ireland by the HSE

Page last reviewed: 9 August 2022
Next review due: 9 August 2025