What causes obesity? Genes, biology and environment

Key points

Several causes act together. Genes, the brain’s appetite system, the food around you, sleep, stress, medicines and life stages all add up.
Genes play a large part. Twin studies estimate that 47 to 90 % of the differences in BMI between people are linked to inherited factors.
The body defends its weight. After weight loss, hunger and fullness hormones were still changed about a year later in a clinical study.
Your surroundings shape what you eat. In a controlled trial, adults ate about 500 kcal more per day on ultra-processed meals than on unprocessed meals.
It is not a matter of willpower. Experts describe obesity as a problem of the body’s weight-control system, not simply a lack of discipline.
Some causes can be changed. Food, walking, sleep and a medicine review are good starting points. Talk to your doctor first.

How does obesity develop?

Obesity develops when, over months and years, the body stores more energy than it uses. That balance is not a simple sum: the brain steers it through hunger, fullness and energy use.

An Endocrine Society scientific statement describes obesity as a disorder of this weight-control system. In many people, the body seems to defend a higher level of fat mass, whether that setting is inherited or acquired.

This page gives an overview. Each cause has its own page with the full evidence, listed in the table below.

Which factors cause obesity, and what can help?

Most people living with obesity have several of these factors at the same time. The table shows what each one does and where you can read more.

Main causes of obesity at a glance
Factor	What it does	What can help	Read more
Genes and biology	Shape hunger, fullness and how the body stores fat	Knowing it is not your fault; steady habits; medical check	Genetics and biology
Food environment	Cheap, energy-dense food and big portions make eating more very easy	Simple home cooking, more whole foods, fewer sugary drinks	Food environment
Medicines	Some medicines for mood, diabetes or epilepsy, and steroid tablets, can add weight	A medicine review with your doctor; never stop on your own	Medicines and weight
Sleep, stress and mood	Short sleep and stress raise appetite and make moving harder	Regular sleep times, stress support, talking therapy	Sleep and stress
Little daily movement	Jobs, transport and screens need less movement than before	Daily walking at your own pace, building up slowly	Walking
Health conditions and life stages	Thyroid problems, PCOS, pregnancy and menopause can change weight	A check-up and advice that fits your stage of life	PCOS, menopause
Weight stigma	Blame and shame add stress and keep people away from care	Respectful care and support without judgement	Weight stigma

How much do genes and biology matter?

A review of 88 estimates from twin studies, with more than 140,000 twins, found that inherited factors explain a large share of the differences in BMI. The estimates ranged from 47 to 90 %, and were higher in children than in adults.

Genes rarely act alone. They mostly work through appetite and fullness, so the same genes lead to more weight gain where food is everywhere. Read more on genes, hormones and the brain.

How does the food around you add to weight gain?

Food that is cheap, tasty, always within reach and sold in large portions makes it easy to eat more than the body needs. Experts call such surroundings an obesogenic environment.

In one tightly controlled trial, 20 adults could eat as much as they liked. On ultra-processed meals they ate about 500 kcal more per day and gained 0.9 kg in two weeks; on unprocessed meals they lost 0.9 kg.

That is why we focus on simple, filling food rather than strict rules. Read more about the food environment.

Can sleep, stress and medicines play a part?

Yes. A meta-analysis of more than 600,000 people found that short sleepers had higher odds of obesity: about 1.9 times in children and 1.6 times in adults. Studies like these cannot fully prove cause and effect.

Chronic stress, depression and trauma can change eating, sleep and the energy to move. More on sleep, stress and mental health.

Medicines matter too. A review of 257 trials found average gains of about 1 to 3 kg with some antidepressants, antipsychotics and diabetes medicines, compared with placebo. Never stop a medicine yourself; ask your doctor about medicines and weight gain.

Which health conditions and life stages are linked to weight gain?

Medical conditions: an underactive thyroid (hypothyroidism), polycystic ovary syndrome (PCOS) and some rare genetic syndromes.
Life stages: pregnancy, menopause, stopping smoking and getting older.
Early life: factors before birth and in early childhood can influence weight later on.

If your weight changed quickly without a clear reason, ask your doctor for a check-up. Our doctor visit checklist helps you prepare.

Why is it not just about willpower?

Telling people to simply eat less and move more ignores the biology and surroundings described above. It also feeds shame, which makes it harder to ask for help.

The body also pushes back after weight loss. In one study, people lost about 13.5 kg on average, yet a year later their appetite hormones still favoured regaining it. Read why on weight regain.

What can you do about the causes you can change?

Talk to your doctor first, especially if you take medicines or have a health condition. Together you can see which causes play a role for you.

Ask for a medicine review if your weight rose after starting a new medicine.
Build regular sleep times and a calm evening routine.
Start walking a few minutes a day, at a pace that feels right.
Cook simple meals from whole foods; our free eating plan shows how.

Need help to get started? Sign up for free for eating plans, workout videos and a free video meeting (in English).

Unsure about a word? See the obesity glossary.

More questions about causes and treatment? See our frequently asked questions.

Frequently asked questions

Is obesity genetic?

Genes play a significant role. They influence appetite, fullness and how the body stores energy. But genes interact with the environment: the same genetic risk leads to more obesity in an environment with plenty of cheap, energy-dense food.

Can medication cause weight gain?

Yes. Some medicines, such as certain antidepressants, antipsychotics, corticosteroids and some diabetes medicines, can lead to weight gain. Never stop a medicine on your own; discuss alternatives with your doctor.

Why is it so hard to keep weight off?

After weight loss, the body often increases hunger hormones and lowers energy use to defend its previous weight. This is a biological response, not a lack of discipline, and it is why long-term support matters.

Why do some people gain weight more easily than others?

People differ in their genes, appetite hormones, sleep, stress, medicines and surroundings. Twin studies suggest that inherited factors explain a large part of the differences in BMI between people. So two people can eat and move in a similar way and still end up with a different weight.

Can someone eat healthily and still gain weight?

Yes. Medicines, short sleep, stress, hormonal conditions such as an underactive thyroid or PCOS, and life stages such as menopause can all lead to weight gain. Portion size also counts, even with healthy food. If your weight rises without a clear reason, ask your doctor for a check-up.

What causes obesity?