What causes obesity?

The short version

Obesity develops when energy intake exceeds energy expenditure over an extended period — but why this imbalance arises is complex. Research points to five major contributing areas that interact:

1. Genetics and biology
2. Environment and food availability
3. Sleep
4. Stress and mental health
5. Habits and lifestyle

No single factor "causes" obesity on its own.

1. Genetics and biology (50–70%)

Twin and adoption studies show that 50–70% of the variation in BMI between individuals is explained by genetics. Hundreds of gene variants have been identified — the vast majority affect the brain's regulation of hunger and satiety, rather than fat cells directly. This means that people with a certain genetic profile feel hungrier on the same amount of food, find it harder to feel full, and have a lower resting metabolic rate.

In children, heritability is equally high. See is obesity hereditary?.

2. Environment and food availability

The sharp increase in obesity over the last 40 years cannot be explained by genetics — our genes have not changed. What has changed is the environment:

• Energy-dense, processed food is cheap and readily available.
• Marketing aimed at children influences food choices.
• Portion sizes have increased.
• Sugary drinks (fizzy drinks, juice, sports drinks) contribute significantly to energy intake without promoting satiety.
• Sedentary behaviour has replaced physical labour and active transport.

This is often referred to as an "obesogenic environment" — it makes it easier to develop obesity regardless of individual willpower.

3. Sleep

Too little or poor sleep is an independent risk factor for weight gain. Lack of sleep affects the hormones leptin and ghrelin (satiety and hunger), increases cravings for sweet and fatty foods, and reduces the energy for physical activity. School-age children need 9–11 hours per night.

4. Stress and mental health

Chronic stress elevates cortisol levels, which promotes abdominal fat accumulation and increases cravings for quick energy. Anxiety, depression and trauma — including ACEs (adverse childhood experiences) — are associated with a higher risk of obesity. The relationship is bidirectional: poor mental health increases the risk of obesity, and obesity-related stigma worsens mental health.

5. Habits and lifestyle

Meal patterns, screen time, choice of drinks and daily physical activity all play a role — but they are not the whole picture. Two families with identical habits can have completely different BMIs due to genetics and biology. Habits are nevertheless what is easiest to influence, which makes them the first step in treatment.

Why children in particular?

In children, additional factors interact:

• Growth and puberty — certain stages lead to increased fat storage.
• Parental habits — children inherit both genes and mealtime patterns.
• Lack of sleep — common among school-aged children and teenagers.
• Screen time — has increased dramatically since smartphones.

See why do children gain weight quickly.

It is not a matter of willpower

This is the most important conclusion from modern obesity research: obesity is a chronic, biologically rooted disease. The WHO and Socialstyrelsen classify it as a disease — not a lifestyle choice. This also means that treatment requires long-term support, not quick-fix diets. In children, treatment is always directed at the whole family, not at the child's body alone.

For children with obesity, many Swedish paediatric outpatient clinics offer structured treatment. Many use digital treatment support from Evira — on evira.se you can see which clinics are connected.

Related questions

• Is obesity hereditary?The heritability of BMI is 40–70%. But genes do not decide alone — here is how the family can influence the risk.
• Why is my child gaining weight so fast?Common causes of rapid weight gain and when you as a parent should seek healthcare.
• What is childhood obesity?Childhood obesity is an iso-BMI of 30 or higher — a chronic disease that should be treated within healthcare.
• What is childhood obesity?Childhood obesity is an iso-BMI of 30 or higher — a chronic disease that should be treated within healthcare.

See all questions and answers →