What Causes Obesity? It’s Not That Simple

Last updated: 4 March, 2026

According to the NHS, around 1 in every 4 adults in the UK are living with obesity.

Obesity means carrying excess body fat, and it is most commonly measured using body mass index (BMI). A BMI over 25 is classed as overweight, and a BMI over 30 is classed as obesity. But measures like waist circumference can also help diagnose obesity.

Obesity can increase health risks. In 2019, high BMI was linked to around 5 million deaths from non-communicable diseases. This includes stroke, heart disease, and certain cancers, according to the World Health Organisation (WHO).

But what causes obesity? It’s not as straightforward as you might think.

What Are the Causes of Obesity?

Obesity carries a lot of stigma. It is sometimes viewed as a personal or moral failure, rather than a chronic condition.

But if you are living with obesity or overweight, and you’ve tried everything to lose weight, it does not mean you are lazy or lack willpower.

Living with obesity is rarely down to just one thing. The UK government describes obesity as a “complex and multifactorial disease”. This simply means lots of different things can contribute to it, including:

• Your biology
• How much you exercise
• Eating habits
• Psychology
• The environment

Your Biology

If you’re living with obesity or with overweight, it’s not because you lack willpower. Biology can play a significant role in your appetite and the food choices you make.

According to the government’s Tackling Obesities – Future Choices report, research into the biological drivers of obesity has found a number of genes associated with obesity:

• One gene (called an FTO gene) that may contribute to a higher risk of weight gain
• Genes that influence pathways and “feedback loops” in the brain and body
• Genes that influence how much the body wants to hold onto body weight

Hormones play a big role in our appetite levels and decisions we make around food. To learn more about how our biology influences our food choices and body weight, check out our Ask the Expert podcast episode on endocrinology (the study of hormones).

Underlying health conditions

Some medical conditions can lead to weight gain, which may contribute to obesity.

If you have an underactive thyroid (hypothyroidism), this can cause weight gain, tiredness, and low mood. It is usually treated with daily hormone replacement tablets called levothyroxine, which increase thyroxine levels.

Cushing’s syndrome is caused by excess cortisol in the body and is strongly linked with weight gain. Many people with the condition have obesity. Symptoms can include increased fat around the chest and tummy, a build-up of adipose tissue on the back of the neck and shoulders, and a red, puffy, rounded face.

Polycystic ovary syndrome (PCOS) is an obesity-related condition that affects women. Obesity can contribute to PCOS developing, and PCOS can also make weight gain more likely and weight loss more difficult.

If you think an underlying health condition may be affecting your weight, it is worth speaking to your GP.

Food Production

Food production has changed rapidly over the past century. Just take a wander down any supermarket and look at all the brightly-coloured packets.

A lot of food now being produced and sold in the UK is convenient but unhealthy ultra-processed foods (UPFs). These are foods low in nutrients and high in salt, fat and sugar. According to the British Medical Journal (BMJ), UPFs make up 57% of household food intake in the UK. 

Unfortunately, higher intakes of UPFs, particularly added sugars, are associated with an increased risk of obesity and other health conditions.

If you find that ultra-processed foods (UPFs) are prevalent in your diet, it’s likely due to their convenience, affordability, and ease of preparation.

To create a more balanced diet, consider incorporating more whole foods such as fruits, vegetables, healthy fats, and lean proteins. These options not only offer essential nutrients but can also be a fulfilling way to diversify your meals.

The Environment

As well as food production, the world we live in has changed dramatically over the past 100 years – new technologies, rising car ownership and a decrease in manual jobs.

These environmental factors have had an impact on exercise levels and what we eat. Living less active lives and eating convenient processed foods can contribute to weight gain. There’s even a term for this environment, which encourages us to eat more and move less: an obesogenic environment.

Financial and social circumstances also play a big role in determining how healthy people are. So, if you live in an area with poor street lighting, and without access to parks, gyms and pavements, it can also make it harder to be physically active.

Behaviour

What – and How Much – You Eat

People gain weight when they consume more food than their body needs. When this happens, the body stores the excess energy as fat.

The energy we get from food is measured in calories. According to the NHS, men typically need around 2,500 calories a day to maintain a healthy weight. Women typically need about 2,000 calories a day. Although energy levels vary depending on how much exercise you do.

While the theory behind weight gain centres on the balance of calorie intake and expenditure, the reality is: there are a number of factors driving people to consume more calories than needed.

These can include emotional eating, social influences, lifestyle choices, and even advertising.

Additionally, accessibility to high-calorie, convenience foods often makes it easier to overeat. Understanding weight gain is complex and multifaceted, extending beyond simple energy equations.

Exercise Levels

Doing physical activity not only helps you burn calories but can also boost your metabolic rate. This can lead to greater overall energy expenditure.

Beyond weight management, regular exercise offers a range of benefits, including improved cardiovascular health, enhanced mood, and increased strength and flexibility.

The Department of Health and Social Care recommends that adults aged 19 and over aim for at least 150 minutes of moderate-intensity activity each week. Or at least 75 minutes of vigorous-intensity activity, or a combination of both.

Moderate-intensity activity is where your breathing and heart rate increase but you can still talk. This might include brisk walking, cycling, or swimming. Vigorous activity is where it is difficult to talk, such as running, climbing stairs, or sports like tennis or football.

If time is a challenge, small changes can still help. Try taking the stairs instead of the lift or parking further away so you walk more. Any movement is better than none. The most helpful exercise is often the one you can stick with, whether that is 20 minutes or an hour.

Other Causes of Obesity

Certain medications

All medicines can have side effects, and some medications can contribute to weight gain. Medicines commonly linked with weight gain include:

• Steroids: often used to treat inflammatory conditions such as asthma, cancer, and arthritis. Studies show weight gain can be a side effect, especially with longer-term use.
• Diabetes medicines: some treatments used to treat type 1 or type 2 diabetes, including insulin and sulphonylureas, have been linked to weight gain.
• Epilepsy medicines: some anti-epileptic medicines are associated with weight gain, including gabapentin, pregabalin, valproic acid, and vigabatrin.
• Antidepressants: research has found a higher risk of weight gain in people taking commonly prescribed antidepressants compared with those who are not.

A healthy lifestyle may help reduce the risk of weight gain with some medicines. This can include eating a balanced diet, stopping smoking, reducing alcohol, and being physically active.

If you are worried about weight gain linked to a medicine, speak to a doctor or pharmacist. In some cases, an alternative may be available. Do not stop taking prescribed medication without medical advice, as this can be harmful.

Key Takeaways

• Obesity increases the risk of non-communicable diseases like heart disease, stroke, and certain cancers. But it isn’t just caused by overeating. It’s not a personal or moral failure. It’s a complex, long-term health condition.
• Is obesity genetic? Biology and genes can make weight gain more likely. Certain genes and hormone pathways affect appetite and cravings, how full you feel, and how strongly your body defends its weight.
• Modern food production, our environment and biology can lead to weight gain and obesity. Ultra-processed foods are designed to be easy to overeat. Technological developments have impacted activity levels.
• Conditions like hypothyroidism, Cushing’s syndrome, and PCOS can cause weight gain.
• Certain medications, including steroids, diabetes treatments, epilepsy drugs, and antidepressants, can lead to weight gain as a side effect.
• Not getting enough exercise lowers the number of calories you burn, making it easier to gain weight.

References

World Health Organization (2020). Obesity. [online] WHO. Available at: https://www.who.int/health-topics/obesity.

Department of Health and Social Care (2023). Physical activity guidelines. [online] GOV.UK. Available at: https://www.gov.uk/government/collections/physical-activity-guidelines.

Government office for science (2007). Tackling Obesities: Future Choices – Project Report 2nd Edition Government Office for Science FORESIGHT. [online] Available at: https://assets.publishing.service.gov.uk/media/5a759da7e5274a4368298a4f/07-1184x-tackling-obesities-future-choices-report.pdf. 

Loos, R.J.F. and Bouchard, C. (2008). FTO: the first gene contributing to common forms of human obesity. Obesity Reviews, 9(3), pp.246–250. doi: https://doi.org/10.1111/j.1467-789x.2008.00481.x. 

Rauber, F., Louzada, M.L. da C., Martinez Steele, E., Rezende, L.F.M. de, Millett, C., Monteiro, C.A. and Levy, R.B. (2019). Ultra-processed foods and excessive free sugar intake in the UK: a nationally representative cross-sectional study. BMJ Open, 9(10), p.e027546. doi: https://doi.org/10.1136/bmjopen-2018-027546. 

‌Burgoine, T., Mackenbach, J.D., Lakerveld, J., Forouhi, N.G., Griffin, S.J., Brage, S., Wareham, N.J. and Monsivais, P. (2017). Interplay of Socioeconomic Status and Supermarket Distance Is Associated with Excess Obesity Risk: A UK Cross-Sectional Study. International Journal of Environmental Research and Public Health, [online] 14(11), p.1290. doi: https://doi.org/10.3390/ijerph14111290.

NHS (2023). Causes – obesity. [online] NHS. Available at: https://www.nhs.uk/conditions/obesity/causes/. 

Liu, D., Ahmet, A., Ward, L., Krishnamoorthy, P., Mandelcorn, E.D., Leigh, R., Brown, J.P., Cohen, A. and Kim, H. (2013). A practical guide to the monitoring and management of the complications of systemic corticosteroid therapy. Allergy, Asthma & Clinical Immunology, [online] 9(1), p.30. doi: https://doi.org/10.1186/1710-1492-9-30.

www.kcl.ac.uk. (n.d.). King’s study finds link between antidepressant use and weight gain | Website archive | King’s College London. [online] Available at: https://www.kcl.ac.uk/archive/news/sphes/kings-study-finds-link-between-antidepressant-use-and-weight-gain.

Editor (2019). A number of widely prescribed diabetes medications have been linked with increased weight gain. [online] Diabetes. Available at: https://www.diabetes.co.uk/weight/weight-gain-on-medication.html.

Antel, J. and Hebebrand, J. (2011). Weight-Reducing Side Effects of the Antiepileptic Agents Topiramate and Zonisamide. Handbook of Experimental Pharmacology, pp.433–466. doi: https://doi.org/10.1007/978-3-642-24716-3_20.

Jallon, P. and Picard, F. (2001). Bodyweight Gain and Anticonvulsants. Drug Safety, 24(13), pp.969–978. doi: https://doi.org/10.2165/00002018-200124130-00004.

NIH (2021). What causes obesity & overweight? [online] National Institute of Health. Available at: https://www.nichd.nih.gov/health/topics/obesity/conditioninfo/cause.

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