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Obesity causes are complex. Food and activity matter, but so do genetics, sleep, medicines, health conditions and the environment. Understanding these factors helps you seek useful support rather than relying on blame or promises of rapid weight loss.
This guide explains five groups of contributing factors, how adults use BMI as a screening measure and what a weight-management assessment can involve.
What are overweight and obesity?
Obesity is a complex chronic disease involving excess body fat that can affect health. Body weight alone does not describe a person’s health, and an assessment should consider more than appearance.
The WHO fact sheet describes obesity as a condition with multiple biological, environmental and social influences. Energy intake and expenditure are part of the picture, but the factors shaping them differ between people.
Five groups of obesity causes and contributing factors
Several factors can operate together. The NIDDK guide to weight and health provides background on the following contributors.
1. Eating, activity and everyday surroundings
Regularly taking in more energy than the body uses can lead to fat storage. Food cost and availability, work routines, long periods of sitting and access to safe activity spaces influence everyday choices.
2. Sleep and stress
Too little sleep can affect hunger and food choices. Long-term stress and mental health difficulties can also influence weight. These issues deserve support rather than being treated as a lack of effort.
3. Family history and genetics
Genes can influence appetite, fat storage and where fat is carried. Shared surroundings and routines also matter. A family history does not determine every outcome, but it is useful information for a clinical assessment.
4. Medicines
Some medicines used for mental health conditions, epilepsy or diabetes, and corticosteroids, can contribute to weight gain. Effects vary. Ask your prescriber about a suspected change; do not stop necessary treatment yourself.
5. Health conditions
Conditions such as hypothyroidism, Cushing’s syndrome and polycystic ovary syndrome can contribute to weight gain. A clinician can decide whether symptoms or your history justify further investigation.
BMI: a useful starting point with limits
Body mass index compares weight with height. For most adults, the usual screening categories are:
| BMI (kg/m²) | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Healthy weight range |
| 25–29.9 | Overweight |
| 30 or above | Obesity |
BMI cannot distinguish muscle from fat. NICE recommends lower thresholds for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African–Caribbean backgrounds: overweight at 23–27.4 and obesity at 27.5 or above. These are UK guideline thresholds, not a universal personal diagnosis.
See NICE’s assessment guidance. Clinicians may also consider waist measurements, symptoms, medical history and test results. Children need age-appropriate assessment rather than this adult table.
How to calculate BMI correctly
BMI = weight in kilograms ÷ (height in metres × height in metres).
For example, a person weighing 80 kg and measuring 1.70 m has a BMI of 80 ÷ (1.70 × 1.70) = 27.7 kg/m², rounded to one decimal place. Multiply the height by itself first, then divide the weight by that result.
This calculation provides a screening value. It is not a weight-loss calculator or a complete measure of fitness. Ask a clinician how to interpret it for you.
Health risks and reasons to seek support
Living with obesity can increase the risk of type 2 diabetes, cardiovascular disease, fatty liver disease and joint problems. Some people also experience breathlessness, fatigue, snoring or difficulty moving. Those symptoms can have other causes and should not automatically be attributed to weight.
The NHS adult overweight and obesity guide explains the risks and available support. Seek an assessment if weight concerns, symptoms or an eating disorder are affecting your health. Our diabetes guide explains related warning signs and testing.
What treatment can involve
A care plan may include eating and activity changes, behavioural support, weight-management medicines or surgery when appropriate. These options need an assessment of benefits, risks and suitability; they are not interchangeable quick fixes.
NIDDK’s treatment overview explains the main approaches. Ask what professional support and follow-up are available locally. Improving health, mobility and everyday wellbeing matters alongside changes on the scale.
For practical habit-building ideas, read our weight-management guide. Keto is a restrictive option with specific safety concerns, rather than a default solution; our ketogenic diet guide explains those considerations.
Preparing for a weight-management appointment
A short written summary can make your visit more useful. Include:
- When your weight changed and what else was happening at the time.
- Your current medicines and supplements.
- Typical meals, sleep and activity, including difficulties with access or cost.
- Symptoms or mobility problems you want assessed.
- Previous approaches, what helped and what was difficult.
Choose a goal you can discuss clearly, such as walking more comfortably, organising meals around shifts or understanding a medicine-related change. Ask which outcomes to track and when to review progress.
Finally, agree on a realistic next step. That might be an assessment, a referral or one manageable habit. A plan should fit your circumstances and make space for setbacks and ongoing support.
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About this information
HealthyCura shares health and wellness information for general reading. This article does not replace professional medical advice, diagnosis or treatment.
