If you are obese, it means you’re very overweight which can increase your health risks.

By Wallace Health I Medically reviewed by Adrian Roberts.
Page last reviewed: October 2018 I Next review due: October 2023

What is obesity?

The definition of obesity for adults is a body mass index (BMI) of over 30. You can work out your BMI by dividing your weight in kilograms by the square of your height in metres. Or you can use an online calculator.

Obesity usually means you are eating and drinking more calories than your body can use. The extra calories are stored as fat.

Around one in four adults and one in five children aged 10-11 in the UK is obese.

Obesity is not a disease. However, the effects of obesity can be severe as you are more likely to have serious health problems, such as diabetes, gout, cancer and heart disease. Obesity can also increase your risk of developing arthritis. If your BMI is over 40, you are morbidly obese, which means your obesity may shorten your life.

You can reduce the health risks caused by obesity by losing weight.

Obesity symptoms

Obesity symptoms include breathlessness and being very overweight with a lot of body fat.

Some people have a high BMI but are not obese. That is because they have more muscle or don’t have much fat.

If you are worried about your weight, ask your GP for advice.

Obesity and BMI

The relationship between your BMI and your weight indicates whether your weight is healthy or not. In general, if your BMI is: 

  • Between 18.5 and 24.9, then you have a healthy weight
  • Between 25 and 29.9, then you are overweight
  • Between 30 and 39.9, then you are moderately obese
  • Over 40, then you are morbidly obese

If your BMI is over 25, additionally measuring your waist size can indicate whether the excess fat you are carrying will cause health problems. In general, men with a waist size of 94 cm or more and women with a waist size of 80 cm or more are at greater risk of developing health problems due to obesity. 

Other body measures, considered alongside your BMI, can also help determine whether your weight may harm your health. This includes your waist-to-hip ratio (WHR), your waist-to-height ratio (WtHR), and how much fat you have and how it is distributed. 

Talk to your doctor if you’re concerned about symptoms

You can book an appointment with a Spire private GP today.

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Diagnosis and tests for obesity

Your GP will be able to explain what is a healthy weight and measurement for you. They may calculate your BMI and take your waist measurement, as well as ask you about your general health and lifestyle. This may include asking you about: 

  • How much physical activity you do
  • How much alcohol you drink 
  • What your diet includes
  • Whether you smoke
  • Whether you have any medical conditions or are taking any medications — certain medical conditions and medications can make it easier to gain weight
  • Your family history — obesity and other health conditions (eg diabetes) can run in families

They may also ask you how motivated you feel to lose weight and how you feel about your weight eg whether your weight makes you feel depressed

Your GP may then refer you for tests to check for health conditions including type 2 diabetes and an underactive thyroid.

The Edmonton scale

If you are diagnosed as obese, your doctor may further assess you using the Edmonton scale. This scale relates your weight to your health and has five stages: 

  • Stage 0 — no health problems related to your weight
  • Stage 1 — mild health problems related to your weight eg borderline high blood pressure or occasional aches and pains
  • Stage 2 — you have a chronic (long-term) disease related to your weight (eg high blood pressure, osteoarthritis, sleep apnoea or type 2 diabetes) or you have moderate problems with completing daily activities and feeling healthy
  • Stage 3 — serious health problems related to your weight (eg heart attack, heart failure, stroke or other serious health conditions)
  • Stage 4 — severe, life-threatening chronic health problems related to your weight

If your doctor does not use the Edmonton scale, they will still explain how your weight is affecting your health. 

Causes of obesity

Obesity is a growing problem due to modern lifestyles ie easy access to cheap, high-calorie foods and spending more time sitting. Obesity develops as a result of eating and drinking too many calories without exercising enough to burn them off. It is more likely if your diet is high in fat and sugar eg if your diet includes a lot of alcohol, high-sugar drinks, takeaways or ready-made meals.

Excess calories and poor diet

An average, physically active man needs to consume around 2,500 calories each day to maintain a healthy weight, while an average, physically active woman needs around 2,000 calories each day.

Ready-made meals and takeaways can easily account for the majority of these calories eg one meal that includes a large takeaway hamburger, fries and a milkshake can contain around 1,500 calories. Other ways to easily eat too many calories include: 

  • Comfort-eating — eating to feel better when you are depressed or if you have low self-esteem
  • Drinking too much alcohol — heavy drinkers are often overweight due to the high amount of calories in alcohol
  • Drinking too many sugary drinks eg soft drinks and fruit juice
  • Eating a lot of highly processed foods (eg tinned foods) or fast food — these are often high in fat and sugar; this includes hidden sugars ie foods that you would not expect to be sugary but are nonetheless
  • Eating excessively large portions — this is more likely to occur if you eat with others whose meal sizes are too large; this can cause weight gain even if your diet is healthy ie contains lots of fruits, vegetables, whole grains and water
  • Eating meals out a lot — restaurant food may be higher in sugar and fat than a home-cooked equivalent and you may also be more tempted to eat a starter or dessert 

Obesity can run in families. This may be genetic but also due to unhealthy eating habits that you learnt as a child and continued to follow as an adult.

Leading a sedentary lifestyle

If you eat too many calories than you need and then do not burn them off through physical exercise, those excess calories will be stored as fat. 

The UK's Department of Health and Social Care recommends that adults perform at least two and a half hours of moderate aerobic exercise (exercise that increases your heart and breathing rate) every week. You can split this up into shorter stretches of daily exercise such as brisk walking, running or cycling. 

If you are overweight or obese, you may need to perform more than two and half hours of exercise every week to lose excess weight. However, make sure you do not overexert yourself — gradually increase the amount and intensity of exercise you do each week. 

Physical activity will affect your hormones and your hormones affect how you process food. Physical activity has been shown to help stabilise insulin levels — unstable insulin levels are linked to weight gain.

Not sleeping enough

Poor sleep increases the risk of weight gain in both children and adults. This is because sleep deprivation increases levels of the hormone ghrelin and decreases levels of the hormone leptin. Ghrelin stimulates your appetite while leptin suppresses it. 

Genetics and obesity

Most people can lose weight as obesity is usually caused by lifestyle factors eg poor eating habits and a lack of physical activity. However, there are very rare genetic conditions that can cause obesity eg Prader-Willi syndrome.

Medical reasons

If you have a healthy lifestyle but still find it difficult to lose weight, it might be caused by a medical condition eg Cushing’s syndrome where your body produces excessively high levels of steroid hormones or an underactive thyroid (hypothyroidism) where your body does not produce enough hormones. 

However, if these conditions are treated, they are unlikely to cause weight problems.

Medications and weight gain

Certain medications can make it easier to gain weight. These include: 

  • Anticonvulsants and mood stabilisers eg gabapentin 
  • Atypical antipsychotics eg olanzapine, quetiapine and risperidone
  • Certain antidepressants
  • Glucocorticoids for treating rheumatoid arthritis 
  • Hypoglycaemic medications eg tolbutamide 

Risks of obesity

Day-to-day problems caused by obesity

The day-to-day risks of obesity are caused by the extra strain put on your bones, joints and organs. They include: 

  • Breathlessness
  • Difficulty performing physical activities and often feeling extremely tired
  • Increased sweating
  • Joint and back pain 
  • Low confidence and self-esteem and feeling isolated
  • Snoring

Serious health conditions caused by obesity

Obesity causes high blood pressure, high cholesterol and atherosclerosis (fatty deposits that narrow your arteries). These conditions all increase your risk of coronary heart disease and stroke. 

Obesity also increases your risk of certain diseases, including: 

  • Cancer — this includes bowel, breast and womb cancer
  • Gastro-oesophageal reflux disease (GORD) — frequent acid reflux and heartburn
  • Liver and kidney disease
  • Metabolic syndrome — a combination of diabetes, high blood pressure and obesity 
  • Osteoarthritis — pain and stiffness in your joints
  • Type 2 diabetes 

If you are obese you are also at greater risk of developing: 

  • Asthma 
  • Gallstones 
  • Inflammation that damages your cells
  • Sleep apnoea — interrupted breathing while you sleep; this can cause fatigue and increases your risk of diabetes, high blood pressure and heart disease

Obesity can also decrease your fertility. If you are pregnant and obese, you are at greater risk of complications during your pregnancy, including gestational diabetes and pre-eclampsia (a dangerous increase in your blood pressure). 

Overall, obesity reduces your life expectancy by, on average, three to 10 years, depending on its severity. Being overweight or obese is estimated to contribute to at least one in every 13 deaths in Europe. 

Even if you are unable to reach a healthy weight, losing any amount of excess weight and maintaining that weight loss will reduce your risk of obesity-related health conditions. 

Common treatments for obesity


Your GP will suggest you follow a healthy, balanced reduced-calorie diet. They may refer you to a dietitian to help you plan your meals. This will involve avoiding high-fat, high-sugar and high-salt foods — salt can increase your blood pressure, which is particularly dangerous if you are obese. 

You will also need to check how many calories each of your meals contain so you do not exceed your daily limit — this includes calories in both food and drink. Make sure you eat slowly as this will help you feel full by the end of your meal. 

Take extra care when eating out at restaurant meals or having take-out meals, as they can contain a high amount of calories. 

Losing excess weight can get harder because once you have lost around 10 kg, your body's metabolism will slow down. This means you will burn fewer calories during physical exercise. It is therefore important to prevent weight gain and seek help from healthcare professionals and support groups when you are trying to lose weight. 


Physical activity will help you lose excess weight and maintain that weight loss. It also reduces your risk of several other serious health conditions, including type 2 diabetes. 

Your GP may suggest you join a local weight loss group, exercise class or gym to help you stay motivated. 

You can also try moderate-intensity activities, such as brisk walking, jogging, swimming or tennis for at least two and half hours and up to five hours every week. Alternatively, you can try one hour and 15 minutes of vigorous-intensity activity every week or a combination of moderate and vigorous activity. 

Other useful strategies

  • Avoiding situations where you think you will be tempted to overeat
  • Being more aware of what and when you are eating — it is easier to overeat when you are distracted eg watching TV
  • Getting professional psychological support to help you change your relationship with food for the better
  • Involving your family and friends with your weight loss plans — they can provide emotional support and may also join you in eating healthier meals or exercising
  • Recording your weight regularly to help track your progress
  • Setting realistic weight loss goals 


If it is very challenging for you to lose weight, your GP may prescribe the medication orlistat to help. 

Orlistat reduces how much fat you absorb by about a third when digesting your food. The fat which is not absorbed will pass out of your body via your stools. This approach may help you avoid gaining further weight but may not help you lose weight. 


Your doctor may suggest you have weight loss (bariatric) surgery such as a gastric bypass or gastric sleeve if you: 

  • Are morbidly obese ie have a BMI of 40 or higher 
  • Are moderately obese ie have a BMI of 35–39.9 and have a serious health condition that may improve with weight loss
  • Are obese and have been unable to lose weight by making changes to your diet and lifestyle

You will only be able to go ahead with the surgery if you are: 

  • Medically fit for surgery and anaesthetic
  • Willing to adopt a healthy diet and lifestyle after your surgery 
  • Willing to attend long-term follow-up appointments 

Frequently asked questions

Is obesity an eating disorder?

Obesity is not an eating disorder, although there may be psychological aspects to why an individual has developed unhealthy eating habits that have caused excess weight gain.

What weight is classed as obese?

There is no specific weight that is classed as obese. Obesity depends on both your weight and height. You can get an indication of whether or not you are obese by calculating your body mass index (BMI) — this is your weight in kilograms divided by the square of your height in metres. Having a BMI greater than 25 means you are overweight and a BMI greater than 30 means you are obese.

How does obesity cause cancer?

Obesity causes your body to develop excess body fat. This fat sends out signals to the rest of your body eg by releasing fat or chemicals into your blood. This affects other cells and can cause them to divide more, which may lead to cancer. Excess fat can cause increases in the levels of insulin, growth factors, inflammatory chemicals and oestrogen in your body, all of which may trigger cells to divide more.

How does obesity cause diabetes?

The exact mechanism behind how obesity causes diabetes is not fully understood. However, research suggests that in obese people, fat cells are more likely to release chemicals that cause inflammation. This can make your body less sensitive to insulin, which causes type 2 diabetes. Obesity may also change your body's metabolism, causing your fat cells to release fat into your blood, which causes cells to become less sensitive to insulin.

What body fat percentage is obese?

A body fat percentage that is 32% or more is considered obese.

Can obesity cause low oxygen levels?

Obesity puts you at greater risk of heart disease and heart failure, which can reduce oxygen levels in your blood. If your obesity causes sleep apnoea, this can also cause your oxygen levels to drop when you sleep.