Obesity is an excess of calories stored in the form of fat. It is an increasing public health and medical problem associated with reduced quality and length of life.

What is obesity?

Obesity is a state of excess body fat that presents a risk to one’s health.

How is obesity diagnosed?

Rather than measuring body fat directly to define obesity, the approach most frequently used is a measurement called Body Mass Index (BMI). BMI is calculated using measurements of your weight in kilograms and height in metres. It tells us whether a person’s weight is appropriate for their height. Depending on your BMI you may be classified as being underweight, healthy weight, overweight or obese (see table). A diagnosis of obesity is considered when the BMI is more than 30 Kg/m2. BMI is a helpful measurement because it predicts your risk of developing obesity related health problems. However, BMI has some limitations for instance it does not directly measure body fat and can inappropriately classify people who are very muscular as being overweight and obese.


Body Mass Index (BMI)


<18.5 Kg/m2


18.5 to 24.9 Kg/m2


25.0 to 29.9 Kg/m2


>30.0 Kg/m2

(Based on WHO classification)

Although BMI is the main tool used to define obesity it does not provide information about the location or distribution of excess fat on a person’s body. In some people, excess fat is evenly distributed around the body while in others they store most of their excess fat in the middle portion of their body (abdomen).  Waist circumference (girth) is a very useful measurement that can describe fat distribution. This is important information because we know that storing excess fat in the middle portion of your body increases your risk of obesity related health problems when compared to storing it in other regions of the body.

What causes obesity?

In simple terms obesity develops when the number of calories (energy) we eat is greater than the number of calories that our body uses, when this happens the excess or unused energy is stored as fat. There are many factors that contribute to the imbalance between energy intake and energy output and it is usually a combination of these factors that lead to obesity. These include the following:

Genes - Research shows that obesity tends to run in families and studies with twins and adopted children have shown that genes play a key role in this. Genes can affect the amount and types of food that we prefer to eat, how much energy our bodies require and how fat is distributed around the body.

Environment - It is important to note that most of the increase in obesity rates has occurred over the last 50 years, during which time our genes have not changed significantly. This means that for most people, while their genes may drive their tendency to put on weight in a certain environment, it is the environment itself that has a greater influence, in particular changes in diet and activity levels.

Diet - The energy we put into our bodies is determined by the amounts and types of food that are available to eat. Portion sizes are getting bigger, usually at very little extra cost and there is increased use of convenient pre-packaged food, fast food and soft drinks, which are often high in calories. 

Inactivity - In general, we are living more inactive lifestyles than we did in the past which means we burn fewer calories. The increased use of cars, changes in work practices and inactive pastimes, such as watching television and surfing the internet mean that people require fewer calories. Of course, an individual’s activity level can also be limited by factors that are beyond their control such and pain and other physical disabilities.

Medical problems - There are some infrequent medical causes for obesity, although these are uncommon. Some of these include glandular or hormonal problems (an underactive thyroid, known as hypothyroidism),  reduced sex hormones (such as in the menopause), rare genetic causes (e.g. Prader-Willi syndrome) and some drugs (oral contraceptives, anti-psychotic drugs, anti-epileptics and steroids, and some diabetes medications). 

What are the signs and symptoms of obesity?

The most obvious sign of obesity is when someone is carrying excess weight and has a BMI greater than 30 Kg/m2.

How common is obesity?

In the United Kingdom, the most recent estimates indicate that 29% of adults (~1 in 4) and 20% of children (~1 in 5) are obese. Obesity rates have increased significantly over recent decades, this increase is due to changes in our environment over that time. This in not just a problem in the UK and rates of obesity are high worldwide. 

Is obesity inherited?

Differences in genes (DNA) between people can increase their risk of becoming obese. However, as previously mentioned, although genetic factors may drive an individual’s tendency to put on weight in a certain environment, it is the environment itself which has a greater influence. There are a number of rare cases of obesity that develop because of an abnormality in a specific gene, these individuals usually present as severe obesity that develops early in life.

How is obesity treated?

There are many different strategies that are put forward to treat obesity and people can find it very difficult to lose weight. Losing weight is a long-term commitment and individuals need to find a strategy that works for them and that they can maintain over time. Before making any significant changes to lifestyle, it is advisable to visit the GP. 

Obesity is treated in a number of different ways:

  1. Dietary programmes – achieving a reduction in calories (energy) intake is still the most important way of achieving long-lasting weight loss. By reducing the amount of energy that we eat our bodies will have to find another source of energy to meet its requirements. To overcome this the body will use the energy that is stored as fat which will reduce the amount of excess fat on a person’s body.
  1. Very low calorie diets (VLCD) – these involve reducing calorie intake to 800 calories per day or fewer. VLCDs include the replacement of normal diet with meal substitutes, these usually take the form of nutritionally balanced meal replacement products. Such short-term interventions (~12 weeks) are very effective at achieving weight loss and are followed by a food reintroduction phase. These diets should be carried out under medical supervision.

  2. Exercise programmes – Exercise is vital to any weight management programme because it helps build muscle mass, increase metabolic activity and improve general health. Aerobic exercise (such as brisk walking or swimming) is of the greatest value for individuals who are obese. The minimum goal should be to achieve 30–60 minutes of continuous aerobic exercise at least five times per week.  
  1. Behavioural changes – this requires a trained professional to have an in-depth discussion with the person regarding the changes required, such as non-meal eating and snacking. The success depends on both a highly motivated person and a dedicated counsellor who is willing to maintain long-term follow-up.

  2. Medications – The most widely used anti-obesity drug in the UK is Orlistat. Orlistat is a capsule to be taken at mealtimes that blocks the action of pancreatic enzymes (lipases), this reduces the absorption of dietary fat from the gut. Treatment with Orlistat is indicated in individuals who have a BMI less than 30 Kg/m2 or more than 28 Kg/m2 with weight related complications. It should be combined with diet and exercise interventions. Other medical therapies that are licenced in Europe include Liraglutide (Saxenda) which is a daily injectable therapy and naltrexone with bupropion which is used in tablet form. These medications most likely induce weight loss by reducing appetite and food intake.

  3. Surgical care – surgery for obesity (bariatric surgery) is a very effective treatment for obesity and results in a significant and sustained reduction in weight. In the United Kingdom, individuals with the following indications may be considered for surgery:

    • BMI > 40 kg/m2
    • BMI > 35 kg/m2 and obesity related health complications
    • BMI > 30 kg/m2 and a recent diagnosis of type 2 diabetes mellitus

    However, not all individuals who fulfil these criteria will be suitable for bariatric surgery and should be assessed by a multidisciplinary team in order to determine if surgery is appropriate and safe. The two most commonly used bariatric surgical procedures are a sleeve gastrectomy and a Roux-en-Y gastric bypass (RYGB). Sleeve gastrectomy involves the removal of a large part of the stomach, leaving behind a gastric sleeve which is much smaller in size than the stomach. The length of the gut is not reduced by this procedure (see diagram). RYGB is a surgical procedure where the stomach is reduced to the size of a small pouch. The small intestine is joined to the pouch, allowing the remaining stomach to be bypassed. This will shorten the length of the gut. The type of surgery undertaken is a decision made in consultation with the surgeon and multidisciplinary team. Although the surgeries are quite different both are effective at reducing hunger and increasing the feeling of fullness which should reduce the amount of calories that are eaten and enable weight loss.

Are there any side-effects to the treatment?

Many of the treatments for obesity involve having a healthy, balanced and active lifestyle, all of which impact positively on health

VLCD’s are generally well tolerated however, in addition to hunger, some people experience altered bowel habit, cramps and dizziness.

Orlistat may be associated with flatulence (wind) and abdominal discomfort. It may also change the consistency and frequency of bowel motions.

Bariatric surgery has a risk associated with it, between a 0.1% and 0.5% mortality rate, depending on the type of procedure undertaken. Patients are at risk of developing deficiencies in vitamin and minerals after the operation, these are usually prevented by monitoring and supplements, this is mostly seen with RYGB. In the case of sleeve gastrectomy patients may develop acid reflux after the surgery. Following either surgery, some patients can develop episodes of hypoglycaemia (low blood sugar) after meals or dumping syndrome which is the development of symptoms of nausea, sweating, diarrhoea or weakness after eating. All patients who undergo bariatric surgery require long-term follow-up.

What are the longer-term implications of obesity?

Obesity is associated with a range of serious health problems, which have a negative impact on quality of life and life expectancy. These include conditions such as:

Metabolic and hormonal complications

  • Type 2 diabetes
  • Unhealthy cholesterol profile
  • High blood pressure,
  • Heart disease
  • Stroke
  • Cancers such as endometrial, prostate, gall bladder, breast, colon and pancreas
  • Non-alcoholic fatty liver disease (NAFLD)
  • Reduced fertility
  • Erectile dysfunction

Mechanical complications

  • Reflux oesophagitis
  • Obstructive sleep apnoea
  • Osteoarthritis
  • Reduced mobility

Psychological complications

  • Depression
  • Social isolation

Last reviewed: Jul 2019