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. Overweight and obesity are defined as an excess body weight for height. Men have a body fat percentage of 15–20% and women have approximately 25–30%, but  the body mass index (BMI) is used far more commonly than body fat percentage to define obesity. The BMI of a normal weight adult is between 20.0 and 24.9. If someone is overweight, their BMI will be between 25.0 and 29.9. Obesity is defined as having a BMI of between 30.0 and 39.9. Severe (or morbid) obesity is defined as a BMI greater than or equal to 40. 

What causes obesity?

The causes of obesity are far more complicated than the simple imbalance between energy intake and energy output. There are many factors to be considered, including a combination of genes, environment and behaviour. 

Genes can affect the amounts and types of food that we prefer to eat. We all know that some people stay thin whatever they seem to eat and some become obese. 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. 

Our environment affects the amounts and types of food that are available to eat. Portion sizes are getting bigger, usually at very little extra cost. More people are eating pre-packaged food, fast food and soft drinks, which are often high in calories, fat, salt and sugar. We are also living more inactive lifestyles. The use of cars is increasing and inactive pastimes, such as watching television and surfing the internet, are becoming more popular.

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.

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), ovary-syndrome/'>polycystic ovary syndrome, reduced sex hormones (such as in the menopause), 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 symptoms are if someone is carrying excess weight and has a BMI greater than 30. Other non-specific symptoms can include tiredness and fatigue. Depression may be a consequence or a cause of excessive dietary intake and reduced activity. Almost 30% of patients who are obese have eating disorders such as bingeing or food-seeking behaviour.

BMI and waist and hip circumference are useful ways of assessing weight. Skin changes can also occur, and include excess hair growth and darker, thicker skin around the armpits and neck. Knee and spinal osteoarthritis are also more common in obesity. 

How common is obesity?

Obesity is becoming more and more common, having tripled in the last 25 years in the UK. In England, the percentage of men who are obese has risen from 14% in 1994 to 25% in 2006, and for women from 19% in 1994 to 29% in 2006. These increases in obesity are also being seen among children. Between 1995 and 2006, the percentage of obesity among English boys increased from 11% to 17%, and from 12% to 15% among English girls.  

Is obesity inherited?

Given that many children of obese parents become obese themselves and both identical twins tend to be obese rather than just one, there does seem to be a genetic cause for obesity. 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.      

How is obesity diagnosed?

The World Health Organization criteria for obesity are based on BMI. BMI is calculated by your weight in kilograms divided by height in metres squared (kg/m2). For adults, overweight is a BMI of 25.0–29.9. Obesity is a BMI of 30.0–39.9 kg/m2. Severe or morbid obesity is defined as a BMI greater than or equal to 40 kg/m2

X-ray of the upper gastroduodenal tract of a 26-year-old female patient, showing resection (removal) of two-thirds of the stomach after bariatric surgery to treat obesity. The arrow indicates the new junction between the oesophagus and reduced stomach.

X-ray of the upper gastroduodenal tract of a 26-year-old female patient, showing resection (removal) of two-thirds of the stomach after bariatric surgery to treat obesity. The arrow indicates the new junction between the oesophagus and reduced stomach.

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 calorie intake is still the most important way of achieving long-lasting weight loss.
  2. Exercise programmes – aerobic exercise (such as brisk walking or swimming) is of the greatest value for individuals who are obese. The ultimate minimum goal should be to achieve 30–60 minutes of continuous aerobic exercise at least five times per week. Exercise is vital to any weight management programme because it helps build muscle mass, increase metabolic activity and improve general health.
  3. Public policy – although management of obesity in the individual is important, obesity is also a public health problem. Educating the public regarding healthy eating as well as encouraging regular participation in exercise and outdoor activities is important.
  4. Very low calorie diets – these involve reducing calorie intake to 800 calories per day or fewer. They are only suitable for treatment in people who are very obese and should only be carried out under medical supervision. When used in the right conditions, they can achieve weight loss of 1.5–2.5 kg per week, with a total loss of as much as 20 kg over 12 weeks. Although these diets can lead to a significant short-term weight loss, it is extremely difficult to maintain the diet beyond a few weeks. Close supervision is needed to avoid breaking the diet. Most people often quickly regain all the weight they lose and often gain more.
  5. 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.
  6. Medications – not many medications are available for the treatment of obesity, and those that are available have minimal long-term effectiveness. In the UK, the only currently available anti-obesity drug is orlistat. Orlistat blocks the action of pancreatic enzymes, reducing fat absorption from the gut. Trials have shown weight loss of 9–10% over two years, although this is often regained on stopping the medication (unless lifestyle changes – such as increased exercise and decreased food intake – have occurred).
  7. Surgical care – surgical therapy for obesity (bariatric surgery) is only proven to be linked with sustained weight loss in people with unhealthy severe obesity or obesity associated with other conditions. Evidence shows that well-performed bariatric surgery in carefully selected patients with good multidisciplinary support (support from a team of specialists) substantially improves the problems linked with severe obesity. Therefore, although bariatric surgery is the only therapeutic method associated with consistently proven sustained weight loss, it is not the answer for everybody.

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.

Bariatric surgery has a risk associated with it, between a 0.2% and 1.0% mortality rate, depending on the type of procedure undertaken.  

What are the longer-term implications of obesity?

Obesity is associated with many longer-term consequences, which have a negative impact on quality of life and life expectancy. These include conditions such as:

  • type 2 diabetes
  • reduced mobility
  • high blood pressure, heart disease and stroke
  • cancers such as endometrial, prostate, gall bladder, breast, colon and pancreas
  • depression
  • obstructive sleep apnoea
  • reduced fertility
  • osteoarthritis
  • fatty liver and reflux oesophagitis.

Last reviewed: Feb 2018