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Definition: obesity from Merriam-Webster's Collegiate(R) Dictionary

(ca. 1611) : a condition characterized by the excessive accumulation and storage of fat in the body

Summary Article: Obesity
from Black's Medical Dictionary, 42nd Edition

A condition in which the energy stores of the body (mainly fat) are too great. It is commonplace in prosperous countries and is increasing in developing countries, particularly amongst children and young people. Many believe that it represents the most serious public health threat of our age.


Whatever the causes, the fact remains that energy intake (in the form of food and drink) must have exceeded energy output (in the form of activity and exercise) over a sufficiently long period of time.

Obesity tends to run in families, leading to a suggestion that some people inherit a ‘thrifty’ gene which lowers their energy output and so puts them at risk of obesity in later life. However, although a genetic component is possible, it is unlikely to be great or to prevent weight loss from being feasible in most people if they reduce their energy intake. Environmental influences, such as eating habits, are believed to be more important in explaining the familial association in obesity.

An inactive lifestyle plays a minor role in the development of obesity, but it is unclear whether people are obese because they are inactive or are inactive because they are obese. Unfortunately, it can be possible to identify by the time of their first birthday many of the children destined to be obese. In the UK, childhood obesity increased from 10 to 14 per cent between 1995 and 2003, with 27 per cent classified as overweight by the later date.

Rarely, obesity has an endocrine basis and is caused by hypothyroidism (See under THYROID GLAND, DISEASES OF), HYPOPITUITARISM, HYPOGONADISM or CUSHING'S SYNDROME.


Obesity can make you ill and is even a cause of death in some, from conditions such as cardiovascular disease and non-insulin-dependent DIABETES MELLITUS. It may lead to diseases of the gall-bladder (see GALL-BLADDER, DISEASES OF), OSTEOARTHRITIS, HERNIA, GOUT and possibly certain cancers (i.e. colon, rectum and prostate in men, and breast, ovary, endometrium and cervix in women). Menstrual irregularities and ovulatory failure are often experienced by obese women. Obese people are also at greater risk when they undergo surgery. With the exception of gall-stone formation, weight loss will reduce these health risks.


The key is persistence over a prolonged period with a slimming diet. Eating 1,000 kcal/day less than one expends in energy (see CALORIE) will produce a loss of 1 kg of fat/week. Theoretically, this could also be achieved by increasing energy expenditure (exercise) but, in practise, this is relatively ineffective.

Anorectic drugs and bariatric surgery such as gastric stapling and jaw-wiring are sometimes used to treat severe obesity. The long-term effectiveness of surgery is still uncertain and it is debatable whether the modest reduction in weight achieved by use of anorectic drugs is worthwhile. The most effective is probably ORLISTAT; this reduces the amount of fat the gastrointestinal tract absorbs from food.


The BODY MASS INDEX, which relates weight in kilograms (W) to height2 in metres (H2), is a widely accepted way of classifying obesity according to severity. For example:

Grade of obesity

BMI (W/H2)







not obese


Other measurements are useful in assessing overall risk, in particular because excessive fat deposition around the waist seems to create the greatest risk for future health problems. The waist/hip ratio is, therefore, a relevant measurement to take.

Copyright © A & C Black Publishers Ltd

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