The degree to which a person is overweight is generally described by the body mass index (BMI). "Overweight" is defined as a BMI of 25 or more, thus it includes pre-obesity defined as a BMI between 25 and 30 and obesity as defined by a BMI of 30 or more. Pre-obese and overweight however are often used interchangeably, thus giving overweight a common definition of a BMI of between 25–30. There are, however, several other common ways to measure the amount of adiposity or fat present in an individual's body.

- Body mass index

- Body volume index

- Simple weighing

- Skinfold calipers or "pinch test"

- Bioelectrical impedance analysis

- Hydrostatic weighing

- Dual-energy X-ray absorptiometry (DEXA)

The most common method for discussing this subject and the one used primarily by researchers and advisory institutions is BMI. Definitions of what is considered overweight vary by ethnicity. The current definition proposed by the US National Institutes of Health (NIH) and the World Health Organization (WHO) designates whites, Hispanics and blacks with a BMI of 25 or more as overweight. For Asians, overweight is a BMI between 23 and 29.9 and obesity for all groups is a BMI of 30 or more.

BMI, however, does not account extremes of muscle mass, some rare genetic factors, the very young, and a few other individual variations. Thus it is possible for an individuals with a BMI of less than 25 to have excess body fat, while others may have a BMI that is significantly higher without falling into this category. Some of the above methods for determining body fat are more accurate than BMI but come with added complexity.

If an individual is overweight and has excess body fat it can create or lead to health risks. Reports are surfacing, however, that being mildly overweight to slightly obese – BMI being between 24 and 31.9 – may be actually beneficial and that people with a BMI between 24 and 31.9 could actually live longer than normal weight or underweight persons.

Being overweight is having more body fat than is optimally healthy. Being overweight is especially common where food supplies are plentiful and lifestyles are sedentary.

Excess weight has reached epidemic proportions globally, with more than 1 billion adults being either overweight or obese in 2003. In 2013 this increased to more than 2 billion. Increases have been observed across all age groups.

A healthy body requires a minimum amount of fat for proper functioning of the hormonal, reproductive, and immune systems, as thermal insulation, as shock absorption for sensitive areas, and as energy for future use. But the accumulation of too much storage fat can impair movement, flexibility, and alter the appearance of the body.

Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health. It is defined by body mass index (BMI) and further evaluated in terms of fat distribution via the waist–hip ratio and total cardiovascular risk factors. BMI is closely related to both percentage body fat and total body fat.

In children, a healthy weight varies with age and sex. Obesity in children and adolescents is defined not as an absolute number but in relation to a historical normal group, such that obesity is a BMI greater than the 95th percentile. The reference data on which these percentiles were based date from 1963 to 1994, and thus have not been affected by the recent increases in weight. BMI is defined as the subject's weight divided by the square of their height and is calculated as follows.

BMI is usually expressed in kilograms per square metre, resulting when weight is measured in kilograms and height in metres. To convert from pounds per square inch multiply by .

The most commonly used definitions, established by the World Health Organization (WHO) in 1997 and published in 2000, provide the values listed in the table.

Some modifications to the WHO definitions have been made by particular organizations. The surgical literature breaks down class II and III obesity into further categories whose exact values are still disputed.

- Any BMI ≥ 35 or 40 kg/m is "severe obesity".

- A BMI of ≥ 35 kg/m and experiencing obesity-related health conditions or ≥40–44.9 kg/m is "morbid obesity".

- A BMI of ≥ 45 or 50 kg/m is "super obesity".

As Asian populations develop negative health consequences at a lower BMI than Caucasians, some nations have redefined obesity; Japan have defined obesity as any BMI greater than 25 kg/m while China uses a BMI of greater than 28 kg/m.

Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have a negative effect on health. People are generally considered obese when their body mass index (BMI), a measurement obtained by dividing a person's weight by the square of the person's height, is over , with the range defined as overweight. Some East Asian countries use lower values. Obesity increases the likelihood of various diseases and conditions, particularly cardiovascular diseases, type 2 diabetes, obstructive sleep apnea, certain types of cancer, osteoarthritis and depression.

Obesity is most commonly caused by a combination of excessive food intake, lack of physical activity, and genetic susceptibility. A few cases are caused primarily by genes, endocrine disorders, medications, or mental disorder. The view that obese people eat little yet gain weight due to a slow metabolism is not generally supported. On average, obese people have a greater energy expenditure than their normal counterparts due to the energy required to maintain an increased body mass.

Obesity is mostly preventable through a combination of social changes and personal choices. Changes to diet and exercising are the main treatments. Diet quality can be improved by reducing the consumption of energy-dense foods, such as those high in fat and sugars, and by increasing the intake of dietary fiber. Medications may be used, along with a suitable diet, to reduce appetite or decrease fat absorption. If diet, exercise, and medication are not effective, a gastric balloon or surgery may be performed to reduce stomach volume or length of the intestines, leading to feeling full earlier or a reduced ability to absorb nutrients from food.

Obesity is a leading preventable cause of death worldwide, with increasing rates in adults and children. In 2015, 600 million adults (12%) and 100 million children were obese. Obesity is more common in women than men. Authorities view it as one of the most serious public health problems of the 21st century. Obesity is stigmatized in much of the modern world (particularly in the Western world), though it was seen as a symbol of wealth and fertility at other times in history and still is in some parts of the world. In 2013, the American Medical Association classified obesity as a disease.

Abdominal obesity, also known as central obesity, is when excessive abdominal fat around the stomach and abdomen has built up to the extent that it is likely to have a negative impact on health. There is a strong correlation between central obesity and cardiovascular disease. Abdominal obesity is not confined only to the elderly and obese subjects. Abdominal obesity has been linked to Alzheimer's disease as well as other metabolic and vascular diseases.

Visceral and central abdominal fat and waist circumference show a strong association with type 2 diabetes.

Visceral fat, also known as organ fat or "intra-abdominal fat", is located inside the peritoneal cavity, packed in between internal organs and torso, as opposed to subcutaneous fat, which is found underneath the skin, and intramuscular fat, which is found interspersed in skeletal muscle. Visceral fat is composed of several adipose depots including mesenteric, epididymal white adipose tissue (EWAT) and perirenal fat. An excess of visceral fat is known as central obesity, the "pot belly" or "beer belly" effect, in which the abdomen protrudes excessively. This body type is also known as "apple shaped", as opposed to "pear shaped", in which fat is deposited on the hips and buttocks.

Researchers first started to focus on abdominal obesity in the 1980s when they realized it had an important connection to cardiovascular disease, diabetes, and dyslipidemia. Abdominal obesity was more closely related with metabolic dysfunctions connected with cardiovascular disease than was general obesity. In the late 1980s and early 1990s insightful and powerful imaging techniques were discovered that would further help advance the understanding of the health risks associated with body fat accumulation. Techniques such as computed tomography and magnetic resonance imaging made it possible to categorize mass of adipose tissue located at the abdominal level into intra-abdominal fat and subcutaneous fat.

Lack of exercise along with sugar-sweetened foods and drinks have caused one out of six five-year-olds in Sweden to be overweight or obese. The breakdown is 12.9% of children are considered overweight and 4.3% are considered obese.

There are various ways of measuring abdominal obesity including:

- Absolute waist circumference (> in men and > in women)

- Waist–hip ratio (the circumference of the waist divided by that of the hips of >0.9 for men and >0.85 for women)

- Waist-to-height ratio

- Sagittal Abdominal Diameter

In those with a BMI under 35, intra-abdominal body fat is related to negative health outcomes independent of total body fat. Intra-abdominal or visceral fat has a particularly strong correlation with cardiovascular disease.

BMI and waist measurements are well recognized ways to characterize obesity. However, waist measurements are not as accurate as BMI measurements. For this reason, it is recommended to use both methods of measurements.

While central obesity can be obvious just by looking at the naked body (see the picture), the severity of central obesity is determined by taking waist and hip measurements. The absolute waist circumference in men and in women) and the waist-hip ratio (>0.9 for men and >0.85 for women) are both used as measures of central obesity. A differential diagnosis includes distinguishing central obesity from ascites and intestinal bloating. In the cohort of 15,000 people participating in the National Health and Nutrition Examination Survey (NHANES III), waist circumference explained obesity-related health risk better than the body mass index (or BMI) when metabolic syndrome was taken as an outcome measure and this difference was statistically significant. In other words, excessive waist circumference appears to be more of a risk factor for metabolic syndrome than BMI. Another measure of central obesity which has shown superiority to BMI in predicting cardiovascular disease risk is the Index of Central Obesity (waist-to-height ratio - WHtR), where a ratio of >=0.5 (i.e. a waist circumference at least half of the individual's height) is predictive of increased risk.

Another diagnosis of obesity is the analysis of intra-abdominal fat having the most risk to one's personal health. The increased amount of fat in this region relates to the higher levels of plasma lipid and lipoproteins as per studies mentioned by Eric Poehlman (1998) review.

An increasing acceptance of the importance of central obesity within the medical profession as an indicator of health risk has led to new developments in obesity diagnosis such as the Body Volume Index, which measures central obesity by measuring a person's body shape and their weight distribution. The effect of abdominal adiposity occurs not just in those who are obese, but also affects people who are non-obese and it also contributes to insulin sensitivity

Several studies have shown that obese men tend to have a lower sperm count, fewer rapidly mobile sperm and fewer progressively motile sperm compared to normal-weight men.

Obesity in North Africa and the Middle East is a notable health issue. In 2005, the World Health Organization measured that 1.6 billion people were overweight and 400 million were obese. It estimates that by the year 2015, 2.3 billion people will be overweight and 700 million will be obese. The Middle East, including the Arabian Peninsula, Eastern Mediterranean, Turkey and Iran, and North Africa, are no exception to the worldwide increase in obesity. Subsequently, some call this trend the New World Syndrome. The lifestyle changes associated with the discovery of oil and the subsequent increase in wealth is one contributing factor.

Urbanization has occurred rapidly and has been accompanied by new technologies that promote sedentary lifestyles. Due to accessibility of private cars, television, and household appliances, the population as a whole is engaging in less physical activity. The rise in caloric and fat intake in a region where exercise is not a defining part of the culture has added to the overall increased percentages of overweight and obese populations. In addition, women are more likely to be overweight or obese due to cultural norms and perceptions of appropriate female behavior and occupations inside and outside of the home.

The body mass index (BMI) does not capture information about percentage body fat (PBF), which is a better predictor of risk due to obesity.

The term "metabolically obese normal weight" (MONW) refers to people with normal weight and BMI, but who display some metabolic characteristics that may increase the risk of developing metabolic syndrome, in the same way as obesity. MONW subjects have excess visceral fat, and are predisposed to insulin resistance, hypertension and cardiovascular disease. These people show benefits from energy restriction and weight loss, for example a 4- to 12-week period of diet and exercise.

Some studies have suggested that the main factor which explains the metabolic abnormalities in MONW individuals is fat distribution. On the basis of these studies, a scoring method has been proposed to identify MONW individuals, based on the presence of associated diseases or biochemical abnormalities related to insulin resistance.

Obesity in Pakistan is a health issue that has attracted concern only in the past few years. Urbanisation and an unhealthy, energy-dense diet (the high presence of oil and fats in Pakistani cooking), as well as changing lifestyles, are among the root causes contributing to obesity in the country. According to a list of the world's "fattest countries" published on "Forbes", Pakistan is ranked 165 (out of 194 countries) in terms of its overweight population, with 22.2% of individuals over the age of 15 crossing the threshold of obesity. This ratio roughly corresponds with other studies, which state one-in-four Pakistani adults as being overweight.

Research indicates that people living in large cities in Pakistan are more exposed to the risks of obesity as compared to those in the rural countryside. Women also naturally have higher rates of obesity as compared to men. Pakistan also has the highest percentage of people with diabetes in South Asia.

According to one study, "fat" is more dangerous for South Asians than for Caucasians because the fat tends to cling to organs like the liver instead of the skin.

The medical condition of being overweight or obesity is defined as "abnormal or excessive fat accumulation that may impair health". It is measured through the Body Mass Index (BMI), defined as a person's weight, in kilograms, divided by the square of the person's height, in meters. If an individual has a BMI of 25–29, he or she is overweight. Having a BMI of 30 or more means an individual is obese. The greater the BMI, the greater the risk of chronic diseases as a result of obesity. These diseases include cardiovascular diseases, diabetes, musculoskeletal disorders, cancer, and premature death.

Normal weight obesity is the condition of having normal body weight, but with a high body fat percentage, leading to some of the same health risks as obesity.

The problem of diagnosing a person with the condition is hard as there is no reference value for the appropriate lean mass. This is because healthy people tend to show variations in their composition due to their body types.

The symptoms are basically the same as that of sarcopenia and obesity. The individual may show a BMI that is appropriate and healthy to his or her age but will look fat in appearance.

Obesity in China is a major health concern according to the WHO, with overall rates of obesity below 5% in the country, but greater than 20% in some cities. This is a dramatic change from times when China experienced famine as a result from ineffective agriculturalization plans such as the Great Leap Forward.

Currently, obesity in China is mostly confined to the cities where fast food culture and globalization have taken over, in comparison to poorer rural areas. Despite this concentration of obesity, the sheer size of China's population means that over one fifth of all one billion obese people in the world come from China.

Like many other medical conditions, obesity is the result of an interplay between environmental and genetic factors. Studies have identified variants in several genes that may contribute to weight gain and body fat distribution; although, only in a few cases are genes the primary cause of obesity.

Polymorphisms in various genes controlling appetite and metabolism predispose to obesity under certain dietary conditions. The percentage of obesity that can be attributed to genetics varies widely, depending on the population examined, from 6% to 85%. As of 2006, more than 41 sites on the human genome have been linked to the development of obesity when a favorable environment is present. The involvement of genetic factors in the development of obesity is estimated to be 40–70%. Some of these obesogenic or leptogenic genes may influence obese individuals response to weight loss or weight management.

The term "non-syndromic obesity" is sometimes used to exclude these conditions. In people with early-onset severe obesity (defined by an onset before 10 years of age and body mass index over three standard deviations above normal), 7% harbor a single locus mutation.

TOFI (thin-outside-fat-inside) is used to describe lean individuals with a disproportionate amount of fat (adipose tissue) stored in their abdomen. The figure to illustrate this shows two men, both 35 years old, with a BMI of 25 kg/m. Despite their similar size, the TOFI had 5.86 litres of internal fat, whilst the healthy control had only 1.65 litres.

Subjects defined as TOFI with body mass index (BMI) <25 kg/m have increased levels of many of the risk factors associated with the metabolic syndrome. This phenotype is a further refinement of “metabolically-obese normal-weight" (MONW).

Subjects defined as TOFI have been described as being at higher risk of developing insulin resistance and type II diabetes due to the fact that they have reduced physical activity/VOmax, reduced insulin sensitivity, higher abdominal adiposity, and a more atherogenic lipid profile. Another important characteristic observed in this cohort is elevated levels of liver fat. It is shown that overconsumption of fructose can lead to TOFI by inducing inflammation associated cortisol release.

To classify an individual as TOFI, it is essential to measure their internal fat content. This done by using magnetic resonance Imaging (MRI) or CT scanning. The parameters of the MRI scanner are manipulated to show fat as bright (white) and lean tissue as dark.

Indirect methods such as waist circumference are not suitable as individuals with an identical waist circumference can have vastly different levels of internal fat. The figure clearly shows that despite having an identical waist circumference (in this example all men had a waist of 84 cm), there is considerable variation in the amount of visceral fat (volumes shown on the image in litres) present.

People tend to gain weight in lipedematous areas and lose it in non-lipedematous areas, though there are cases where weight loss has resulted in improvement of the condition. Obese lipedema patients who undergo bariatric surgery lose fat primarily from the waist up. While lipedema presents itself in various ways, diagnosis is possible as early as pre-puberty when inner thigh pads present and at any age when fat gathers and drapes at knees.

Symptoms of lipedema include disproportionately large, column-like legs, legs unusually large to the knees, disproportionate hips, stomach or buttocks. As lipedema progresses, patients become increasingly heavy in the lower body. The additional, expanding fat cells interfere with the pathways of lymphatic vessels, and patients can develop secondary lymphedema, a condition known as lipo-lymphedema. Many lipedema patients cannot tolerate the compression garments associated with conventional lymphedema treatment because the underlying lipedematous fat is very painful, and those patients therefore are at risk for the side effects of uncontrolled lymphedema, including recurring blood infections and fibrosis. If not kept in check through a healthy lifestyle, lipo-lymphedema can worsen, and patients will become progressively less mobile.

There are often no visible symptoms of hyperinsulinemia unless hypoglycaemia (low blood sugar) is present.

Some patients may experience a variety of symptoms when hypoglycaemia is present, including:

- Temporary muscle weakness

- Brain fog

- Fatigue

- Temporary thought disorder, or inability to concentrate

- Visual problems such as blurred vision or double vision

- Headaches

- Shaking/Trembling

- Thirst

If a person experiences any of these symptoms, a visit to a qualified medical practitioner is advised, and diagnostic blood testing may be required.

These depend on poorly understood variations in individual biology and consequently may not be found with all people diagnosed with insulin resistance.

- Increased hunger

- Lethargy (tiredness)

- Brain fogginess and inability to focus

- High blood sugar

- Weight gain, fat storage, difficulty losing weight – for most people, excess weight is from high subcutaneous fat storage; the fat in IR is generally stored in and around abdominal organs in both males and females; it is currently suspected that hormones produced in that fat are a precipitating cause of insulin resistance

- Increased blood cholesterol levels

- Increased blood pressure; many people with hypertension are either diabetic or pre-diabetic and have elevated insulin levels due to insulin resistance; one of insulin's effects is to control arterial wall tension throughout the body

The clinical presentation is similar to people with congenital lipodystrophy: the only difference is that AGL patients are born with normal fat distribution and symptoms develop in childhood and adolescence years and rarely begins after 30 years of age. Females are more often affected than males, with ratio being 3:1.

The hallmark characteristics are widespread loss of subcutaneous fat, ectopic fat deposition, leptin deficiency, and severe metabolic abnormalities such as insulin resistance. Subcutaneous fat loss in AGL patients are visible in all parts of the body. AGL mostly affects face and the extremities and may look sunken or swollen in the eyes. However, the degree and location of severity may vary by person. Especially, intra-abdominal fat loss is variable. As subcutaneous fat is lost, affected areas show prominent structures of veins and muscle. Those with panniculitis-associated AGL may present erythematous nodules.

Metabolic complications include insulin resistance, high metabolic rate, and uncontrolled lipid levels such as hypertriglyceridemia, low HDL, and high LDL. Patients may develop diabetes mellitus secondary to insulin resistance.

Recent case reports reveled that lymphoma is present in some patients but its prevalence is not known at this time.

Possible causes include:

- Neoplasm

- Pancreatic cancer

- Polycystic ovary syndrome (PCOS)

- Trans fats