Being overweight or fat is having more body fat than is optimally healthy. Being overweight is especially common where food supplies are plentiful and lifestyles are sedentary. As of 2003, excess weight reached epidemic proportions globally, with more than 1 billion adults being either overweight or obese. 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; however, the accumulation of too much storage fat can impair movement, flexibility, and alter the appearance of the body. Classification
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 29.9 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 and 29.9. There are, however, several other common ways to measure the amount of adiposity or fat present in an individuals body. Body mass indexThe body mass index (BMI) is a measure of a persons weight taking into account their height. It is given by the following formula: BMI equals a persons weight (mass) in kilograms divided by the square of the persons height in metres.
Katherine Flegal et al., however, found that the mortality rate for individuals who are classified as overweight (BMI 25 to 29.9) may actually be lower than for those with an "ideal" weight (BMI 18.5 to 24.9), noting that many studies show that the lowest mortality rate is at a BMI close to 25. The specific conclusions appear to depend on what other factors are controlled for, and Flegal has accordingly alleged that the findings from the 2016 review are driven by bias toward preconceived opinions.Being overweight has been identified as a risk factor for cancer, and Walter Willett predicts that being overweight will overtake smoking as the primary cause of cancer in developed countries as cases of smoking-related cancer dwindle. Being overweight also increases the risk of oligospermia and azoospermia in men.Psychological well-being is also at risk in the overweight individual due to social discrimination. Being overweight has been shown not to increase mortality in older people: in a study of 70 to 75-year old Australians, mortality was lowest for "overweight" individuals (BMI 25 to 29.9), while a study of Koreans found that, among those initially aged 65 or more, an increase in BMI to above 25 was not associated with increased risk of death. Causes
Being overweight is generally caused by the intake of more calories (by eating) than are expended by the body (by exercise and everyday activity). Factors that may contribute to this imbalance include:

Alcoholism
Eating disorders (such as binge eating)
Genetic predisposition
Hormonal imbalances (e.g.