In 2015, 600 million adults (12%) and 100 million children were obese in 195 countries. Obesity is more common in women than in men. Public health officials view it as one of the most serious public health problems of the 21st century. Today, obesity is stigmatized in most of the world. Conversely, some cultures, past and present, have a favorable view of obesity, seeing it as a symbol of wealth and fertility. Nevertheless, in 2013, several medical societies, including the American Medical Association and the American Heart Association, classified obesity as a disease. Classification
Obesity is typically defined as a substantial accumulation of body fat that could impact health. Medical organizations tend to classify people as obese based on body mass index (BMI) – a ratio of a persons weight in kilograms to the square of their height in meters. For adults, the World Health Organization (WHO) defines overweight as a BMI 25 or higher, and obese as a BMI 30 or higher. For children, obesity measures take age into consideration along with height and weight. For children aged 5–19, the WHO defines obesity as a BMI two standard deviations above the median for their age (a BMI around 18 for a five-year old; around 30 for a 19-year old). For children under five, the WHO defines obesity as a weight three standard deviations above the median for their height.
Those who quit smoking gain an average of 4.4 kilograms (9.7 lb) for men and 5.0 kilograms (11.0 lb) for women over ten years. However, changing rates of smoking have had little effect on the overall rates of obesity.In the United States, the number of children a person has is related to their risk of obesity. A womans risk increases by 7% per child, while a mans risk increases by 4% per child. This could be partly explained by the fact that having dependent children decreases physical activity in Western parents.In the developing world urbanization is playing a role in increasing rate of obesity. In China overall rates of obesity are below 5%; however, in some cities rates of obesity are greater than 20%. In part, this may be because of urban design issues (such as inadequate public spaces for physical activity). Time spent in motor vehicles, as opposed to active transportation options such as cycling or walking, is correlated with increased risk of obesity.Malnutrition in early life is believed to play a role in the rising rates of obesity in the developing world. Endocrine changes that occur during periods of malnutrition may promote the storage of fat once more food energy becomes available. Gut bacteria
The study of the effect of infectious agents on metabolism is still in its early stages. Gut flora has been shown to differ between lean and obese people. There is an indication that gut flora can affect the metabolic potential.