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.
Obesity is a medical condition, sometimes considered a disease, in which abnormal or excess body fat has accumulated to such an extent that it may have a negative effect on health. People are classified as obese when their body mass index (BMI)—a measurement obtained by dividing a persons weight by the square of the persons height (despite known allometric inaccuracies)—is over 30 kg/m2; the range 25–30 kg/m2 is defined as overweight. Some East Asian countries use lower values to calculate obesity.Obesity is a major cause of disability and is correlated with various diseases and conditions, particularly cardiovascular diseases, type 2 diabetes, obstructive sleep apnea, certain types of cancer, and osteoarthritis. High BMI is a marker of risk for, but not a direct cause of, diseases caused by diet and physical activity. A reciprocal link has been found between obesity and depression, with obesity increasing the risk of clinical depression, and also depression leading to a higher chance of developing obesity.Obesity has individual, socioeconomic, and environmental causes. Some of the known causes are diet, physical activity, automation, urbanization, genetic susceptibility, medications, mental disorders, economic policies, endocrine disorders, and exposure to endocrine-disrupting chemicals. Epidemiologic studies of overweight and obesity in children and adults covering 195 countries have shown that the prevalence of obesity has steadily increased in most countries, doubling in 73 countries between the years 1980 and 2015. As of 2015, the United States and China had the largest numbers of obese adults, and China and India had the largest numbers of obese children.