Overeating and lack of physical activity are the main causes of obesity, especially in combination. But many factors contribute to obesity.
The number of overfed people on the planet is at its highest level. For the first time in history (year 2000) the world's total of overweight people equals the total of those who are underfed. Up to 1.2 billion people eat more than they need, and as many go hungry each day.
Obesity is a chronic disease. It develops over a period of years. Obesity becomes a health problem when it leads to disability, illness or increased risk of death, and these effects are difficult to separate or quantify. Some of the consequences of obesity are controversial. However, serious overweight appears to reduce life expectancy. Obesity strongly predisposes to maturity-onset (type 2) diabetes because it can lead to insulin resistance and glucose intolerance, conditions that often precede the onset of type 2 diabetes. The evidence linking it to hypertension is somewhat controversial but obesity is considered to be a major risk factor for coronary heart disease. It also causes disabilities, including osteoarthritis of weight-bearing joints, musculo-skeletal complaints such as backache, potential loss of vision (via diabetes), varicose veins, breathing difficulties (including sleep apnoea), increased surgical, obstetric and cancer risks (liver, kidney, oesophagus, gallbladder, pancreas, colon and rectum, cervix, ovary, uterus, breast (in postmenopausal women), prostate, and stomach (in men) as well as non-Hodgkin's lymphoma and multiple myeloma), and numerous psycho-social problems. Overweight and obese people reportedly have slower cognitive abilities, increased pain and limited mobility.
Generally, obesity was recognized as a serious health problem during the 1970s in industrialized countries and among affluent groups elsewhere, and its prevalence appears to be increasing in most western-style countries. The average body weights in industrialized countries have increased by 4 to 9 kg over the last 30 years.
In 1995 in Europe, the incidence of overweight people was around 33%, and had risen steadily since 1980. Cultural differences may be marked: Eastern Europe, Britain, Benelux and Germany have higher proportions of overweight people than the Mediterranean countries, Scandinavia, and Finland. In Czechoslovakia there was a striking increase in the incidence of obesity in men and women between 1956 and 1972. In 1996, there were more overweight men (one in four) than women (slightly over one in five) in the European Union as a whole. The record in this respect was held by Greek men. In the UK, body fat increased by about 10% in all age groups over the last four decades of the twentieth century. In 1991, in the UK the percentage of obese men and women had risen to 13% and 15% respectively, compared to 7% and 12% in 1987; there was a 30% increase in individual obesity in the last two decades. This trend may be encouraged by the ever larger official recommended weights for people that doctors and insurers accept.
People in non-developed countries are also gaining weight. China, where famine recurs in history, is an example. Since 1993, the proportion of overweight men in China tripled. In 2001, fifteen percent of China's adult population was overweight.
One national survey in Canada estimated that more than 60% of people of both sexes aged over forty were overweight. This was called middle-age spread. Another estimate made in 2001 is that one out of every seven Canadians is obese. This survey found that weight seems to have a bigger impact on women than on men. Women view their own health state as poor if they are overweight while men perceive their health to be compromised once they become morbid obese. Men rated the severity of obesity alongside migraines, but women said it is more serious than being diabetic or having a stroke.
Americans are the heaviest society in the world, and probably in the history of the world. The incidence figures for overweight vary greatly, from 30% to 60% of the population, depending on the source and the definition of overweight. Undeniable is the recent massive rate of increase in overweight, a jump of nearly 50% from 1991 to 1998. According to the Centers for Disease Control and Prevention in 2001, more than 50% of American adults are overweight and approximately 20% are obese. A government estimate in 1999 was that more than 60% of adults were overweight or obese. These numbers translate into at least 70 million obese people, including more than a third of all adults and one in five children. One in 200 people are morbidly overweight.
At the same time, only 39% of US adults considered themselves overweight. Women are affected slightly more than men. Mexican-American women and African-American women have a higher weight than the average for American women; American males show little variation for race. Nearly 30% of patients diagnosed as overweight in 2000 have been 35 years old or younger. Between 1989 to 1999, the obesity rate among Americans aged 18 to 29 more than doubled from 5 percent to 12 percent.
In 1988 the US Surgeon General declared being overweight the most significant nutrition problem in the USA, and it has remained so. Obesity and related conditions were the second leading preventable cause of death in the USA in 1997, according to some medical experts. As of 1998, an estimated 300,000 people die prematurely each year of conditions brought on by obesity, but this figure is contested. The weight problem is estimated to cost the health care industry an estimated $70 billion annually. In 1995, one third of the women's cancer deaths in one study, particularly breast, colon and endometrial cancers, were due to being overweight. Implicated were bile acids from dietary fat and oestrogen in the body fat.
In the Netherlands in 1997, the medical costs for overweight patients were estimated to be $500 million per year.
Dietary changes may be responsible for rising obesity in some countries in the South Pacific region. Especially the consumption of refined carbohydrates has increased, and the incidence of hypertension, diabetes and gout rose in parallel.
The increase in seat size and strength in cars and public transport over the past 20 years tacitly acknowledges the increased average size of people.