Obesity is a known risk factor for numerous health problems, including hypertension, high cholesterol, diabetes, cardiovascular diseases, respiratory problems (asthma), musculoskeletal diseases (arthritis) and some forms of cancer. At an individual level, several factors can lead to obesity, including excessive calorie consumption, lack of physical activity, genetic predisposition and disorders of the endocrine system. Because obesity is associated with higher risks of chronic illnesses, it is linked to significant additional health care costs.
The most frequently used measure of overweight and obesity is based on the body mass index (BMI), which is a single number that evaluates an individual's weight status in relation to his or her height (weight/height2, with weight in kilograms and height in meters). Based on the WHO current classification, adults with a BMI between 25 and 30 are defined as overweight, and those with a BMI over 30 as obese.
The BMI classification may not be suitable for all ethnic groups, who may be exposed to different levels of health risk for the same level of BMI. The thresholds for adults are also not suitable to measure overweight and obesity among children.
For most countries, data on obesity are self-reported through population-based health interviews. The exceptions are Australia, the Czech Republic (2005), Japan, Luxembourg, New Zealand, the Slovak Republic (2007), the United Kingdom and the United States; in these countries, data are derived from health examinations whereby actual measures are taken of people's height and weight. These differences in data collection methodologies seriously limit comparability of estimates across countries. Estimates of obesity based on health examinations are generally higher and more reliable than those coming from health interviews.
Half or more of the adult population is now defined as being either overweight or obese in 13 OECD countries: Mexico, the United States, the United Kingdom, Australia, Greece, New Zealand, Luxembourg, Hungary, the Czech Republic, Portugal, Ireland, Spain and Iceland. By comparison, overweight and obesity rates are much lower in Japan and Korea and in some European countries (France and Switzerland), although overweight and obesity rates are also increasing in these countries. The prevalence of obesity (which presents greater health risks than overweight) varies by a factor of ten among OECD countries, ranging from a low of 3% in Japan and Korea to over 30% in the United States and Mexico.
Based on consistent measures of obesity over time, the rate of obesity has more than doubled over the past twenty years in the United States, while it has almost tripled in Australia and more than tripled in the United Kingdom. Some 20 to 24% of adults in the United Kingdom, Australia, Iceland and Luxembourg are obese, about the same rate as the one prevailing in the United States in the early 1990s. Obesity rates in many European countries have increased substantially over the past decade.
In all countries, more men are overweight than women. However, in almost half of all OECD countries, more women are obese than men. Taking overweight and obesity together, the prevalence for women exceeds that for men only in Mexico and Turkey.