Clinicians typically use factors such as the body mass index to define obesity. The BMI is a person's weight divided by the square of that person's height, and is typically measured in units of kilograms per square meter. Clinicians use the absolute BMI to measure obesity in adults, but use a relative value of the BMI to measure obesity in children. For example, the World Health Organization considers adults with a body mass greater than 30 kg/m^2 to be obese. Children with an index in the top five percent of their gender and age group may also be considered obese.
The WHO classifies a BMI of at least 30 kg/m^2 but less than 35 kg/m^2 as class I obesity. A reading of at least 35 but less than 40 is class II obesity and class III obesity is a BMI of at least 40.
Experts often subdivide class III obesity into further categories, such that a classification of at least 40 but less than 50 is morbid obesity and a BMI of at least 50 is super obesity.
The health of Asians is generally affected at a lower BMI than that of Caucasians, causing some Asian countries to use different standards than the WHO. For example, Japan, defines obesity as greater than 25, while China sets the threshold for obesity at 28.
Obesity is one of the leading causes of preventable death throughout the world. Health professionals view it as a serious health problem due to its increasing prevalence in most countries. A 2005 report in The Lancet concluded that obesity reduces a person's life expectancy by about 6.5 years, on average. The study also concluded that obesity in the United States causes between 112,000 and 365,000 deaths each year.
A large-scale study published in a 2010 issue of the New England Journal of Medicine showed that non-smokers with a BMI between 20 and 25 kg/m^2 had the lowest mortality risk. The lowest risk for smokers was between 24 and 27. A 1995 study in the NEJM showed that women with an index greater than 32 had double the normal mortality rate.
Obesity has a strong correlation with heart diseases such as myocardial infarction and angina. A 2002 report from the WHO showed that 21 percent of all ischemic heart disease can be attributed to obesity. A BMI above 30 kg/m^2 doubles your risk of heart failure, according to a 2002 study in the NEJM. The 2005 Lancet study on obesity showed that 85 percent the people with blood hypertension have a BMI above 25 kg/m^2. The risk of hypertension is five times higher in obese people, compared to those of normal body weight. Obesity also increases the risk of venous thromboembolism by a factor of 2.3, according to a study in a 2008 issue of Circulation.
Type II diabetes is also strongly linked to obesity. It is the cause of 64 percent of diabetes cases in men and 77 percent of diabetes cases in women, according to Clinical Obesity in Adults and Children, a study published in 2005. Polycystic ovarian syndrome is also associated with obesity and 60 percent of patients with PCOS in the United States have a BMI greater than 30 kg/m^2, according to The Journal of Clinical Endocrinology & Metabolism.
Obesity increases estrogen production, which increases infertility rates for men and women. Excess estrogen reduces sperm production in men and impairs ovulation in women. A 2005 study on obesity estimated that six percent of infertility cases are caused by obesity. A 2008 study in Obesity Reviews also established a correlation between obesity and complications during pregnancy such as hemorrhaging, infections and increased care requirements for the infant.