Definitions on This Page
The Body Mass Index (BMI) was originally designed in the 1800s as a way to show the various proportions of the human build. By the 1970s, the BMI evolved into a quick and inexpensive way for epidemiologists to classify participants based on body mass in public health studies. In 1986, the National Institute of Health started encouraging its use for defining “obesity”
The BMI is calculated by dividing an individual’s body weight in kilograms by their height in squared meters. It is a measure of total body mass. It does not calculate a person's body fat, nor does it take into consideration a person's natural set point range. The assumption that obesity directly causes disability and death has led to the belief that the higher your BMI, the higher your health risk. However, there is no consistent or reliable data to show that losing weight and lowering one’s BMI actually decreases risk of disease or increases life expectancy.
The current guidelines that define a "normal" BMI were lowered based on recommendations set by the Obesity Task Force in 1998; however these recommendations were not supported by research evidence. It is important to note that weight gain, or increases in BMI, over time may be normal and healthy. Not everyone labeled "overweight" or "obese" is unhealthy and needs to lose weight, and not everyone labeled "normal" is healthy. Since this tool does not reveal the individual's level of fitness, their quality of life, or the quality of their dietary habits, it is not effective at measuring an individual’s health.
The label of “obesity” is problematically defined and can be stigmatizing for the individuals it is associated with. The World Health Organization defines "obesity" as a BMI of 30 and above. Please see Body Mass Index (BMI) for information on its limitations. It is assumed that all people who are “obese”, as per BMI standards, have a higher risk of death. However, an association between obesity and increased mortality is actually clustered in the extreme obesity range (BMI 35 and above), which is higher than where most people's BMI falls. In addition, government statistics have indicated that obesity rates have stabilized and leveled off between 1999 and 2003, while life expectancy continues to increase. Therefore, we can conclude that "obesity crises" may be exaggerated.
One of the more harmful effects of people being labeled "obese" is often the prescribed recommendation to lose weight. Attempts to lose weight have been associated with lean tissue loss, a higher risk of preoccupation with food and eating disorders, and other psychological consequences. Often people use “diets” in an effort to lose weight despite newer research showing dieting can predict weight gain; in fact, two-thirds of those who lose weight through dieting often regain all the weight, if not more, within 1 to 5 years.
A person's weight may be influenced by numerous contributors beyond diet, physical activity, and genetics. Therefore it should not be assumed a person labeled "obese" is necessarily unhealthy. Most health indicators can actually be improved by changing health behaviours, regardless of weight loss. Therefore, one’s health cannot be assessed by whether or not they meet the BMI’s criteria for obesity. A more accurate measure of one’s health is to look at their health behaviors, including enjoyable exercise habits, intuitive eating habits, and whether they smoke cigarettes or consume alcohol.
The World Health Organization defines "overweight" as a BMI 25-30. Please see Body Mass Index (BMI) for information on its limitations. It’s assumed that people above a certain BMI have a higher risk of death. However, increased mortality is actually clustered in the more extreme obesity range (BMI 35 and above), which is higher than where most people's BMI falls. A BMI in the "overweight" category actually shows a protective affect against risk of death.
The use of the label “overweight” as a health indicator rouses similar concerns to that of the “obesity”. Please see Obesity for more information on its limitations.