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Importance of Body Mass Index

Body mass index (BMI) is a statistical measure of your weight scaled according to your height. It is a simple index of weight-for-height and is widely used by medical, health and fitness professionals to classify underweight, overweight and obesity in adults. BMI is a useful tool and for most individuals is an accurate way to classify weight, but it should be used along with other measurements as it does have limitations. However, you can calculate their BMI without the use of expensive equipment or special knowledge.

BMI is calculated by dividing weight (in kilograms) by height (in meters) squared. The World Health Organization defines overweight as a BMI of 25.0 to 29.9 and obesity as a BMI greater than 30. A BMI value of 19.5 to 24.9 is considered normal, and less than 18.5 is defined as underweight.

BMI is a simple, quick calculation meant for use in classifying people who are not physically active and who have an average body composition, and is generally considered a good indicator of whether you're at a healthy, normal weight for your height. Numerous research studies have related BMI, especially the degree of overweight, to an increased risk of developing various diseases as well as premature death.

Before BMI was commonly used, health professionals calculated ideal body weight for women using a simple mathematical formula. A weight of 100 lb. was allotted for the first five feet of height, and 5 lb. was added for every additional inch of height. This system was limited because it was based on the concept of one ideal weight for all women of a given height.

While BMI does not directly measure body fat, it is more accurate at approximating degree of body fatness than weight alone. In addition, because it uses ranges instead of absolute numbers, it does not suggest one absolute "ideal body weight" for each person. Two women of the same height could differ in weight by 10 or 15 lb. and still fall within the normal weight range. Neither ideal body weight nor BMI accounts for differences in body composition (lean vs. fat mass) or waist, chest or hip measurements.

Though BMI has limitations, it is an important measurement tool. It's an rough indicator of current health status and disease risk, based upon weight and degree of obesity. The American Cancer Institute says an abnormal BMI (overweight) in women with breast cancer is associated with lower survival rates; and according to the American Heart Association, as BMI during childhood climbs, so does the risk of coronary heart disease in adulthood. Additionally, as BMI rises above 25, both blood pressure and cholesterol increase while high-density lipoprotein (HDL) or "good" cholesterol decreases.

References (3)

  • International Journal of Obesity; BMI-Based Body Size Guides for Women and Men; February 2008
  • Postgraduate Medicine; The Medical Risks of Obesity; X. Pi-Sunyer
  • Connecticut Medicine; How Much Should I Weigh?; J. Gadzik

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This article reflects the views of the writer and does not necessarily reflect the views of Jillian Michaels or JillianMichaels.com.