Body mass index is derived from the calculation weight in kilograms divided by height in meters squared. BMI evaluates your weight in relation to your height but does not measure your fat directly. Instead this measure has been validated by research as a simple method to assess body fat and its relation to developing health problems in the future. BMI does not diagnose diseases. Pregnant and lactating women should not use BMI.
Interpreting BMI Values
BMI values for adults over the age of 20 are divided into categories determined by the level of risk for weight-related health issues associated with each BMI value. BMI values for adults are classified as: Underweight for values less than 18.5; normal weight for values between 18.5 and 24.9; overweight for values between 24.9 and 29.9; and obese for values 30.0 and above. BMI for children and teenagers differs in its assessment of body fat by looking at both age and sex in addition to height and weight and by categorizing the values into percentiles.
Underweight Categories
If your BMI is less than 18.5, you are considered to be at an increased risk for health-related problems. For example, a BMI of less than 18.5 can mean you are at risk for undernutrition, osteoporosis, infertility or respiratory disease. A BMI of less than 18.5 is also viewed by health professionals as a possible indicator for eating disorders or other underlying illnesses. A BMI less than 18.5 however, is not automatically an indication of a health problem for those people who are naturally very lean.
Normal Weight Categories
If you have a BMI between 18.5 and 24.9 you are considered to be at a normal weight and have the lowest risk for developing weight-related health problems. As all BMI values within this range are considered to be equally low risk, BMIs that fall at the lower end of this range should not be thought of as your target weight. Having a BMI within the normal range is not a guarantee that you are healthy, it is only an indication that your risk for weight related-health problems is lower than for people who have BMIs less than 18.5 or greater than 25.
Overweight Categories
If your BMI falls between 25.0 and 29.9, you are categorized as having an increased risk for health issues such as Type 2 diabetes, high blood pressure and heart disease. This is not always true for athletes and others with muscular body builds, as BMI is not able to take into account how much of your weight is lean muscle mass. Lean muscle mass does not increase your risk for weight-related health problems and therefore people with a muscular physique may have a BMI over 25 without being at an increased risk.
Obese Categories
BMIs over 30 are further categorized as: Obese Class I for those 30.0 to 34.9; Obese Class II for those 35.0 to 39.9; and Obese Class III for those with a BMI of 40.0 and above. Your associated risk for health problems based on your class of of obesity is high risk, very high risk and extremely high risk, respectively.
Examples of health risks related to obesity include Type 2 diabetes, heart disease, gallbladder disease, obstructive sleep apnea, osteoarthritis, impaired fertility and certain cancers.
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