What do these indicators tell us?
The values for body mass index (BMI) are age-independent for adult populations and are the same for both genders. BMI may not, however, correspond to the same degree of fatness in different populations due, in part, to different body proportions. The health risks associated with increasing BMI are continuous, and the interpretation of BMI grading in relation to risk may differ for different populations.
How are they defined?
BMI is a simple index of weight-to-height commonly used to classify underweight, overweight and obesity in adults. It is defined as the weight in kilograms divided by the square of the height in metres (kg/m2). For example, an adult who weighs 58 kg and whose height is 1.70 m will have a BMI of 20.1: BMI = 58 kg/(1.70 m ´ 1.70 m) = 20.1
- BMI < 17.0 indicates moderate and severe thinness
- BMI < 18.5 indicates underweight
- BMI 18.5-24.9 indicates normal weight
- BMI = 25.0 indicates overweight
- BMI = 30.0 indicates obesity
What are the consequences and implications?
Moderate and severe thinness: A BMI < 17.0 indicates moderate and severe thinness in adult populations. It has been linked to clear-cut increases in illness in adults studied in three continents and is therefore a further reasonable value to choose as a cut-off point for moderate risk. A BMI < 16.0 is known to be associated with a markedly increased risk for ill health, poor physical performance, lethargy and even death; this cut-off point is therefore a valid extreme limit.
Underweight: The cut-off point of 18.5 for underweight in both genders has less experimental validity as a cut-off point for moderate and severe thinness but is a reasonable value for use pending further, comprehensive studies. The proportion of the population with a low BMI that is considered a public health problem is closely linked to the resources available for correcting the problem, the stability of the environment and government priorities. About 3-5% of a healthy adult population have a BMI < 18.5.
Overweight: Overweight (BMI = 25) is a major determinant of many noncommunicable diseases, including non-insulin-dependent diabetes mellitus, coronary heart disease and stroke, and increases the risks for several types of cancer, gallbladder disease, musculoskeletal disorders and respiratory symptoms. In some populations, the metabolic consequences of weight gain start at modest levels of overweight.
Obesity: Obesity (BMI = 30) is a disease that is largely preventable through lifestyle changes. The costs attributable to obesity are high, not only in terms of premature death and health care but also in terms of disability and a diminished quality of life.
Cut-off values for public health significance
Prevalence cut-off values for public health significance
Adult BMI < 18.5
5-9%: Low prevalence (warning sign, monitoring required)
10-19%: Medium prevalence (poor situation)
20-39%: High prevalence (serious situation)
= 40%: Very high prevalence (critical situation)
Reference: WHO, 1995.
WHO. WHO Global Database on Body Mass Index (BMI). Department of Nutrition for Health and Development (NHD), Geneva, Switzerland.
WHO. Obesity and other diet related chronic diseases, list of publications. http://www.who.int/nutrition/publications/obesity_chronicdiseases/en/index.html.
WHO. Physical status: the use and interpretation of anthropometry. Report of a WHO Expert Committee. Technical Report Series No. 854. Geneva, World Health Organization, 1995.