Moderate and severe thinness, underweight, overweight and obesity

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. However, BMI may not correspond to the same degree of fatness in different populations, in part because of different body proportions. The health risks associated with increasing BMI are continuous, and the interpretation of the BMI grading in relation to risk may differ for different populations.

Proportions of underweight in women aged 15–49 years and of overweight in women aged 18 years or more are included as intermediate outcome indicators in the core set of indicators for the Global nutrition monitoring framework. Adult overweight is also included in the NCD global monitoring framework, and in the WHO Global reference list of 100 core health indicators. Underweight in women aged 15–49 years is included as an additional indicator in the WHO Global reference list of 100 core health indicators.

How are these indicators defined?

BMI is a simple index of weight-to-height that is 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 has a height of 1.70 m will have a BMI of 20.1, where BMI = 58 kg/(1.70 m ´ 1.70 m) = 20.1. BMI values indicate the following:

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 clearly linked to increases in illness in adults studied in three continents; therefore, it is a 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; therefore, this cut-off point is a valid extreme limit.

Underweight – The cut-off point of a BMI 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 to be 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 has a BMI <18.5.

Overweight – A BMI ≥25 signifies overweight; it is a major determinant of many NCDs (e.g. non-insulin-dependent diabetes mellitus, coronary heart disease and stroke), and it 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 – A BMI ≥30 signifies obesity, which 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

BMI: body mass index

Source: WHO (1995).

Sources of data

WHO. Global Health Observatory (GHO) data repository. Body mass index (BMI). (http://apps.who.int/gho/data/node.main.BMIANTHROPOMETRY?lang=en).

NCD Risk Factor Collaboration (NCD-RisC). Data downloads. (http://www.ncdrisc.org/data-downloads.html).

Further reading

NCD Risk Factor Collaboration (NCD-RisC). Worldwide trends in body-mass index, underweight, overweight, and obesity from 1975 to 2016: a pooled analysis of 2416 population-based measurement studies with 128.9 million participants. Lancet. 2017;Dec 16;390(10113):2627–2642. doi:http://dx.doi.org/10.1016/S0140–6736(17)32129–3.

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 (http://whqlibdoc.who.int/trs/WHO_TRS_854.pdf).

WHO, UNICEF. Global nutrition monitoring framework: operational guidance for tracking progress in meeting targets for 2025. Geneva: World Health Organization; 2017 (http://www.who.int/nutrition/publications/operational-guidance-GNMF-indicators/en/).

WHO. Global reference list of 100 core health indicators (plus health-related SDGs). Geneva: World Health Organization; 2018 (https://www.who.int/healthinfo/indicators/2018/en/).

Internet resources

WHO. Obesity and other diet-related chronic diseases list of publications. (http://www.who.int/nutrition/publications/obesity/en/).

WHO. e-Library of Evidence for Nutrition Actions (eLENA). Interventions by global target (http://www.who.int/elena/global-targets/en).

Target 7: Halt the rise in diabetes and obesity (http://www.who.int/elena/global-targets/en/#diabetesobesity).