Body mass index

Body Mass Index or BMI is a number calculated using height and weight, and sometimes gender or age (in children) that aims to measure whether someone's weight is within a healthy weight range, or if they are underweight, overweight or obese. BMI values are used as a screening tool to aid diagnosis of certain illnesses, for example the presence or severity of anorexia nervosa, and metabolic syndrome, and to assess risk factors and demographic associations for many illnesses, and for some medical procedures.

BMI ranges in adults
The United States CDC uses these ranges for adults. For children and adolescents, alternative ranges are used.

The CDC regards a Body Mass Index of 30 or higher in adults as obesity.

Criticisms
BMI evolved from an attempt to develop better health risk assessments for the United States health insurance industry than the height/weight tables used in the early 1900s, and were largely based on values for White adult men. In the 1990s, the World Health Organization adopted BMI as a medical measure and published ranges. Today BMI is widely regarded as as measure of "fat-ness" and has been found to contribute to both social stigma and bias or stigma from doctors and clinicans.

Key criticisms include:
 * the EPIC study of over 350,000 adults in Europe (aged 25 to 70) found that increased mortality was associated with a BMI of less than 21 or over 30, which is significantly different from the BMI ranges in use, men with a BMI of 26.5 had the lowest risk, similar results were found in the larger NIH–American Association of Retired Persons study
 * BMI is a relatively poor measure of fat-ness, and does not take into account waist circumference or muscle mass, it may assess some people with a high proportion of fat as in a healthy range and some with a healthy proportion as overweight or even obese
 * Older adults typically have reduced muscle mass, and higher amounts of fat, which means BMI may be too high for them
 * Mortality is often associated with a higher BMI, but a higher mortality in "wasting diseases" such as tuberculosis is associated with a lower BMI
 * Risk of developing diabetes, metabolic syndrome and coronary heart disease has been found to be more closely associated with accumulated fat stores in the upper body than overall

Alternatives
A number of alternative measures exist that may be more accurate or provide additional information when screening for health risks:
 * waist circumference
 * abdominal/hip circumference ratio
 * skin-fold thickness measurements in different areas of the body
 * percentage body fat can be measured by underwater weighing, air displacement and density determination (using a Bod Pod), a bioelectrical impedance analyzer, and a determination of the isotopically labeled water mass
 * a DEXA x-ray scan can estimate fat accumulation around organs

ME/CFS
Body mass index is not considered a risk factor or an indicator for disease progression or for severity in ME/CFS - people with moderate ME/CFS typically have a higher BMI than those with severe ME/CFS. Studies looking at the small minority who have recovered from ME/CFS have not found BMI or any other factors predictive of recovery, except for short illness duration and milder illness. 

Most people with ME/CFS have a BMI in the healthy range, although obesity is relatively common in United States patients with ME/CFS.

Cost and availability
BMI is extremely easy to measure and calculate, and can be assessed by patients at home.

Calculators

 * BMI calculator with adjustments for ethnic group and gender - British Heart Foundation
 * BMI calculator - CDC
 * BMI and waist circumference calculator - Mayo Clinic

Learn more

 * Why BMI is a flawed health standard, especially for people of color - Washington Post
 * Why use BMI? - Havard Health
 * In BMI We Trust: Reframing the Body Mass Index as a Measure of Health - Social Theory and Health Journal