Body Mass Index (BMI) Measurement


Definition


Body mass index (BMI) is a measure of weight relative to height and is often used to approximate the level of body fat in individuals. BMI is used in research to identify trends in people and populations, and to evaluate the need for lifestyle changes and health interventions. It can be used to identify under and over nutrition in adults and children. In addition to calculating BMI, a waist circumference measurement should be used if the research is on individuals and not populations. Waist circumference measurements provide an excellent indicator of obesity related health risks, especially cardiovascular disease.


Relevant Characteristics


Anthropometry is the science that deals with the physical dimensions, proportions, and composition of the human body, as well as the study of related variables that affect them. Anthropometric measurements, such as body mass index, are used to assess body size and physical parameters in order to identify diet related conditions such as being underweight or overweight, wasting and stunting. BMI does not measure body fat directly but research has shown BMI correlates to direct measures of body fat. [1] In children and adults measurements can be used to assess growth and development, disease risk and body composition changes in regards to nutritional status.[2] It is important to note that while the BMI calculation does not change, the interpretations differ when used on children, adolescents, and adults. Simply, these changes in interpretation are due to differing growth rates in childhood, and rates of sexual maturation in adolescence.

Children in particular must be measured carefully and compared to a standard growth curve for the population to which they belong. The World Health Organization (WHO) Global Database on Child Growth and Malnutrition standards have been decided upon as the most accurate and widely accepted in estimating underweight, stunting, wasting, overweight and obesity.[3] The WHO has compiled data from around the world since 1986 in order to determine standards of identification for child growth and development patterns.[4] Stunting, or low height for age, is caused by long-term insufficient nutrient intake. Stunting generally occurs before age two, and effects are largely irreversible. Wasting, or low weight for height, is usually the result of acute significant food shortage and/or disease. Wasting is a strong predictor of emergency situations.[5] Underweight, overweight, stunting, wasting, and obesity are all determined based on the number of standard deviations the individual falls below or above the median weight for age of the reference population.

Measurements to assess body composition include height, weight, and percent of body fat. These measurements are often combined with age and gender to form indices that indicate what is considered healthy, unhealthy or normal in a given population. BMI calculations use weight, height and waist circumference measurements. Waist circumference should be measured in conjunction with BMI in order to adequately assess obesity related health risks. Waist circumference measurements may provide a stronger indicator of obesity and related health risks in individuals and should be used especially when assessing health risks in a population as opposed to identifying nutrition status.

Large sample sizes should be used in order to gain a representative sample of the population. Cluster sampling may produce misleading results due to the uneven distribution of abnormal BMI. Since BMI data does not have a normal distribution, median rather than mean values should be cited and other methods of presentation than mean values should be used.[6]


“Method Made Easy”


To measure BMI of an individual the researcher would weigh the subject on a previously calibrated scale. The researcher would then measure the standing height of the subject using a tape measure. Outside of the US, the metric scale is used for all of these measurements and the calculations.

Body Mass Index (BMI)= weight (kg) / [height (m)]2

Waist circumference is particularly useful for individuals with a BMI of 25-34. If a waist circumference measure was being taken, the researcher would measure around the abdomen of the individual at the widest point, typically just about the hip bones, while the subject is relaxed. Individuals are at a higher risk for health problems if their waist circumference is above 94cm or 37 inches for men and above 80cm or 31.5 inches for women. Typically, a waist circumference above 102cm or 40 inches for men and 88cm or 34.5 inches for women denotes abdominal obesity and puts the individual at very high risk for health problems.[7]




Advantages for research


  • Useful for measuring across large populations. The results are not exact body fat percentages; rather they are approximations that can provide an understanding of trends in a population when measurements are averaged, better than it can provide exact measures of an individual
  • Inexpensive and quick, which makes it ideal for use in the field
  • Noninvasive: researchers need only a scale and tape measure to perform measurements
  • Requires little-to-no training. Many calculators are available on the Internet to assist researchers in performing the necessary calculations
  • Useful for identifying at-risk individuals, who may then require more in-depth follow up

Advantages for use as a nutritional status indicator

  • Provides a practical and numerical way of describing the problem
  • Standard proxy measure of human welfare reflecting dietary inadequacies, infectious diseases and other environmental health risks that affect body size
  • Strong predictors, at individual and community levels, of poor health trends and mortality
  • Can be used to assess the success or failure of interventions directed toward the many economic and environmental factors underlying nutrition deprivation[8]


Limitations


  • Does not directly measure level of body fat. BMI is only an approximation.
  • Individual assessment accuracy: BMI does not differentiate between fatty mass and muscular or lean mass. Therefore, individuals with a high proportion of muscle tissue can be categorized as overweight or obese. Also, elderly people or people who have lost muscle mass may receive underestimates of their body fat.
  • Population assessment accuracy, cannot account for differences in size based on race. For example, some groups have smaller, stockier statures while others aretall and lean. BMI measurement does not differentiate between the two.
  • Does not measure the distribution of body weight on an individual. For instance, it does not differentiate between someone with excess abdominal weight and someone with uniform or peripheral fat distribution. This can be a problem because studies have shown excess abdominal rate is a risk factor for disease. Adding a waist circumference measurement can easily resolve this limitation.
  • BMI does not always change in response to lifestyle changes.[9] If an individual becomes physically active and begins losing adipose tissue while gaining muscle tissue, this may not register in the BMI calculation. Subjects must be informed of this to prevent them from becoming discouraged. Also, it is helpful to include qualitative data with the quantitative BMI collection in order to pick up on such instances.
bmi comparison guys.jpg


Analysis


The Centers for Disease Control and Prevention[10] , define the standard cutoff BMI for underweight adults in the United States as being below 18.5 kg/m2.
  • The normal range is between 18.5 and 24.9 kg/m2.
  • Overweight individuals have BMIs between 25.0 and 29.9 kg/m2.
  • Obese individuals are characterized as having a BMI of 30.0 kg/m2 or above.

BMI calculations are performed the same way for all individual measures, however the results are interpreted differently dependent on sex, race and age. BMI age and sex specific percentiles are used because the amount of body fat changes with age, and the amount differs between males and females (Centers for Disease Control and Prevention, 2011). There are indices available which chart what BMI’s per specific group are considered below average, normal, overweight, obese, and morbidly obese. These indices are used to identify people in need of intervention.


bmi-chart2.jpg


Method in Context


BMI measurement is a technical method requiring precision in measurement and careful calculations. It is typically used in medical or biological anthropology. Measurements are often made relatively quickly in the field and equations are calculated later. Modern technology makes this easier because the Internet offers calculators that allow you to input data while it performs the equation for you.
There is some controversy over the interpretation of BMI measurements and classifications. Whether or not researchers agree with the classifications of overweight or obesity based on BMI, and whether they are accurate measures for all body types, it is generally agreed upon that BMI is a valuable tool for assessing risk. There is a certain amount of stigma and societal judgment associated with individuals outside of the ‘normal range’, which has led to controversy over whether the measurements actually indicate poor health or if they are just indicators of societal value. However, as a first level diagnosis of possible health risks, BMI remains a field standard measure. Below are some sources for further reading on the topic.

This is a lecture with slides, that provides an excellent example of BMI measures being used in anthropological research:
2007 Kuzawa CW, "Early Nutrition, Development and Health: Evolutionary Perspectives on the Metabolic Syndrome", in Nesse, R. (ed.), Evolution and Medicine: How New Applications Advance Research and Practice, The Biomedical & Life Sciences Collection, Henry Stewart Talks Ltd, London. Online at http://www.hstalks.com/?t=BL0141542-Kuzawa


Online Resources





World Health Organization: Global Database on Body Mass Index, an interactive surveillance tool for monitoring nutrition transition. Available at http://apps.who.int/bmi/index.jsp

Obesity Panacea: a PLOS blog. Online at http://blogs.plos.org/obesitypanacea/2012/02/10/why-the-body-mass-index-bmi-is-a-poor-measure-of-your-health/

“Does my BMI look big in this?” University of Oxford. Online at http://www.ox.ac.uk/media/science_blog/130116.html


Further Reading


Rutherford JN, McDade TW, Lee NR, Adair LS, Kuzawa CW.
2010 “Change in waist circumference over 11-years and current waist circumference independently predict elevated CRP in Filipino women”, Amer J Hum Biol 22(3):310-5.

The Obesity Prevention Source. Harvard School of Public Health (n.d.). Online at http://www.hsph.harvard.edu/obesity-prevention-source/obesity-definition/obesity-definition-full-story/


References


  1. ^ Mei Z, Grummer-Strawn LM, Pietrobelli A, Goulding A, Goran MI, Dietz WH.
    2002 Validity of body mass index compared with other body-composition screening indexes for the assessment of body fatness in children and adolescents. American Journal of Clinical Nutrition.
  2. ^ Fryer CD, Gu Q, Ogden CL. Anthropometric reference data for children and adults: United States, 2007-2010. National Center for Health Statistics. Vital Health Stat 11(252). 2012. http://www.cdc.gov/nchs/data/series/sr_11/sr11_252.pdf
  3. ^ Padula, Gisel, Analía I Seoane and Susana A Salceda
    2012 Variations in estimates of underweight, stunting, wasting, overweight and obesity in children from Argentina comparing three growth charts. Public Health Nutrition 15, pp 2086-2090. doi:10.1017/S136898001200095X.
  4. ^ Onis, Mercedes and Monika Blossner
    2003 The World Health Organization Global Database on Child Growth and Malnutrition: methodology and applications. Int. Journal of Epidemiology 32 (4): 518-526.
  5. ^ UNICEF
    n.d. Progress for Children: A world fit for children statistical review. Retrieved from http://www.unicef.org/progressforchildren/2007n6/index_41401.htm
  6. ^ Bailey, K. V. & Ferro-Luzzi, A.
    1995 Use of body mass index of adults in assessing individual and community nutritional status. Bulletin of the World Health Organization; 73(5):673-680.
  7. ^ National Institutes of Health. Department of Health and Human Services.
    n.d. Assessing Your Weight and Health Risk: Body Mass Index (BMI). Retrieved from http://www.nhlbi.nih.gov/health/public/heart/obesity/lose_wt/risk.htm#limitations
  8. ^ UN ACC/SCN (United Nations Administrative Committee on Coordination: Sub-committee on Nutrition. 1992. Second Report on the World Nutrition Situation, Volume 1. Suffolk, England: The Lavenham Press Ltd.
  9. ^ Ross R, & Janiszewski PM.
      • 2008 Is weight loss the optimal target for obesity-related cardiovascular disease risk reduction? The Canadian journal of cardiology, 24 Suppl D PMID: 18787733.
  10. ^ Centers for Disease Control and Prevention.
    2011 Healthy Weight- it’s not a diet, it’s a lifestyle! Division of Nutrition, Physical Activity, and Obesity. Last updated Sept. 13, 2011. Accessed on March 20, 2013. Retrieved from http://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html#Interpreted