BMI vs. IBW

Why we Quantify

When it comes to getting healthier, understanding the difference between Body Mass Index (BMI) and Ideal Body Weight (IBM) is key. BMI gives you a number by comparing your weight and height, while Ideal Body Weight sets a target range based on things like height and gender. This quick look aims to make things clear for everyone, breaking down the differences between these measures and helping you on your journey to better health.

Body Mass Index

Body Mass Index (or BMI) was developed in the early 19th century as a means to discover “social ideals” of human anthropometry. (Eknoyan, 2007) It is derived from the height and weight of a person. 

  BMI = weight / height2

Used for ages 21 and older, it will categorize people as underweight, normal weight, overweight, or obese. Classification continues into BMI percentiles, which are based upon relation between BMI and disease/death. (WHO Expert Committee, 1995)

The overarching concept is that BMI is related to measures of body composition, namely body fat percentage. However, it does not directly measure body composition, and many do not like the use of BMI. It truly has its limitations as a useful measurement. For example, due to racial differences, individuals of Asian descent will have lower thresholds for BMI statuses. (WHO Expert Consultation, 2004) Furthermore, because BMI is proportional to body weight but inversely proportional to the square of height, taller people tend to have BMIs higher than would be expected by their body fat levels. If body weight scales as it should, BMI doubles instead of remaining the same. Thus, individuals who are taller will register larger BMIs relative to their body metrics than will their shorter counterparts. Moreover, if the individual is short in height or athletic (higher muscle mass), BMI can also under- or overestimated body fat levels.

There are several other methods of anthropometry, including the Ponderal Index, Corpulence Index, New BMI, BMI Prime, Ideal Body Weight, and even waist circumference. In fact, regarding heart attack, stroke, or death, measures like waist-to-height ratio or waist-to-hip ratio may outperform BMI. (Schneider et al., 2010)

Let’s take a look at Ideal Body Weight.

Ideal Body Weight

Ideal Body Weight (IBW) was originally created to allow for drug dosage estimations for obese patients. As an obese individual carries predominantly more fat mass than a lean, same-height individual, the obese individual can be presumed to have similar mass of other soft tissues (e.g., muscle, liver, kidney). Thus, their pharmacological needs would remain the same as those of their leaner counterparts.

IBW is estimated via two formulas, the Devine formula and the Hamwi method. Both are functions of body mass and height.

 Devine Formula shown below: (Pai & Paloucek, 2000)
    Male IBW = 50kg + 0.9kg*(height(cm) – 152)
    Female IBW = 45.5kg + 0.9kg*(height(cm) – 152)

Ideal Body Weight possesses its own limitations, just like BMI. IBW does not estimate body composition. And like BMI, IBW may overestimate athletic individuals, deeming them overweight when they are of normal weight relative to their muscle mass. Nevertheless, IBW still remains in use in medicine, mostly related to drug dosing.

Take Home Message

Much like in other scientific fields, different methods have different preferred uses. All methods of assessing body anthropometrics have strengths and flaws, especially when it comes to using these methods to assess body composition. Despite being often used for the individual, BMI is more so useful for populations. Indeed, even a NY Times article reports opinion that “the B.M.I. tables are excellent for identifying obesity and body fat in large populations, but they are far less reliable for determining fatness in individuals.” (Brody, 2010) In addition, IBW is actually a very specific estimation used in drug dosing. Why do some use it for their own health goals then? In fact, when it comes to assessing one’s own anthropometrics as it relates to body health, all measures are tools in a toolkit, just like the bathroom scale. It measures total weight but does not tell the composition of that weight nor how much water a person is holding that may be affecting the number on the scale. On the other hand, body girth measurements will tell size of different body locations, but again does not show muscle vs. fat vs. bone composition. (Nor is it very easy to assess oneself!)

At the end of the day, different methods will overestimate and underestimate different people. Most of these measurements are used for a specific purpose. And as with most things in life, it’s important to not get caught up in comparisons, especially to a measurement that is best used for the group, not just for you alone.

References

• Brody, J. E. (2010). Weight Index Doesn’t Tell the Whole Truth. Personal Health. Retrieved Feb 14, 2024, from https://www.nytimes.com/2010/08/31/health/31brod.html
• Eknoyan, G. (2007). Adolphe Quetelet (1796–1874)—the average man and indices of obesity. Nephrology Dialysis Transplantation, 23(1), 47-51. https://doi.org/10.1093/ndt/gfm517
• Pai, M. P., & Paloucek, F. P. (2000). The Origin of the “Ideal” Body Weight Equations. Annals of Pharmacotherapy, 34(9), 1066-1069. https://doi.org/10.1345/aph.19381
• Schneider, H. J., Friedrich, N., Klotsche, J., Pieper, L., Nauck, M., John, U., Dörr, M., Felix, S., Lehnert, H., Pittrow, D., Silber, S., Völzke, H., Stalla, G. n. K., Wallaschofski, H., & Wittchen, H.-U. (2010). The Predictive Value of Different Measures of Obesity for Incident Cardiovascular Events and Mortality. The Journal of Clinical Endocrinology & Metabolism, 95(4), 1777-1785. https://doi.org/10.1210/jc.2009-1584
• WHO Expert Committee. (1995). Physical status: the use and interpretation of anthropometry. Report of a WHO Expert Committee. World Health Organ Tech Rep Ser, 854, 1-452.
• WHO Expert Consultation. (2004). Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet, 363(9403), 157-163. https://doi.org/10.1016/S0140-6736(03)15268-3