Originally appeared in the Behavioral Health Nutrition Student Blog
Clinicians and the general public alike have been troubled by the question, “How much should I weigh?” The answer is not simple. Numerous factors play into creating that number including height, age, gender, current health status, and health goals. All retain the purpose of reducing a person’s risk of disease throughout the lifespan. Because this is a rather complex determination, several equations have been created over the years in an attempt to simplify the calculation. These are known as Ideal Body Weight (IBW) equations. They are able to predict a recommended weight as a linear function of height. This means that as a person’s height increases, their recommended weight will also increase at an even rate.
Linear relationship between weight and height
While these are useful tools, they do not always offer the most accurate information. IBW equations notoriously underestimate recommended body weight at shorter heights and overestimate at taller heights. Even though many factors are built into an IBW equation, it is still impossible to account for all variances. Risk of disease varies greatly among different demographics and ethnicities. One ideal weight could never serve in everyone’s best interest. In addition, most health care professionals agree that it is better to recommend a target range of body weights instead of a singular weight. Many opt for the use of Body Mass Index (BMI) to determine a healthy body weight range for their patients. BMI calculations, however, are a bit complicated and require several steps.
Peterson et al. aimed to rectify the concerns raised about IBW calculations in a recent article published in the American Journal of Clinical Nutrition. They aimed to combine the simplicity of the linear relationship of IBW equations with the clinical practicality of BMI estimations into one universal equation. Below is the result:
Wt(lb) = [5 X BMI +(BMI ÷ 5)] X (Ht – 60in)
Unlike previous equations, this calculation uses a target BMI to estimate IBW. In this way, an IBW can be estimated for any given BMI. This can also be used to create a range of IBWs. For example, to estimate IBW for a normal BMI range, one would enter 18.5 for the low end and 24.9 for the high end. Since the relationship is linear, the same principle will hold for other BMI ranges as well. This could be useful in counseling individuals for whom a normal BMI may not be reasonable.
This universal equation also maintains a much higher accuracy than its predecessors. Not only does it map onto BMI closely, but it also maintains accuracy at heights on the low and high ends of the spectrum. Likewise, it is not fazed by increasing BMI. This helps to reduce the problems of over- and underestimation encountered with previous methods.
Because the universal equation is so closely tied to BMI, it also falls victim to the same limitations. BMI often misidentifies those who are exceptionally muscular as being overweight. This means that this new equation may recommend an IBW that is too low for these individuals. The target BMI used in this scenario would have to be adjusted to account for the individuals increased muscle mass.
The universal equation proposed is an excellent tool that is easy to use and highly accurate for most body types. It is precise enough to be applied clinically to determine drug dosages while being simple to calculate making it ideal for anyone just looking to keep up on their health goals.
Source:
Peterson, C. M., Thomas, D. M., Blackburn, G. L., & Heymsfield, S. B. (2016). Universal equation for estimating ideal body weight and body weight at any BMI. The American journal of clinical nutrition, 103(5), 1197-1203.