Measuring Success: A Comparison of Weight Loss Calculations

This column is dedicated to providing information on the medical management of obesity, which includes diet, exercise, behavioral change, and medication.

Column Editors: Craig Primack, MD, FAAP, Medical Bariatrician/Certified Medical Obesity Specialist/
Co-Medical Director, Scottsdale Weight Loss Center PLLC, Scottsdale, Arizona; Wendy Scinta, MD, MS, FAAFP, FASBP, Medical Director, Medical Weight Loss of NY, BOUNCE Program for Childhood Obesity, Manilus, New York; Clinical Assistant Professor of Family Medicine, Upstate Medical University,Syracuse, New York

This month: Measuring Success: A Comparison of  Weight Loss Calculations

by Wendy Scinta, MD, MS, FAAFP,FASBP

Funding: No funding was provided in the preparation of this manuscript.

Financial disclosures: The author reports no conflicts of interest relevant to the content of this article.

Bariatric Times. 2012;9(7):18–20

ABSTRACT
The manner in which we discuss and report weight loss has been debated for years. Current formulas, such as percent weight loss, percent excess weight loss, and percent excess body mass index loss, vary significantly in their calculations, presenting difficulty when comparing weight loss across different modalities and studies. Efforts for standardization are underway, and must include strict definition of the variables, regardless of the method or methods selected.

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Is a patient successful when he or she has lost 10 percent of his or her body weight, or reached a normal body mass index (BMI)? Is success after bariatric surgery apparent when the patient shows reversal of obesity-related comorbidities, or when he or she reaches his or her goal weight? What if the patient still has excess body weight when compared to a normal BMI after completing a medical program?

The manner in which we discuss and report weight loss has been debated for years. From the number of pounds lost to change in BMI, performance index, reduction index, percent weight loss, percent excess weight loss, and percent excess BMI, there has never been a shortage of calculations. Unfortunately, the variation and ambiguity in these methods are significant, making it difficult to compare weight loss across different studies and modalities. So, why is it so challenging to determine (and stick with) one weight loss formula?

Percent Weight Loss
One of the key issues is this debate is determining what constitutes successful weight loss. Currently, the medical community prefers the calculation of percent weight loss (%WL).

Percent WL is more accurate than pounds of weight lost because it takes into account the fact that those with a high starting weight tend to lose more weight. Percent WL can be helpful to characterize reversal or prevention of obesity-related comorbidities. For example, in one diabetes prevention program, a seven-percent weight loss prevented diabetes in 50 percent of the patients.[1] Also, a 10-percent weight loss will produce improvements in a majority of metabolic and cardiac risk factors.[2]  One of the disadvantages of %WL is that it does not take into account a therapeutic goal, nor does it express a patient’s desire of how much weight he or she might wish to lose.[3]

Percent Excess Weight Loss
In the surgical community, the use of percent of excess weight lost (%EWL) is favored.

An advantage of %EWL is that it expresses weight loss that has been achieved relative to a defined goal. A concern about %EWL is that the definition of preoperative weight and ideal body weight can be ambiguous. The %EWL calculation varies considerably if pre-operative weight is the weight of the first visit, or the highest weight between first visit and the day of surgery.[4] Ideal body weight (usually captured through the Metropolitan Life Tables) [5,6] also varies depending upon which size body frame is used. As the Metropolitan Life Tables were originally created in the 1940s and have not been updated since 1983, many feel this method is outdated.[7,8]

Percent Excess Body Mass Index Loss
Experts from both the medical and surgical communities have proposed alternatives to %WL and %EWL. Percent excess BMI loss (%EBMIL) is one measure that is frequently used outside the United States and is favored by some experts.[3,8]

BMI is thought to be the easiest index of “fatness” when compared with hydrodensitometry studies.[8,9] In addition, BMI outside the range of 21 to 25kg/m2 has been associated with increased risk of death.[]8 The accuracy of BMI continues to be challenged, however, particularly as it relates to individuals with normal weight obesity (defined as a combination of normal BMI and high body fat content) and muscular body types. Furthermore, when compared to dual-energy x-ray absorptiometry (DXA), a recent paper revealed that BMI misclassified 25 percent of men and 48 percent of women.[10] It is for this reason that many experts feel that the accuracy of BMI in diagnosing obesity is extremely limited.[11]

Comparing Metrics
There is no perfect metric, yet this lack of standardization has made it increasingly challenging to compare weight loss between studies, and particularly between medical and surgical modalities. Consider Table 1, which describes cases of hypothetical patients. The first patient lost 16 pounds, which translates to a %WL of 8 (the number the bariatrician would report). In the same patient, %EWL is 31 (the number that the bariatric surgeon would report). The %EBMIL is 59 (the number proposed as an alternative to both), which is seven times higher than %WL. Clearly, there is a significant variability in the percentages reported depending upon the method used.

Discussion
So, where do we go from here? It has been recently reported that the United States Food and Drug Administration (FDA) Advisory Committee is supporting the notion of abandoning %EWL in favor of %WL.[7]

At the same time, the National Institutes of Health (NIH) Guidelines Panel is meeting to revise the guidelines for the management of obesity and cardiac risk reduction. Three years in the making, these guidelines are expected to be released for public comment some time during the Summer of 2012. This panel is also discussing how to standardize reporting, possibly favoring several methods of reporting (such as total weight loss and %BMIL).

Final Thoughts
Regardless of the method chosen, it is imperative that the bariatric community adopt strict definitions of the variables, such as ideal body weight, pre-operative or starting weight, and benchmark BMI. It may take dual reporting to appeal to both communities; however, once established, agreement on standardization for reporting weight loss will be a welcome change for all obesity specialists.

References
1.    Knowler WC, Barrett-Connor E, Fowler SE, et al. Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. N Eng J Med. 2002;346(6):393–403.
2.    National Heart, Lung, and Blood Institute. Obesity Education Initiative Expert Panel on the identification, evaluation, and treatment of overweight and obesity in adults. clinical guidelines. Obes Res. 1998;6:51S–63S.
3.    Bray GA, Bouchard C, Church TS, et al. Is it time to change the way we report and discuss weight loss? Obesity (Silver Spring). 2009;17(4):619–621.
4.    Montero PN, Stefanidis D, Norton HJ, et al Reported excess weight loss after bariatric surgery could vary significantly depending on calculation method: a plea for standardization. Surg Obes Relat Dis. 2011;7(4):531–534. Epub 2010 Oct 11.
5.    Metropolitan Life Insurance Company. New weight standards for men and women. Stat Bull Metrop Life Insur Co. 1959;40:1–10.
6.    1983 Metropolitan Height and Weight Tables. Stat Bull Metropolitan Life Insurance Co. 1984;64:2–9.
7.    Sharma A. FDA AdCom Strongly Supports Abandoning Excess Weight Loss. Dr. Sharma’s Obesity Notes. Monday, May 14, 2012.
8.    Deitel M. Comment on: Reported excess weight loss after bariatric surgery could vary significantly depending on calculation method: a plea for standardization. Surg Obes Relat Dis. 2011;7(4):534–535. Epub 2010 Oct 15.
9.    Keys A., Flamino F., Kervonen MJ et al. Indices of relative weight and obesity. J Chronic Dis . 1972;25:329-43.
10.    Shah NR, Braverman ER. Measuring adiposity in patients: the utility of body mass index (BMI), percent body fat, and leptin. PLoS One. 2012;7(4):e33308. Epub 2012 Apr 2.
11.    Romero-Corral A, Somers VK, Sierra-Johnson J, et al. Accuracy of body mass index in diagnosing obesity in the adult general population. Int J Obes (Lond). 2008 ;32(6):959–966. Epub 2008 Feb 19.

Category: Medical Methods in Obesity Treatment, Past Articles