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Obtaining standardized weights in long-term studies can be difficult. Self-reported weights are more easily obtained, but less accurate than those from a calibrated scale and may be inaccurately reported. Previous studies have reported that women tend to under-report their weight more than men with the degree of misreporting related to body mass index (BMI) whereby overweight individuals tend to under-report while under-weight individuals tend to over-report.1–4 However, in a study of female gastric bypass candidates, self-reported pre-surgical weights averaged 0.3 kg more than measured weights and misreporting was not significantly related to BMI.5
This study investigated whether self-reported weights following bariatric surgery differed from weights obtained by study personnel using a standard scale.
The Longitudinal Assessment of Bariatric Surgery-2 is a ten center observational cohort study of 2458 adults undergoing an initial Roux-en-Y gastric bypass (RYGB), laparoscopic adjustable gastric band (LAGB), or other bariatric procedure.6 This report utilizes data collected between April 2010 and November 2012 at annual assessments 1 to 5 years after RYGB or LAGB. Each center had institutional review board approval and all participants provided written informed consent.
Annually, participants were asked to report on mailed questionnaires two postoperative weights and the dates weights were measured: (1) weight from last medical office or weight loss program visit (self-reported medical weight) and (2) last self-weighing (self-reported personal weight). Self-reported weights could be from any scale with or without shoes or bulky clothing. Using standardized data collection protocols, study personnel measured height before surgery using a stadiometer and measured weights before surgery and annually afterwards on a standard scale (Tanita® Body Composition Analyzer, model TBF-310) without shoes and bulky clothing (measured weight).
Participants with postoperative measured weights and self-reported weights from no more than 30 days before the measured weight were included. If both self-reported weights met this criterion then both were included. If participants had self-reported and measured weights meeting this criterion at multiple time points, weights from one randomly selected time point were used.
Statistical significance of weight differences was assessed using t-tests for each type of self-reported weight and normal mixed models for all self-reported weights combined. Analyses were conducted using SAS (version 9.2). Two-sided P-values less than 0.05 are considered statistically significant.
Of the 992 participants with weights meeting the inclusion criteria, 4 were excluded because of suspected recording error. The 988 participants included 164 with a self-reported medical weight, 580 with a self-reported personal weight and 244 with both self-reported weights. Included participants are characterized in the Table.
Across the two types of self-reported weight, women and men under-reported their weight by an average of 1 kg or less (range: 10.9 kg under-report to 11.8 kg over-report) and the degree of under-reporting was not significantly different between women and men (Table). Self-reported medical weights were significantly closer to measured weights than were self-reported personal weights for both women and men. Weight differences did not vary systematically by measured BMI or percent weight change from baseline (Figure).
Differences between self-reported and measured weights may be due to differences in clothing, inaccurate personal scales, time between measurements, or intentional misrepresentation.
In a general population survey, obese men and women under-reported their weight, on average, by 1.32 kg and 2.99 kg, respectfully.3 We found smaller differences. Self-reported weights after bariatric surgery may be more accurate because participants who undergo surgery to lose weight may be especially attentive to their weight.
A limitation of this study is that it used a convenience sample of participants whose self-reported weights were no more than 30 days before a measured weight. Those participants who anticipated being weighed by study personnel may have been more likely to report accurately.
In conclusion, self-reported weights following bariatric surgery were close to measured weights. This suggests that self-reported weights may not unduly affect study results of surgically-induced weight change and can be used when measured weights are not available.
Funding/Support: LABS-2 was a cooperative agreement funded by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Grant numbers: Data Coordinating Center – U01 DK066557; Columbia University Medical Center – U01-DK66667 (in collaboration with Cornell University Medical Center CTRC, Grant UL1-RR024996); University of Washington – U01-DK66568 (in collaboration with CTRC, Grant M01RR-00037); Neuropsychiatric Research Institute – U01-DK66471; East Carolina University – U01-DK66526; University of Pittsburgh Medical Center – U01-DK66585 (in collaboration with CTRC, Grant UL1-RR024153); Oregon Health & Science University – U01-DK66555.
Role of the Sponsor: The NIDDK scientists contributed to the design and conduct of the study. The project scientist from the NIDDK served as a member of the Steering Committee, along with the principal investigator from each clinical site and the Data Coordinating Center. The decision to publish was made by the Longitudinal Assessment of Bariatric Surgery Steering Committee, with no restrictions imposed by the sponsor. As a coauthor, an NIDDK scientist contributed to the interpretation of the data and preparation, review, and approval of the manuscript.
Authors Contributions: Dr. Christian had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.Study concept and design: Christian, Belle
Acquisition of data: Courcoulas
Analysis and interpretation of data: Christian, King, Yanovski, Belle
Drafting of the manuscript: Christian
Critical revision of the manuscript for important intellectual content: Christian, King, Yanovski, Courcoulas, Belle
Statistical analysis: Christian, King, Belle
Obtained funding: Courcoulas, Belle
Study supervision: Courcoulas, Belle.
Conflict of Interest Disclosures: Dr. Courcoulas has received research grants from Allergan, Pfizer, Covidien, EndoGastric Solutions, Nutrisystem®, and is on the Scientific Advisory Board of Ethicon J & J Healthcare system. All other authors declare no conflicts of interest.
Nicholas J. Christian, University of Pittsburgh Graduate School of Public Health.
Wendy C. King, University of Pittsburgh Graduate School of Public Health.
Susan Z. Yanovski, National Institute of Diabetes and Digestive and Kidney Diseases.
Anita P. Courcoulas, University of Pittsburgh Medical Center.
Steven H. Belle, University of Pittsburgh Graduate School of Public Health.