Body Composition and Fat Analysis in Prader-Willi Syndrome 

Merlin G. Butler¹, Ron Price², William Riddle², Irene Feurer², Michael Carlson², Travis Thompson ³, Section of Medical Genetics and Molecular Medicine, Children’s Mercy Hospital and University of Missouri-Kansas City School of Medicine¹, 2401 Gillham Rd., Kansas City, MO 64108 ; Vanderbilt University, Nashville, TN² ;

Kansas University Medical Center, Kansas City, KS³. 

Prader-Willi syndrome (PWS) is characterized by infantile hypotonia, hypogonadism, feeding difficulties in infancy, early childhood obesity, mental deficiency, short stature, small hands and feet, and a de novo 15q11-q13 deletion or maternal disomy 15 (UPD).  PWS is the most common cause of morbid obesity with obesity being the most significant health problem.  To further characterize the obesity, we undertook body composition and fatness pattern studies in subjects with Prader-Willi syndrome and simple obesity.  We also examined for potential influences of the PWS genetic subtypes (deletion and UPD).  The participants were part of a larger study recruited at Vanderbilt University.  Our study population consisted of 48 subjects with PWS (21 males, 27 females; 45 Caucasians, 2 African-Americans; 27 deletions, 21 UPD) with an average age of 22 years and 27 obese comparison subjects without a known cause of their obesity with an average age of 28 years (10 males, 17 females; 20 Caucasians, 7 African-Americans).   

Body composition was assessed by a combination of methods including bioelectrical impedance (BIA), dual energy x-ray absorptiometry (DEXA) and magnetic resonance imaging (MRI).  Regional fat distribution was determined by cross-sectional MRI scans at the level of the umbilicus.  As expected, subjects with PWS were shorter and weighed less than the obese controls.  Total body fat as measured by DEXA was not different between the two PWS genetic subtypes or obese controls (51% in PWS-del; 48% in PWS-UPD; 46% in obese); however, lean body mass in kilograms was significantly reduced (p <0.0001) in both PWS genetic subtypes compared with obese controls (34 in PWS-del; 36 in PWS-UPD; 48 in obese).  Cross-sectional MRI scans provided measures of subcutaneous fat (SFA), visceral fat (VFA) and the ratio (SFA:VFA).  No significant differences were detected in regional fat distribution between the two PWS genetic subtypes. However, there was a trend for subjects with PWS to have less visceral fat compared with equally obese controls but the ratio of SFA:VFA was similar in the two subject groups (PWS and obese). In obese controls, SFA was significantly associated with weight, body mass index, percent fat, and absolute fat mass.  None of these factors were related to VFA in the obese controls.  This apparent lack of a relationship between SFA and VFA in the obese group suggests that these two fat deposits are differentially regulated in simple obesity.  In contrast, both SFA and VFA were highly correlated with weight, body mass index, percent fat, and absolute fat mass in the PWS group.  In addition, SFA and VFA were significantly related in the PWS group.  The latter findings in the PWS group suggest VFA may be regulated differently in PWS compared to simple obesity, further supporting the notion that differences in fat metabolism regulation play a role in the morbid obesity seen in PWS.

 July 2002