title: Chemical-shift water-fat MRI of white adipose depots: inability to resolve cell size differences.
abstract: Adipocyte cell size varies among individuals and importantly, is inversely correlated with insulin sensitivity, and modifiable by weight loss or pharmaceutical agents. However, there are no non-invasive, in vivo methods for adipocyte cell size determination. Here we apply Chemical-Shift Water-Fat MRI to in vivo measures of subcutaneous (inguinal) and visceral (gonadal) white adipose tissue (WAT) to determine whether the fat-signal fraction (FF) is a sensitive indicator of adipocyte cell size.
  
  
  
  C57BL/6J male mice (8 weeks old) were singly housed and fed a low-fat diet, high-fat diet or very high-fat diet (n= 16 or 15/group) for 8 weeks. Food intake, body weight and composition were measured; CS-MRI was performed on a 9.4 Tesla Bruker magnet with respiratory gating and anesthesia. Histology was acquired for gonadal WAT; both gonadal and inguinal WAT were fixed with osmium tetroxide and then measured through Image J for cell size.
  
  
  
  Mice fed with higher fat content diets gained significantly more body weight, fat and lean mass while maintaining higher energy intakes over the 8 weeks. There was no significant difference in fat fraction for either gonadal (P= 0.1295) or inguinal (P= 0.4704) WAT among the three groups, despite significantly larger adipocytes (P<0.0001) in mice on high fat diets.
  
  
  
  Although diet-induced obesity significantly increased the amount of fat mass, as well as mean and overall white adipocyte cell size, the CS-MRI measured fat fraction between groups were not significantly different. These results do not support the utility of CS-MRI measured FF for in vivo determination of adipocyte cell size.
authors.affiliation: Department of Nutrition Sciences, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: Yongbin
authors.initials: Y
authors.lastname: Yang
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Smith
authors.forename: Houchun H
authors.initials: HH
authors.lastname: Hu
authors.forename: Guihua
authors.initials: G
authors.lastname: Zhai
authors.forename: Tim R
authors.initials: TR
authors.lastname: Nagy
keywords: White adipose tissue; fat fraction; gonadal; inguinal