title: New insight into fat, muscle and bone relationship in women: determining the threshold at which body fat assumes negative relationship with bone mineral density.
abstract: BACKGROUND
  The aim was to investigate the relationships among lean mass (LM), fat mass (FM), and bone mineral density (BMD) in women stratified by body mass index (BMI) (BMI - normal-weight, overweight, obese) and to determine threshold at which body fat assumes negative relationship with BMD.
  
  
  METHODS
  This was a cross-sectional study in 471 healthy Caucasian women, aged 18-67 years. BMD, LM, and FM were measured using dual-energy X-ray absorptiometry. Analysis of variance with Bonferroni corrections was used to test the BMI group differences. Linear regression was used to examine independent contributions of LM and FM on BMD of various skeletal sites (controlling for age and height). In overweight/obese women PROC LOESS plots were used to determine the inflection points at which either LM or FM relationship with BMD changes direction. Separate analyses in pre- and post-menopausal women were conducted as well.
  
  
  RESULTS
  Spine and femoral neck BMD were not different among three BMI groups while total body, femur and radius BMD were statistically different (the highest in the obese group). Linear regression revealed that LM had significant positive association with BMD of various skeletal sites in all groups. FM showed a negative association with BMD of femoral neck and femur in normal-weight and spine in overweight women, but a positive association with radius in obese women. Inflection points showed that body fat between 33% and 38% assumed negative relationship with BMD for most skeletal sites in overweight and obese women.
  
  
  CONCLUSIONS
  Although LM has strong positive relationship with BMD, FM above 33% in overweight/obese women is negatively related to BMD of most skeletal sites. Therefore, overweight/obesity after certain amount of FM, may not be a protective factor against osteoporosis in females. For clinical practice in women, it is important to maintain LM and keep FM accrual below ~30% body fat to maintain good skeletal health.
authors.affiliation: School of Nutrition and Dietetics, University of Akron, Akron, Ohio, USA.
authors.forename: Pei-Yang
authors.initials: PY
authors.lastname: Liu
authors.affiliation: Department of Nutrition, Food and Exercise Sciences, College of Human Sciences, Florida State University, Florida, USA.
authors.forename: Jasminka Z
authors.initials: JZ
authors.lastname: Ilich
authors.affiliation: Department of Geriatrics, College of Medicine, Florida State University, Florida, USA.
authors.forename: Ken
authors.initials: K
authors.lastname: Brummel-Smith
authors.affiliation: Department of Medical Oncology, University of Alberta and Alberta Health Services-CancerControl, Edmonton, Canada.
authors.forename: Sunita
authors.initials: S
authors.lastname: Ghosh
keywords: Body mass index; bone mineral density; fat mass; lean mass; menopause