title: Irisin and FNDC5: effects of 12-week strength training, and relations to muscle phenotype and body mass composition in untrained women.
abstract: OBJECTIVE
  To investigate the effects of strength training on abundances of irisin-related biomarkers in skeletal muscle and blood of untrained young women, and their associations with body mass composition, muscle phenotype and levels of thyroid hormones.
  
  
  METHODS
  Eighteen untrained women performed 12 weeks of progressive whole-body heavy strength training, with measurement of strength, body composition, expression of irisin-related genes (FNDC5 and PGC1α) in two different skeletal muscles, and levels of serum-irisin and -thyroid hormones, before and after the training intervention.
  
  
  RESULTS
  The strength training intervention did not result in changes in serum-irisin or muscle FNDC5 expression, despite considerable effects on strength, lean body mass (LBM) and skeletal muscle phenotype. Our data indicate that training affects irisin biology in a LBM-dependent manner. However, no association was found between steady-state serum-irisin or training-associated changes in serum-irisin and alterations in body composition. FNDC5 expression was higher in m.Biceps brachii than in m.Vastus lateralis, with individual expression levels being closely correlated, suggesting a systemic mode of transcriptional regulation. In pre-biopsies, FNDC5 expression was correlated with proportions of aerobic muscle fibers, a relationship that disappeared in post-biopsies. No association was found between serum-thyroid hormones and FNDC5 expression or serum-irisin.
  
  
  CONCLUSIONS
  No evidence was found for an effect of strength training on irisin biology in untrained women, though indications were found for a complex interrelationship between irisin, body mass composition and muscle phenotype. FNDC5 expression was closely associated with muscle fiber composition in untrained muscle.
authors.affiliation: Lillehammer University College, P.O. Box 952, 2604, Lillehammer, Norway, Stian.ellefsen@hil.no.
authors.forename: S
authors.initials: S
authors.lastname: Ellefsen
authors.forename: O
authors.initials: O
authors.lastname: Vikmoen
authors.forename: G
authors.initials: G
authors.lastname: Slettaløkken
authors.forename: J E
authors.initials: JE
authors.lastname: Whist
authors.forename: H
authors.initials: H
authors.lastname: Nygaard
authors.forename: I
authors.initials: I
authors.lastname: Hollan
authors.forename: I
authors.initials: I
authors.lastname: Rauk
authors.forename: G
authors.initials: G
authors.lastname: Vegge
authors.forename: T A
authors.initials: TA
authors.lastname: Strand
authors.forename: T
authors.initials: T
authors.lastname: Raastad
authors.forename: B R
authors.initials: BR
authors.lastname: Rønnestad
mesh_terms: D000328:Adult; D001835:Body Weight; D005260:Female; D005353:Fibronectins / Q000097:blood / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D018482:Muscle, Skeletal / Q000378:metabolism* / Q000502:physiology; D000071248:Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1-alpha; D010641:Phenotype; D055070:Resistance Training; D013963:Thyroid Hormones / Q000097:blood; D014157:Transcription Factors / Q000097:blood / Q000235:genetics / Q000378:metabolism
chemical_list: C577593:FNDC5 protein, human; D005353:Fibronectins; C486497:PPARGC1A protein, human; D000071248:Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1-alpha; D013963:Thyroid Hormones; D014157:Transcription Factors