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Muscle Mass Index Revisited

  • John A. Batsis
    Affiliations
    Section of General Internal Medicine, Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire
    Geisel School of Medicine at Dartmouth, Hanover, New Hampshire
    Dartmouth Center for Health and Aging, Dartmouth College, Hanover, New Hampshire
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      To the Editor:
      It was with great interest that I read the article by Srikanthan and Karlamangla,
      • Srikanthan P.
      • Karlamangla A.S.
      Muscle mass index as a predictor of longevity in older adults.
      who suggested that high muscle mass index (MMI) was associated with lower mortality. Readers are cautioned on the implications of their findings, noting some limitations to their methodologies.
      Quartiles of MMI were created on an older, although not entirely elderly, cohort. While muscle mass and quality do decrease with age, the decline accelerates in the seventh decade, with rates ranging between 7.5% and 58.3%.
      • Batsis J.A.
      • Barre L.K.
      • Mackenzie T.A.
      • Pratt S.I.
      • Lopez-Jimenez F.
      • Bartels S.J.
      Variation in the prevalence of sarcopenia and sarcopenic obesity in older adults associated with different research definitions: dual-energy X-ray absorptiometry data from the National Health and Nutrition Examination Survey 1999-2004.
      Quartile stratification is an appropriate approach that assists in the head-to-head comparison with other anthropometric measurements. However, their approach differs from that used in other diseases, including osteoporosis, whose cutoffs are based on a younger, healthier population. The quartile cut points presented are based on a referent population that itself has lower muscle mass to begin with, leading to narrower ranges within categories. Whether the differences between adjacent MMI quartiles provide adequate discrimination with changes in function, disability, or mortality, remain unclear. Methodologically, quartile cutoffs based on the entire population or a younger cohort would provide better representation of true mortality estimates.
      Muscle strength may indeed be more important than muscle mass.
      • Schaap L.A.
      • Koster A.
      • Visser M.
      Adiposity, muscle mass, and muscle strength in relation to functional decline in older persons.
      While the authors recognize this phenomenon, a recent meta-analysis demonstrates even more reliable associations with strength than mass. Further, appendicular lean mass is associated with functional decline in longitudinal studies
      • Cesari M.
      • Pahor M.
      • Lauretani F.
      • et al.
      Skeletal muscle and mortality results from the InCHIANTI Study.
      and should be considered in lieu of total fat-free mass. Recent guidelines recommend this measure,
      • Studenski S.A.
      • Peters K.W.
      • Alley D.E.
      • et al.
      The FNIH sarcopenia project: rationale, study description, conference recommendations, and final estimates.
      even after adjusting for body mass index. Accurate body composition assessment with physiologic muscle parameters is urgently needed for incorporation into clinical practice.
      • Batsis J.A.
      • Sahakyan K.R.
      • Rodriguez-Escudero J.P.
      • Bartels S.J.
      • Lopez-Jimenez F.
      Normal weight obesity and functional outcomes in older adults.
      Readers are cautioned that MMI may not be an ideal indicator as compared with muscle strength or appendicular lean mass, and that the capacity to live independently with maintained function may supercede mortality as an important geriatric outcome.

      References

        • Srikanthan P.
        • Karlamangla A.S.
        Muscle mass index as a predictor of longevity in older adults.
        Am J Med. 2014; 127: 547-553
        • Batsis J.A.
        • Barre L.K.
        • Mackenzie T.A.
        • Pratt S.I.
        • Lopez-Jimenez F.
        • Bartels S.J.
        Variation in the prevalence of sarcopenia and sarcopenic obesity in older adults associated with different research definitions: dual-energy X-ray absorptiometry data from the National Health and Nutrition Examination Survey 1999-2004.
        J Am Geriatr Soc. 2013; 61: 974-980
        • Schaap L.A.
        • Koster A.
        • Visser M.
        Adiposity, muscle mass, and muscle strength in relation to functional decline in older persons.
        Epidemiol Rev. 2012 Dec 4; ([Epub ahead of print])
        • Cesari M.
        • Pahor M.
        • Lauretani F.
        • et al.
        Skeletal muscle and mortality results from the InCHIANTI Study.
        J Gerontol A Biol Sci Med Sci. 2009; 64: 377-384
        • Studenski S.A.
        • Peters K.W.
        • Alley D.E.
        • et al.
        The FNIH sarcopenia project: rationale, study description, conference recommendations, and final estimates.
        J Gerontol A Biol Sci Med Sci. 2014; 69: 547-558
        • Batsis J.A.
        • Sahakyan K.R.
        • Rodriguez-Escudero J.P.
        • Bartels S.J.
        • Lopez-Jimenez F.
        Normal weight obesity and functional outcomes in older adults.
        Eur J Intern Med. 2014; 25: 517-522

      Linked Article

      • Muscle Mass Index As a Predictor of Longevity in Older Adults
        The American Journal of MedicineVol. 127Issue 6
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          Obesity (as defined by body mass index) has not been associated consistently with higher mortality in older adults. However, total body mass includes fat and muscle, which have different metabolic effects. This study was designed to test the hypothesis that greater muscle mass in older adults is associated with lower all-cause mortality.
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      • The Reply
        The American Journal of MedicineVol. 127Issue 12
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          We want to address the issues raised by Batsis regarding the methodological rigor of our assessment of muscle mass index.
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