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West NA, Haan MN.West NA, et al.J Gerontol A Biol Sci Med Sci. 2009 Jan;64(1):103-9. doi: 10.1093/gerona/gln006. Epub 2009 Jan 23.J Gerontol A Biol Sci Med Sci. 2009.PMID: 19168781Free PMC article.