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Importance of Sleep Disorders in Assessing the Association Between Coffee Consumption and All-Cause Mortality

      To the Editor:
      Liu et al
      • Liu J.
      • Sui X.
      • Lavie C.J.
      • et al.
      Association of coffee consumption with all-cause and cardiovascular disease mortality.
      have reported an interesting observational study in which they found a positive relationship between very high coffee consumption and all-cause mortality among men and among both men and women aged less than 55 years.
      • Liu J.
      • Sui X.
      • Lavie C.J.
      • et al.
      Association of coffee consumption with all-cause and cardiovascular disease mortality.
      Because the general finding for men appears to be attributable to the increased risk among those younger than 55 years, the authors concluded that “on the basis of these findings, it seems appropriate to suggest that younger people avoid heavy coffee consumption (ie, averaging >4 cups per day).” However, while this study's findings are suggestive, it may be premature to make any clinical recommendations based on these results.
      This study has a number of strengths, including both a relatively large number of participants and adjustment for a broad array of potential confounders, but one key variable that was not included in the study was sleep disorders. Because one of the traditional uses of coffee is to compensate for feelings of sleepiness, it is reasonable to suppose that this may prove an important confounder, with higher consumption positively related to sleep disorders and the presence of sleep disorders positively related to all-cause mortality.
      Daytime sleepiness has been documented to be an independent risk factor for stroke and other vascular disease,
      • Boden-Albala B.
      • Roberts E.T.
      • Bazil C.
      • et al.
      Daytime sleepiness and risk of stroke and vascular disease: findings from the Northern Manhattan Study (NOMAS).
      and both insufficient and excessive sleep have been associated with an increased risk of all-cause mortality.
      • Hublin C.
      • Partinen M.
      • Koskenvuo M.
      • Kaprio J.
      Sleep and mortality: a population-based 22-year follow-up study.
      • Gallicchio L.
      • Kalesan B.
      Sleep duration and mortality: a systematic review and meta-analysis.
      Even more striking, the relationship between sleep disorders and all-cause mortality exhibits an age dependency similar to that reported by Liu et al. Lavie et al
      • Lavie P.
      • Lavie L.
      • Herer P.
      All-cause mortality in males with sleep apnoea syndrome: declining mortality rates with age.
      found that the excess mortality attributable to sleep apnea among men was limited to those younger than 50 years, and similar trends are also found in the relationship between long or short sleep and all-cause mortality.
      • Hublin C.
      • Partinen M.
      • Koskenvuo M.
      • Kaprio J.
      Sleep and mortality: a population-based 22-year follow-up study.
      Of course, it is also possible that the causal pathway flows instead from excessive coffee consumption to insomnia and thence to an increased risk of mortality. Determining whether one or both of these scenarios explains the increased risk of mortality will require further research that takes sleep into account. It will also be important to measure sleep, coffee intake, and mortality at multiple time points throughout the study in order to establish the temporal sequence between sleep and coffee consumption and to account for changes in these variables over time.
      The study by Liu et al
      • Liu J.
      • Sui X.
      • Lavie C.J.
      • et al.
      Association of coffee consumption with all-cause and cardiovascular disease mortality.
      is an important contribution toward a better understanding of the relationship between coffee intake and all-cause mortality, but the state of the science may be too young at this time to enable clinicians or public health authorities to provide specific advice to patients.

      References

        • Liu J.
        • Sui X.
        • Lavie C.J.
        • et al.
        Association of coffee consumption with all-cause and cardiovascular disease mortality.
        Mayo Clin Proc. 2013; 88: 1066-1074
        • Boden-Albala B.
        • Roberts E.T.
        • Bazil C.
        • et al.
        Daytime sleepiness and risk of stroke and vascular disease: findings from the Northern Manhattan Study (NOMAS).
        Circ Cardiovasc Qual Outcomes. 2012; 5: 500-507
        • Hublin C.
        • Partinen M.
        • Koskenvuo M.
        • Kaprio J.
        Sleep and mortality: a population-based 22-year follow-up study.
        Sleep. 2007; 30: 1245-1253
        • Gallicchio L.
        • Kalesan B.
        Sleep duration and mortality: a systematic review and meta-analysis.
        J Sleep Res. 2009; 18: 148-158
        • Lavie P.
        • Lavie L.
        • Herer P.
        All-cause mortality in males with sleep apnoea syndrome: declining mortality rates with age.
        Eur Respir J. 2005; 25: 514-520

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