American Journal of Epidemiology Advance Access originally published online on September 8, 2005
American Journal of Epidemiology 2005 162(9):921; doi:10.1093/aje/kwi298
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Letter to the Editor |
THREE AUTHORS REPLY
1 Center for Health Care Knowledge Management, Department of Veterans Affairs–New Jersey Health Care System, East Orange, NJ 07018
2 Department of Preventive Medicine and Community Health, University of Medicine and Dentistry of New Jersey–New Jersey Medical School, Newark, NJ 07101
3 Department of Medicine, University of Medicine and Dentistry of New Jersey–New Jersey Medical School, Newark, NJ 07101
4 Birmingham VA Medical Center, Birmingham, AL 35233
We thank Dr. Dowell for his comments (1
) on our article (2) and agree that we were unable to conclude that cold winter temperature may be causing seasonal variations in hemoglobin A1c values. We note that the objective of our paper was to study seasonal effects on hemoglobin A1c levels by use of large-scale population information and a more sophisticated analytical method that includes the trigonometric terms sine and cosine and accounts for autocorrelation of monthly hemoglobin A1c levels. We were also interested in knowing whether the empirical seasonal patterns were consistent across various clinically important subpopulations. The results indicated that the seasonal patterns were consistent across "... all sex, race, age, and diabetes severity groups" (2, p. 571).
To understand the potential reasons for such seasonal variations in hemoglobin A1c levels, we considered two mechanisms: a winter "holiday effect" and climate (e.g., colder temperature). A winter holiday effect is clinically relevant because it relates to dietary indiscretion, an important aspect of diabetes management. Because the seasonal patterns found in our study were not consistent across different climate regions, we concluded that, at the population level, there was no evidence from our data to support such a mechanism. Clearly, cold temperatures could be a marker for other environmental characteristics that also vary by season, such as the dark-light variations pointed out by Dr. Dowell. We welcome and appreciate his comments that have added to our discussion of possible mechanisms for seasonal patterns of hemoglobin A1c levels; this may also have an impact on the studies of seasonal variations in other physiologic markers and disease outcomes. We think that studies to assess the various possible mechanisms causing seasonal fluctuations are intriguing and highly interesting. Further studies will be needed to address these complex issues.
ACKNOWLEDGMENTS
Conflict of interest: none declared.
References
- Dowell SF. Re: "Seasonal patterns in monthly hemoglobin A1c values." (Letter). Am J Epidemiol 2005;162:920–1.
[Free Full Text] - Tseng CL, Brimacombe M, Xie M, et al. Seasonal patterns in monthly A1c values. Am J Epidemiol 2005;161:565–74.
[Abstract/Free Full Text]
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