By Mark Cowen, Senior medwireNews Reporter
Greater monthly increases in solar radiation at any given geographic location are associated with an earlier age at onset of bipolar disorder (BD), researchers report.
Michael Bauer (Technische Universität Dresden, Germany) and team found that each 0.1 kWh/m2 per day increase in maximum monthly solar insolation - a measure of the amount of electromagnetic energy striking the earth - was associated with nearly a 6-month decrease in age at BD onset.
Furthermore, the team found that the largest maximum monthly increases in solar insolation occurred mostly in spring in diverse latitudinal locations, such as Norway and California, USA, indicating that latitude is not associated with age at BD onset.
"Clinicians should be aware of the potential for a younger onset in locations that experience a large springtime increase in sunlight, and detailed questioning to identify symptoms of BD, and more frequent monitoring may be indicated," the researchers comment in Bipolar Disorders.
The findings come from a study of 2414 patients with BD from 24 sites in 13 countries spanning latitudes 6.3 to 63.4 degrees from the equator in both hemispheres.
Data on average yearly, seasonal, and monthly solar insolation at these locations were obtained from the US National Aeronautics and Space Administration's Surface Meteorology and Solar Energy Version 6.0 database.
Generalized estimating equation analyses revealed a significant inverse association between the maximum monthly increase in insolation and age at BD onset.
Indeed, comparing extremes, the age at BD onset in locations with the largest maximum monthly increase in solar insolation, such as Santiago in Chile, was about 5 years younger than in locations with the smallest increase, such as Medellín in Columbia.
No significant associations were observed between age at BD onset and latitude, yearly total solar insolation, and maximum monthly decrease in solar insolation.
Although there was a significant association between a country's median age and age at onset of BD - a 1-year increase in median age was associated with more than a 5-month increase in age at BD onset - the relationship between maximum monthly increase in insolation and age at BD onset remained significant after accounting for this variable.
Bauer and team conclude: "The primary finding of this study was that the larger the maximum monthly increase in solar insolation at the location of onset, the younger the age of onset of BD."
They add: "Further research about the impact of sunlight on the onset and course of bipolar disorder is needed."
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