Abstract
BackgroundLow vitamin D status increases the risk of stress fractures. As ultraviolet (UV) light is required for vitamin D synthesis, low UV light availability is thought to increase the risk of vitamin D insufficiency and poor bone health. The purpose of this investigation was to determine if individuals with low UV intensity at their home of record (HOR) or those with darker complexions are at increased risk of developing stress fractures and lower limb fractures during U.S. Army Basic Combat Training (BCT).MethodsThis was a retrospective cohort study using the Armed Forces Health Surveillance Center data repository. All Basic trainees were identified from January 1997 to January 2007. Cases were recruits diagnosed with stress fractures and lower limb fractures during BCT. The recruit’s home of record (HOR) was identified from the Defense Manpower Data Center database. The average annual UV intensity at the recruits’ HOR was determined using a U.S National Weather Service database and recruits were stratified into low (≤3.9); moderate (4.0-5.4), and high (≥5.5) UV index regions. Race was determined from self-reports.ResultsThe dataset had 421,461 men and 90,141women. Compared to men, women had greater risk of developing stress fractures (odds ratio (OR) = 4.5, 95% confidence interval (95%CI) = 4.4-4.7, p < 0.01). Contrary to the hypothesized effect, male and female recruits from low UV index areas had a slightly lower risk of stress fractures (male OR (low UV/high UV) = 0.92, 95%CI = 0.87-0.97; females OR = 0.89, 95%CI = 0.84-0.95, p < 0.01) and were at similar risk for lower limb fractures (male OR = 0.98, 95%CI = 0.89-1.07; female OR = 0.93, 95%CI = 0.80-1.09) than recruits from high UV index areas. Blacks had lower risk of stress and lower limb fractures than non-blacks, and there was no indication that Blacks from low UV areas were at increased risk for bone injuries.ConclusionsThe UV index at home of record is not associated with stress or lower limb fractures in BCT. These data suggest that UV intensity is not a risk factor for poor bone health in younger American adults.
Highlights
Low vitamin D status increases the risk of stress fractures
The incidence rates for stress fractures were 26.0, 31.3 and 31.1 cases/1,000 person years for low, moderate and high UV index regions, respectively, with those in the low UV index group showing a lower risk compared to the high UV index group (OR low UV/high UV = 0.83, 95% confidence interval (95%CI) = 0.80-0.87; p < 0.01)
There was no evidence to support the idea that individuals with dark complexion who were residing in low solar load areas were at added risk of developing stress fractures, as individuals self-reporting to be black or of Hispanic ancestry and residing in low UV index areas prior to entering the military had injury risks that were little different from individuals that had been residing in high solar load areas
Summary
As ultraviolet (UV) light is required for vitamin D synthesis, low UV light availability is thought to increase the risk of vitamin D insufficiency and poor bone health. The purpose of this investigation was to determine if individuals with low UV intensity at their home of record (HOR) or those with darker complexions are at increased risk of developing stress fractures and lower limb fractures during U.S Army Basic Combat Training (BCT). The ability and availability of sunlight to stimulate the synthesis of vitamin D is reduced at northern latitudes and in regions with low ultraviolet radiation [1]. This was not the case for Black and Asian women, nor Hispanic and American Indian women [13]
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