To the Editor:
We read “Sleep, Obesity, and Injury Among US Male Career Firefighters” by Kaipust et al1 with great interest. This paper indicated that body composition may serve as the effect modifier for the association between sleep and injury among on-duty male career firefighters. This association is found in obese firefighters, but not for normal or overweight firefighters. Considering the association between obesity and disordered sleep, coupled with an increased likelihood of occupational accidents in the presence of obstructed sleep,2,3 it is of upmost importance to investigate the role of obesity on sleep–injury association. This study successfully addressed the association between body composition and sleep among this population, however, some limitations exist in their methodology and some clarifications are needed to further support their findings.
First, the methodology used to test physical activity and fitness is needed to further address. The non-exercise VO2max model using the Self-Report of Physical Activity (SRPA)4 lacks accuracy for predicting VO2max when it compared with VO2max equation derived from the International Physical Activity Questionnaire (IPAQ).5 The non-exercise VO2max model is not accurate to predict VO2max among highly fit occupation that requires high physical demands.4 Compared with the non-exercise VO2max model, VO2max equation adapted in IPAQ5 is more accurate and widely accepted to predict VO2max among aerobically well-trained individuals, for example, firefighters with high physical demands.6,7 Additionally, both physical activity and fitness measurements are adapted in SRPA questionnaire. However, in model building process, fitness was not included in the mixed effects logistic regression for injury on sleep. Considering that firefighters with vigorous physical activity more than 3 hours per week were more likely to injury (odds ratio [OR]: 1.76; 95% confidence interval [CI]: 1.06, 2.92) compared with less than or equal to 1 hour moderate physical activity, fitness with correct measure may also reach the significance level. If so, future study may raise up another research question: will fitness modify sleep–injury association?
Second, the authors found that only obese firefighters without enough sleep are twice as likely to develop on-exercise injury compared with those with enough sleep. This significant association was not found in normal or overweight firefighters. The authors did not discuss possible explanations that could contribute to this inconsistent finding. One possible explanation is that the addition of several different moderators, such as lifestyle behaviors, which may offset the significance level. Additionally, previous studies have documented that sleep issues are directly associated with occupational injury,8 that sleep issues are positively associated with obesity risk,9 and that obesity is highly correlated with occupational injury.10 Therefore, beyond the moderator, obesity may serve as a mediator in the temporality between sleep and injury.
Third, this study may not be a strict observational study because the intervention, the Fire Service Joint Labor Management Wellness Fitness Initiative11 as a wellness program, has been included. Additional limitation in their methodology is sleep measurement. Their sleep related questions lack validation and support for measuring the complexities of sleep quality. More information is needed to support the administration of this questionnaire as opposed to a more comprehensive and validated sleeping questionnaire, for example, the Pittsburg Sleep Quality Index.12
Though the findings reported in this paper can provide important information for understanding the association between sleep and body composition among firefighters, some limitations still exist. More support is necessary for defending the method used in measuring these complex variables.
X.G. is the primary author of this letter. N.J.D., K.M., and T.A. reviewed, edited, and revised this letter. All authors contributed equally to this letter.
1. Kaipust CM, Jahnke SA, Poston WS, et al. Sleep, obesity, and injury among US male career firefighters. J Occup Environ Med
2. Garbarino S, Guglielmi O, Sanna A, Mancardi GL, Magnavita N. Risk of occupational accidents in workers with obstructive sleep apnea: systematic review and meta-analysis. Sleep
3. Vgontzas AN, Tan TL, Bixler EO, Martin LF, Shubert D, Kales A. Sleep apnea and sleep disruption in obese patients. Arch Inter Med
4. Jackson AS, Blair SN, Mahar MT, Wier LT, Ross RM, Stuteville JE. Prediction of functional aerobic capacity without exercise testing. Med Sci Sports Exerc
5. Schembre SM, Riebe DA. Non-exercise estimation of VO2
max using the international physical activity questionnaire. Measur Phys Educ Exerc Sci
6. Malek MH, Berger DE, Housh TJ, Coburn JW, Beck TW. Validity of VO2
max equations for aerobically trained males and females. Med Sci Sports Exerc
7. Gledhill N, Jamnik V. Characterization of the physical demands of firefighting. Can J Sport Sci
8. Uehli K, Mehta AJ, Miedinger D, et al. Sleep problems and work injuries: a systematic review and meta-analysis. Sleep Med Rev
9. Knutson KL, Van Cauter E. Associations between sleep loss and increased risk of obesity and diabetes. Ann N Y Acad Sci
10. Kouvonen A, Kivimäki M, Oksanen T, et al. Obesity and occupational injury: a prospective cohort study of 69,515 public sector employees. PLoS One
11. Poston WS, Haddock CK, Jahnke SA, Jitnarin N, Day RS. An examination of the benefits of health promotion programs for the national fire service. BMC Public Health
12. Carpenter JS, Andrykowski MA. Psychometric evaluation of the Pittsburgh sleep quality index. J Psychosom Res