Original ArticleSleep duration and obesity among adults: a meta-analysis of prospective studies
Introduction
Overweight and obesity are severe health burdens both in developed and developing countries. About 500 million adults worldwide were obese in 2005, and this number has doubled since 1980 and is projected to increase to 1.1 billion by 2030 [1]. In simplistic terms, obesity develops when energy intake is greater than energy expenditure. Although diet and physical activity play an important role in the risk of weight gain and obesity, a potential additional factor may be sleep.
During the past couple of decades, several cross-sectional studies have found a significant association between short sleep duration and increased prevalence of obesity or weight gain in adults from various countries [2], [3]. Some of these studies also observed a U-shaped association between sleep duration and body mass index (BMI). By definition, the findings of cross-sectional studies give little information about the direction of causality. Obesity is a strong risk factor for diseases such as sleep apnea and arthritis that may limit an individual's ability to sleep [4]. Longitudinal researches can overcome the problems inherent in cross-sectional studies, and thus can provide more convincing results [5]. However, the results of longitudinal associations between sleep duration and obesity were conflicting [4], [5], [6]. Most recently, a systematic review of a cohort study by Magee and Hale concluded that the association between sleep duration and subsequent weight gain is unclear in adults [7]. However, they did not provide a quantitative risk estimate. Besides, other studies [8], [9], [10], [11] have since been published. Therefore, we conducted a meta-analysis of prospective studies to: (1) first assess the risk of obesity for the short sleep duration vs normal sleep duration as well as the long sleep duration vs normal sleep duration; (2) then evaluate the modification of key covariates to the association between sleep duration and obesity risk among adults; (3) and explore the heterogeneity among studies and publication bias.
Section snippets
Literature search and selection
A search of the literature up to 20 February 2014 was performed using the databases of PubMed and Web of Science using the following search terms: (sleep duration or sleep time), (obesity or weight gain), and (cohort) without restrictions. Moreover, we reviewed the reference lists from retrieved articles to search for further relevant studies.
Two investigators independently reviewed all identified studies, and studies were included if they met the following criteria: (1) a prospective design
Literature search and study characteristics
The detailed steps of our literature search are shown in Fig. 1. On the basis of the search strategy and inclusion criteria, 17 papers seemed to be eligible after reading the title or abstract. After the full texts had been read, two papers [15], [16] were excluded because of duplicate reports from the same study population; two papers [17], [18] were excluded because they provided data only on the prospective association between sleep duration and weight gain only; and two papers [19], [20]
Discussion
This study provides for the first time a meta-analysis of prospective-only studies examining the association between sleep duration and future obesity. The meta-analysis of prospective studies, including 197,906 participants for short sleep duration and 164,016 participants for long sleep duration, strongly identified that short sleep duration was significantly associated with increased risk of obesity, whereas long sleep duration had no contribution to future obesity in adults.
The mechanisms
Funding
This research project was supported by grants from the National Natural Science Foundation of China (81302485) and the Natural Science Foundation of Shandong Province (BS2013SF002).
Conflict of interest
The ICMJE Uniform Disclosure Form for Potential Conflicts of Interest associated with this article can be viewed by clicking on the following link: http://dx.doi.org/10.1016/j.sleep.2014.07.018.
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