Keeping Up With the Clock

Abstract

Estimates of obesity prevalence based on the 2013-2014 National Health and Nutrition Examination Survey average almost 40%, corresponding to a 7% increase from the 1999-2000 cycle. 1 Similar alarming trajectories are reported worldwide, 2 along with increasing concerns for related health risks. The traditional concept that obesity results from a mere imbalance between energy intake and energy consumption has evolved to incorporate novel modulating factors, which may be targeted to develop preventative strategies. One such example is the potential role of disruption of circadian rhythms. In this brief review, the physiology of circadian control of body weight and metabolism, and the implications of altered circadian rhythms for obesity risk in humans will be discussed, together with potential countermeasures that may mitigate such consequences. The focus is on evidence accumulated from human studies. For a comprehensive review on the molecular mechanisms underlying the clock in metabolism and obesity, we direct the interested readers to Eckel-Mahan and Sassone-Corsi. 3 Historical Perspective Circadian systems have evolved as biological mechanisms to facilitate adaptation to the daily change from day to night, with related changes in activity and other behaviors. The intrinsic oscillatory system enables organisms to optimally synchronize their physiological and behavioral rhythms to the 24-hour solar day. Although humankind has been aware of the existence of periodicity in natural phenomena, the origins of chronobiology as a science are relatively recent and can be traced back to the seventeenth century, with the landmark studies of Jean-Jacques de Mairan, a French astronomer. De Mairan noted that the daily leaf movements of the Mimosa plant, synchronous with the day-night cycle, persisted even in the absence of light exposure. Based on these observations, he hypothesized an endogenous botanic rhythm driving the plant movements. Confirmation of this was provided by the Swiss botanist Augustus Pyramus de Candolle, who showed that the period of such internal rhythmicity not only persisted under