Obesity has become a significant public health problem in the United States, with almost $180 billion per year spent on obesity-related healthcare costs. As such, it has become increasingly important to identify the causes and risk factors for obesity. One potential cause of obesity is the disruption of circadian rhythms. In recent years, epidemiological studies have shown that the average amount of overnight sleep has decreased by almost 20 minutes in the last 30 years, while rates of obesity have increased. This report explores the relationship between circadian rhythms, metabolic processes, and the regulation of energy balance.
Our circadian rhythms are mainly controlled by the "master clock" found in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN responds primarily to light cues, although there are peripheral clocks distributed throughout the peripheral nervous system (PNS) that are sensitive to temperature, hormones, feeding or fasting states, physical activity, and more. Our peripheral clocks evolved to synchronize with environmental cues and regulate various physiological processes, including metabolism. Phyllis C. Zee et al.'s review article "Circadian Disruption and Human Health" explores the relationship between disturbed circadian rhythms and many adverse health conditions. Disrupted circadian cycles have strong correlations with neurodegenerative disorders, psychiatric conditions, immune dysfunction, and metabolic disorders such as diabetes and obesity. Even in the absence of a specific disorder, circadian rhythms are an essential component of overall health.
The article "Effect of Circadian Rhythm on Metabolic Processes and the Regulation of Energy Balance" expands upon the correlation found between disturbed sleep rhythms and obesity. This research posits that disturbed circadian rhythms are a cause of obesity and poor energy metabolism, rather than just a correlation as found in other disorders. Energy metabolism involves various hormones and enzymes that are modulated by circadian rhythms. A study involving selective lesions of the SCN in rats showed that insulin secretion is regulated by circadian rhythms. The rats with lesions showed insulin resistance and impaired glucose metabolism, both factors leading to obesity. Leptin, a hormone that regulates long-term appetite and satiety, is secreted by adipose tissue and is also regulated by circadian rhythms with blood levels peaking at night. Poor sleep can reduce the secretion of leptin and can indirectly effect energy homeostasis.
Basal metabolic rate (BMR), or the amount of energy needed for the body to conduct essential body functions, is highly dependent on sleep and circadian rhythms. It is vital that the brain goes through complete sleep cycles, and REM sleep is considered the most important because of its role in memory and emotional processing. While in REM sleep, the sympathetic nervous system becomes active, and energy expenditure and metabolic rate peak. REM sleep deficits cause decreases in BMR, leaving those who work night shifts or sleep late at an increased risk of obesity.
Physical activity has been identified as a successful method of synchronizing peripheral clocks via the physiological changes it induces, including body temperature, hormone regulation, and sympathetic nervous system activation. The timing of the exercise is also a key factor for peripheral clock synchronization. A study has shown that in mice, low-intensity physical activity 2 hours a day for four weeks is sufficient to shift the circadian cycle.
It has been determined that there is a strong correlation between disturbed circadian rhythms and obesity. Disruptions in circadian rhythms can cause various metabolic dysfunctions, including insulin resistance, impaired glucose metabolism, and decreased BMR. The standard treatment for obesity is to reduce energy intake, but consideration of timing and circadian cycles should also be taken into account. Developing strategies to improve circadian rhythms and synchronization of peripheral clocks may be a promising approach to the treatment and prevention of obesity.
References
Fishbein, A. B., Knutson, K. L., & Zee, P. C. (2021, October 7). Circadian disruption and human health. The Journal of Clinical Investigation. Retrieved March 3, 2023, from https://www.jci.org/articles/view/148286
Serin, Y., & Acar Tek, N. (2019). Effect of circadian rhythm on metabolic processes and the regulation of Energy Balance. Annals of Nutrition and Metabolism, 74(4), 322–330. https://doi.org/10.1159/000500071
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