Teens suffering from obesity have a broken circadian clock

Since 1990, obesity among adults has doubled worldwide and has quadrupled among adolescents. There are identified causes, such as the substitution of food with ultra-processed products, but there are other reasons that still aren’t so clear. One of the factors that can influence weight gain is the disruption of routines that affect the circadian clock. This clock helps the body optimize its functioning, such as getting it ready to sleep and starting repair processes when it gets dark, or helping the system digest food at certain times. However, when mealtimes are irregular, or when light confuses the body about the time of rest, everything becomes deregulated… and the risk of becoming overweight increases.

On February 18, the peer-reviewed scientific journal PNAS published a paper which analyzed the influence of the circadian clock in the regulation of what is eaten. The team that carried out the study — led by Mary Carskadon, a professor at Brown University and director of the EP Bradley Hospital Sleep Research Lab — followed 51 teenagers (ages 12 to 15) for 11 days and 10 nights. To separate the effects of the participants’ internal clock from their daily habits, the researchers broke those days into seven 28-hour cycles, in which the teens were awake for 17.5 hours and asleep for 10.5.

The results show that, first and foremost, the circadian clock and lifestyle habits separately influence caloric intake, but that eating habits are more important. They also found that the influence of the circadian rhythm was less prevalent in overweight or obese adolescents than in the rest, which confirms previous results. This indicates that a break with the rhythms that nature sets — by staying up late into the night or working extended shifts — is harmful to health. Additionally, the study shows that adolescents suffering from obesity consumed more calories later in the day. In participants with a healthy weight, the peak of caloric intake came at around 5:30 p.m. — due to the influence of the circadian system — but in those with an unhealthy weight, this peak was delayed.

“We believe that the weaker circadian signal in the obese group is related to the fact that young people with obesity aren’t as synchronized with circadian or other biologically meaningful cues [...] they may instead be more influenced by other factors that drive eating behavior, such as [poor] eating habits, the pleasure of eating, sensory and emotional stimuli, or other aspects of hedonic hunger,” explains David Barker, a professor at Brown University and co-author of the study. Hedonic hunger is when one eats to obtain pleasure, even if there’s no energy deficit.

“This research is consistent with evidence suggesting that the timing of caloric intake is associated with weight,” Barker continues. “Implications for prevention and treatment include working with patients to adjust the timing of their caloric intake, as well as intervening when it comes to circadian synchronization, by reducing light exposure in the later hours of the day and increasing bright light in the morning.” He acknowledges that “more research is needed to understand the optimal timing of caloric intake and how this relates to weight.”

As part of a strategy to reduce consumption as the day progresses, it’s advisable to concentrate a greater part of the caloric intake in the first hours of the day, especially at breakfast. Additionally, maintaining regular meal times could be beneficial. And such a pattern could also be extended to other habits, such as going to bed and waking up at the same time.

Marta Garaulet, a professor of physiology and physiological bases of nutrition at the University of Murcia, Spain, explains that “a characteristic of obesity is a flattening of the circadian cycles,” something that has also been observed in this study. “If you don’t pay attention to the signals of your biological clock, it loses effectiveness… the tissues and organs cannot predict when they’ll receive food,” she warns. To avoid the problems caused by this mismatch, Garaulet, who didn’t participate in the study published in PNAS, explains that it’s necessary to bring the circadian rhythm (and general lifestyle) back into sync. This can be achieved by exposing oneself to sunlight during the daytime and avoiding screens at night, moving physical activity away from bedtime and trying to maintain routines, even on weekends. “You have to give [the body] strong signals that aren’t contradictory, such as taking a very long nap in the middle of the day,” she offers as an example.

The professor also suggests that, although there are evening chronotypes — people who naturally get hungry later in the day and take longer to feel sleepy at night — due to the type of “early rising” societies that we inhabit, it’s better to adjust one’s circadian rhythm, or at least not reinforce late-night habits.

In the case of adolescents such as those who participated in the study, Garaulet emphasizes: “It’s the time [of life] when we’re more nocturnal… although, with age, we tend to become morning people.”

“During sleep,” she continues, “the growth hormone is produced. And this is very important in adolescents for adequate hormonal development.” For some time, it has been suggested that the difficulty of adolescents to adapt to early rising causes them to live in a permanent state of jet lag. Certain changes have been proposed to counter this, such as delaying the start of the school day. Meanwhile, various scientists — such as those that make up the Brown University team led by Carskadon — stress that there’s a need to carry out more studies, in order to better understand the mechanisms that justify the relationships observed between the time at which one eats and the effects on health.

Translated by Avik Jain Chatlani

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