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One of the most important breakthroughs in recent years has been the discovery of “peripheral clocks.” We’ve known about the central clock, the so-called suprachiasmatic nucleus, for decades. It sits in the middle of your brain, right above where your optic nerves cross––allowing it to respond to night and day. Now we know there are semi-autonomous clocks in nearly every organ of our body. Our heart runs on a clock, our lungs run on a clock, our kidneys run on a clock. Up to 80 percent of the genes in our liver are expressed in a circadian rhythm; our entire digestive tract does too.
The rate at which our stomach empties, the secretion of digestive enzymes, and the expression of transporters in our intestinal lining for absorbing sugar and fat all cycle around the clock. So too does the ability of our body fat to sop up extra calories. The way we know these cycles are driven by local clocks, rather than being controlled by our brain, is that you can take surgical biopsies of fat, put them in a petri dish, and still watch them rhythm away.
All this clock talk is not just biological curiosity. Our health may depend on keeping all these clocks in sync. Think of it like a child playing on a swing. Imagine you’re pushing, but you become distracted by other goings-on in the playground, and stop paying attention to your timing. So, you forget to push, or push too early or too late. What happens? Out of sync, the swinging becomes erratic, slows, or even stops. That is what happens when we travel across multiple time zones or have to work the night shift.
The “pusher” in this case are the light cues falling onto our eyes. Our circadian rhythm is meant to get a bright light push every morning at dawn. But if the sun rises at a different time, or we’re exposed to bright light in the middle of the night, this can push our cycle out of sync and leave us feeling out of sorts. That’s an example of a mismatch between the external environment and our central clock. Problems can also arise from a misalignment between the central clock in our brain and all the other organ clocks throughout our body. An extreme illustration of this is a remarkable set of experiments suggesting even our poop can get jet lag.
Our microbiome seems to have its own circadian rhythm. Even though they’re down where the sun don’t shine, there’s a daily oscillation in both bacterial abundance and activity in our colon. Interesting, but who cares? Check this out: if you put people on a plane and fly them halfway around the world, then feed their poop to mice, they grow fatter than mice fed preflight feces. Though it may have just been bad airline food or something, the researchers suggest the fattening flora was a consequence of circadian misalignment. Indeed, several lines of evidence now implicate “chronodisruption”—the state in which our central and peripheral clocks diverge out of sync—as playing a role in conditions ranging from premature aging and cancer to mood disorders and obesity.
Bright light exposure is the synchronizing swing pusher for our central clock. What drives our internal organ clocks that aren’t exposed to daylight? Food intake. That why the timing of our meals may be so important. Removing all external timing cues by locking people away under constant dim light, researchers showed you could effectively decouple central from peripheral rhythms just by shifting meal times. They took blood draws every hour, and even took biopsies of their fat every six hours, to demonstrate the resulting metabolic disarray.
Just as morning light can help sync your central clock in your brain, morning meals can help sync your peripheral clocks throughout the rest of your body. Breakfast-skipping disrupts the normal expression and rhythm of these clock genes themselves, which coincides with the adverse metabolic effects. Thankfully, they can be reversed. Take a group of habitual breakfast skippers, and have them eat three meals at 8am, 1pm, and 6pm, and their cholesterol and triglycerides improves compared to taking meals five hours later at 1pm, 6pm, and 11pm. There’s a circadian rhythm to cholesterol synthesis in the body, too––which is also strongly influenced by food intake, as evidenced by the 95 percent drop in cholesterol production in response to a single day of fasting. That why just a few hours’ shift in meal timing can result in a 20-point drop in LDL cholesterol, thanks to eating earlier meals.
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