Circadian diet another form of intermittent fasting

Dear Doctor: A yoga teacher at our gym has been talking about something called the circadian diet. He says it’s good for your gut microbiome. I’ve heard of circadian rhythms, but I don’t see how that’s connected to the gut. What is a circadian diet anyway?

Dear Reader: At the start of the 18th century, astronomer Jean-Jacques d'Ortous de Mairan’s experiments with mimosa plants opened the door to the idea of a biological clock. Ever since then, researchers have been fascinated by the concept. A few hundred years of study later, they had arrived at a good understanding of what they eventually came to call the circadian clock -- that is, the roughly 24-hour internal cycle that coincides with the flow of day to night.

While this cycle is keyed to the daily pattern of light and dark, it’s actually controlled by the body itself. It’s widely understood today that in addition to the existence of a master biological clock, located in a light-sensitive part of the brain called the hypothalamus, each individual cell in the body also operates on a circadian cycle of its own.

Working as a whole, these timing mechanisms form a complex matrix. They play a role in virtually all of our bodily processes, including sleep-wake cycles; metabolism; hormonal activity; body temperature; the cardiovascular, gastrointestinal and nervous systems; organ function; and the gut microbiota. When our internal clocks and the external cues of light and dark fall out of sync, we suffer physical consequences. Jet lag and the adverse health effects of night shift work are good examples.

Recent research has turned up new and intriguing connections between the gut microbiome and the circadian cycle. A study in mice found that the microbes in their guts produced measurably more of a natural antimicrobial compound during the day than at night. It made them better at fending off possible food poisoning during the hours they were most likely to be eating. This may shed light on why people who experience chronic sleep disruption have been found to be more susceptible to intestinal infections. The results of another study, which restricted the times rodents were given food, suggested that the way in which the body metabolizes fats depended on the time of day that food is consumed.

Elizabeth Ko, MD and Eve Glazier, MD

Humans generally eat during the day and fast at night while we sleep. This is basically a form of intermittent fasting, which research suggests has a range of health benefits such as improved blood sugar control, lower levels of inflammation, better blood pressure numbers and more healthful blood lipid levels.

In the circadian diet, you eat during a 12-hour window -- typically between 7 a.m. and 7 p.m. -- and fast during the other 12 hours. Meal sizes are flipped, with breakfast the largest meal of the day and dinner the smallest. That 12-hour nightly fast eliminates after-dinner snacking and midnight raids on the fridge.

It’s important to note that at this time, while proponents of the circadian diet discuss its benefits with great certainty, there’s a shortage of reliable studies to back their various claims.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)


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