Scientists identify how many hours to eat a day to optimize longevity

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Intermittent fasting. In the last ten years, research has found evidence that restricting the hours in which you eat could reduce the risk for obesity, diabetes, cardiovascular diseases, and other ailments that can stem from an off-balance metabolism.

The body has a network of internal clocks that respond to external cues, one of which is food consumption. Maintaining an eating schedule — and giving your cells a break from processing food — may cut down on circadian disruptions that meddle with the functions of cells and are linked to a host of metabolic health problems. Helpfully enough, the exact time breakdown is already a popular pick for those who practice intermittent fasting.

In a new article on time-restricted eating published this month Endocrine Reviews, scientists argue that the golden ratio of time spent eating to time spent fasting is 8 to 16 hours. The 8:16 time breakdown is a popular choice for adherents of intermittent fasting, but the authors of the paper do draw a distinction between intermittent fasting and time-restricted eating that’s important to call out:

“Intermittent fasting implies some kind of caloric restriction. Time-restricted eating does not,” Emily Manoogian, the paper’s lead author and a researcher at the Salk Institute for Biological Studies, tells Inverse.

“It might happen but it’s not part of the intervention.”

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In time-restricted eating, a person abstains from food during the same window of time each day. Generally, this means carving out 14 to 16 hours free of meals, snacks, and nibbles. For obvious reasons, these no-food hours usually align with sleep.

Why it’s a hack — By not eating for 14 to 16 hours, you give your body a chance to utilize the glucose it accumulated when you last ate, without having to process another load.

The body is full of internal clocks that run on a 24-hour schedule. There is a master clock in the brain, but every organ and every cell has its own peripheral clock.

One function of these clocks is to keep glucose levels in the bloodstream relatively consistent. Consuming food loads the body up with glucose and raises blood sugar levels. Afterward, insulin moves that glucose into cells to burn for energy, and blood sugar dips.

After a while, the body comes to expect a new glucose infusion at certain times, the researchers say, allowing it to better hone its process of doling out glucose and leaving less to linger in the bloodstream or eventually get converted into fat.

When a person is at rest, their insulin levels are low — at the same time, the glucose from a late-night snack can stick around and cause high blood sugar. If it stays that way, high blood sugar can cause diabetes and other metabolic conditions that affect long-term health.

But making energy out of glucose is not the only function of these peripheral clocks — each cell also needs to breaks down unnecessary molecules and toxins, as well as repair damage to its DNA and organelles.

“[This] three part-process cannot happen at once,” Satchin Panda, a professor at the Salk Institute and a co-author of the paper, tells Inverse.

By eating like Liz Lemon at any and all hours, you interrupt the time in which these cells can perform these other functions by calling them back to glucose-processing duty.

Science in action — Most studies of time-restricted eating have so far been done in mice, so it is difficult to translate the results directly to humans.

In a 2012 study, also from researchers at the Salk Institute, two groups were given all-they-could-eat access to high-fat diets, but one had access for only eight hours a day. These mice consumed the same number of calories from the same food sources as the other group but were largely free of high-fat diet-induced obesity and related metabolic illnesses. Researchers also demonstrated improvements to gene expression and metabolic pathways.

The few studies involving humans have been so-so. In one study of male endurance runners, those that completed eight weeks of eight-hour time-restricted eating regimes lost four pounds of weight when compared to a control group. The regimens did not improve their performances when it came to their sport, however.

In a study involving “healthy” males, the time-controlled eating group showed some signs of improved glucose utilization. And in another study of males with obesity, those practicing time-restricted eating for two months consumed 550 fewer calories a day on average without counting calories than the control group.

This new paper calls attention to the fact that the research is just beginning. But Manoogian says the greatest benefits of time-restricted eating should be seen over the lifetime risk of chronic disease.

“It’s an important component ignored for too long,” she says. “We know how optimal cell performance could fight disease and risk over time.”

How this affects longevity — Out-of-whack peripheral clocks lead to sub-optimal cell processing, which can have several health effects, Panda says.

“For example, when our liver or fat cells cannot break down enough stored fat — which occurs during the fasting period of the 24-hour circadian rhythm — we may accumulate too much fat in the liver, leading to fatty liver disease,” he says.

More research is needed to identify which chronic diseases may be eased by time-restricted eating, Manoogian says, but circadian disruptions are linked to a variety of problems that all do lead to chronic disease.

“It increases inflammation,” she says. “It decreases glucose regulation. It increases fat stores. There are a lot of potential problems there. Every affective disorder — depression, bipolar disorder — are linked to circadian disruption.”

Food is more disruptive to the peripheral clock system than other kinds of circadian disruptions, like inconsistent sleep and wake times, she adds.

Intermittent fasting is admittedly trendy, but studies show it may also influence how the body stores and uses up glucose without requiring changes in what or how much a person eats. However, the results so far are preliminary — but they are also exciting, particularly if we think beyond the idea of weight.

“I think its overall benefits to health that are probably more important than a modest amount of weight loss,” Manoogian says.

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Summary: Time-restricted feeding (TRF, animal-based studies) and time-restricted eating (TRE, humans) are an emerging behavioral intervention approach based on the understanding of the role of circadian rhythms in physiology and metabolism. In this approach, all calorie intake is restricted within a consistent interval of less than 12 hours without overtly attempting to reduce calories. This article will summarize the origin of TRF/TRE starting with concept of circadian rhythms and the role of chronic circadian rhythm disruption in increasing the risk for chronic metabolic diseases. Circadian rhythms are usually perceived as the sleep-wake cycle and dependent rhythms arising from the central nervous system. However, the recent discovery of circadian rhythms in peripheral organs and the plasticity of these rhythms in response to changes in nutrition availability raised the possibility that adopting a consistent daily short window of feeding can sustain robust circadian rhythm. Pre-clinical animal studies have demonstrated proof of concept and identified potential mechanisms driving TRF-related benefits. Pilot human intervention studies have reported promising results in reducing the risk for obesity, diabetes, and cardiovascular diseases. Epidemiological studies have indicated that maintaining a consistent long overnight fast, which is similar to time-restricted eating, can significantly reduce risks for chronic diseases. Despite these early successes, more clinical and mechanistic studies are needed to implement TRE alone or as adjuvant lifestyle intervention for the prevention and management of chronic metabolic diseases.