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The reason many blood tests are taken before eating after an overnight fast is that meals can tip our system out of balance, bumping up certain biomarkers for disease such as blood sugars, insulin, cholesterol, and triglycerides. Yet fewer than 1 in 10 Americans may even make it 12 hours without eating. As evolutionarily unnatural as eating three meals a day is, most of us are eating even more than that. One study using a smartphone app to record more than 25,000 eating events found that people tended to eat about every 3 hours over an average span of about 15 hours a day. Might it be beneficial to give our bodies a bigger break?

Time-restricted feeding is “defined as fasting for periods of at least 12 hours but less than 24 hours.” This involves trying to confine calorie intake to a set window of time, typically 3–4 hours, 7–9 hours, or 10–12 hours a day, resulting in a daily fast lasting 12-21 hours. When mice are restricted to a daily feeding window, they gain less weight even when fed the exact same amount. Rodents have such high metabolisms, though, that a single day of fasting can starve away as much as 15 percent of their lean body mass. This makes it difficult to extrapolate from mouse models. You don’t know what happens in humans until you put it to the test.

The drop-out rates in time-restricted feeding trials certainly appear lower than most prolonged forms of intermittent fasting, suggesting it’s more easily tolerable. But does it work? If you have people even just stop eating between 7 p.m. and 6 a.m. for two weeks, they lose about a pound each week, compared to no time restriction. Note: no additional instructions or recommendations were given on the amount or type of food consumed; no gadgets, calorie counting, or record-keeping. They were just told to limit their food intake to the hours of 6 a.m. through 7 p.m., a simple intervention, easy to understand and implement.

The next logical step was to try putting it to the test for months instead of just weeks. Obese men and women were asked to restrict eating to the eight-hour window between 10 a.m. and 6 p.m. Twelve weeks later, they had lost seven pounds. This deceptively simple intervention may be operating from a number of different angles. People tend to eat more food later in the day, and higher-fat foods later in the day. By eliminating eating in the late evening hours, one removes prime-time snacking on the couch, a high-risk time for overeating. And indeed, during the no-eating-after-7-p.m. study, the subjects were inadvertently eating about 250 fewer calories a day. Then, there are also the chronobiological benefits of avoiding late-night eating.

I’m going to do a whole series of videos about the role our circadian rhythms have in the obesity epidemic, how the timing of meals can be critical, and how we can match meal timing to our body clocks. Just to give you a taste, the exact same number of calories at dinner is significantly more fattening than the same number of calories eaten at breakfast.

Calories in the morning cause less weight gain than the same calories given in the evening. A diet with a bigger breakfast causes more weight loss than the same exact diet with a bigger dinner. Nighttime snacks are more fattening than the same snacks in the daytime. Thanks to our circadian rhythms, metabolic slowing, hunger, carbohydrate intolerance, triglycerides, and a propensity for weight gain are all things that go bump in the night.

What about the fasting component of time-restricted feeding? There’s already the double benefit of fewer calories and avoiding night-time eating. Does the fact that you’re fasting for 11 or 16 hours a day play any role, considering the average person may only make it about 9 hours a day without eating? How would you design an experiment to test that? What if you randomized people into two groups, and forced both groups to eat the same number of calories a day and both to eat late into the evening, but with one group fasting even longer—20 hours? That’s exactly what researchers at the USDA and National Institute of Aging did.

Men and women were randomized to eat three meals a day, or to fit all those same calories into a four-hour window between 5 p.m. and 9 p.m. and fast the rest of the day. If the weight-loss benefits from the other two time-restricted feeding studies was due to the passive calorie restriction or avoidance of late night eating, then presumably both these groups should end up the same, because they’re both eating the same amount, and they’re both eating late. But that’s not what happened. After eight weeks, the time-restricted feeding group ended up with nearly five pounds less body fat. About the same number of calories, but they lost more weight. A similar study with an eight-hour window resulted in three pounds more fat loss. So, there does seem to be something to giving your body daily breaks from eating around the clock.

Because that four-hour window was at night, though, they suffered the chronobiological consequences—significant elevations in blood pressures and cholesterol levels—despite the weight loss. The best of both worlds was demonstrated in 2018: early time-restricted feeding, eating with a narrow window earlier in the day, which we’ll cover next.

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