One of the biggest diet and nutrition trends these days is intermittent fasting. Every week, I hear from listeners wanting to know my thoughts on it. I've mentioned intermittent fasting on the podcast before, in an episode on the health benefits of fasting. But that was way back in 2011. At that point, the research was still quite preliminary and most of it had been done in rodents. Nonetheless, researchers were excited about the potential for intermittent fasting to prevent or reverse diabetes, weight gain, DNA damage, and other artifacts of aging.
Based on these promising but preliminary results, lots of people started experimenting with various forms of modified or intermittent fasting—generating a lot of positive anecdotal reports. Over the last few years, more studies have been done—some of them on actual humans.
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How Does Intermittent Fasting Work?
Before I dip into the latest research, let me define some terms. Intermittent fasting is an umbrella term that includes a pretty wide variety of approaches, most of which fall into one of two categories: alternate day fasting and restricted eating windows.
Alternate Day Fasting
Alternate day fasting involves switching back and forth between days when you eat more and days when you eat less. In some versions, you eat nothing or next to nothing on your fast days and as much as you want on your feast days. Other versions have you cut your usual food intake by a third to a half on your fast days and allow you to eat more than your usual food intake on your feast days. (This is sometimes described as calorie cycling.)
The proportion of fast to feast days also varies. Some protocols have you fasting every other day. Another popular variation is the 5:2 diet, where you fast for two non-consecutive days every week.
Restricted Eating Window
The other approach that’s commonly included in discussions of intermittent fasting is the restricted eating window. I talked about this in my episode on timing your meals. Instead of restricting your food intake, you restrict your meal schedule.
Again, there are lots of variations on this approach. Some people follow a four-hour eating window, essentially eating just one meal a day. Others might eat two or three meals within an eight or ten hour window. (If you’re a breakfast skipper, you might already be doing this without even realizing it!)
The timing of the window is also up for debate. Because of our circadian rhythms, it might work better to put your eating window in the first half of the day. But due to our social rhythms, most people who follow this approach prefer to have their eating window in the second half of the day.
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