A: Hey, Mike! These are great questions. Thank you so much for asking!

You’re absolutely right, intermittent fasting has become a household term. If you’re interested in taking a deep dive into the topic, I suggest checking out this article covering the health benefits of intermittent fasting, its different forms, how it seems to work its magic, and if it seems to be safe.

What is Time-Restricted Feeding?

As you’ll see, time-restricted feeding (often abbreviated as TRF and sometimes also called time-restricted eating) is just one type of intermittent fasting. That is, all time-restricted feeding is intermittent fasting. But not all intermittent fasting is time-restricted feeding.

Said differently, intermittent fasting comes in two major subcategories:

1. Fasting for 1 – 4 days per week. Alternate-day fasting (e.g., every other day diet) and periodic fasting (e.g., the 5:2 diet) are prime examples.
2. Fasting daily for an extended period of time, typically between 14 and 20 hours. This is, by definition, time-restricted feeding.

So, time-restricted feeding is a type of daily intermittent fasting where all food is consumed within a condensed “feeding” window. This means a longer-than-normal “fasting” window is observed.

To your point, Mike, depending on who you’re talking to, feeding and fasting windows can vary considerably. Generally speaking, feeding windows vary from 3 – 4 hours up to 10 – 12 hours. (That is, a fasting window of 12 – 21 hours, respectively.)

Although time-restricted feeding is arguably the most popular form of intermittent fasting, alternate-day fasting and periodic fasting (e.g., the 5:2 diet) are the most widely studied. Research on intermittent fasting in its various forms reveal impressive results, such as: ¹

• Reduced body fat
• Reduced resting heart rate and blood pressure
• Increased heart rate variability (e.g., increased parasympathetic activity)
• Reduced triglycerides
• Improved insulin and leptin sensitivity
• Elevated ketone levels
• Reduced markers of inflammation
• And more

Despite being less widely studied in humans, the accumulating evidence—in controlled settings andthe real world—offer tremendous promise for time-restricted feeding. It may be a particularly effective tool to control caloric intake and reduce fat mass.

And when you add that to the substantial research in insects and mammals, researchers believe time-restricted feeding could have profound beneficial effects on overeating disorders, diabetes, obesity, and more.

Before we look at some of the results from recent human studies, it’s important to note the research on animals is quite impressive. While humans are different than animals, this research provides clues to how time-restricted feeding might be helpful and healthful.

If the animal research wasn’t so darn impressive, scientists wouldn’t go the extra mile to test time-restricted feeding with humans. This is worth mentioning because we have a tendency to discount animal studies (as well as in vitro studies). But it plays a crucial part in the research process.

Time-Restricted Eating: Here is the Science

In one of the first studies on time-restricted feeding, researchers from The Salk Institute for Biological Studies (a pioneering research institution on time-restricted feeding led by Dr. Satchin Panda) found that overweight healthy adults who restricted their feeding to a 10-hour window daily lost an average of 4% of their starting body weight. And that was sustained for one year. ²

This was a cool and important study for many reasons. For starters, the researchers had the participants use a smartphone app to track food intake. (Actually, you can sign up for the study, which is ongoing, and the app by visiting myCircadianClock.org.) Here were some of the key findings:

• Generally speaking, people eat frequently and erratically throughout their waking hours.
• The length of overnight fasting was related to time spent in bed.
• Generally speaking, people eat most of their calories at night. For example, <25% of calories were consumed before noon and >35% were consumed after 6:00 pm.
• On average, the typical feeding window was 14 ¾ hours.
• When overweight individuals, who normally had a feeding window of >14 hours, restricted their feeding to a self-selected 10- to 11-hour daily window for 16 weeks, they lost a significant amount of weight (7 pounds on average), reported being more energetic, and slept better.
• The folks who implemented the time-restricted feeding on both weekdays and weekends experienced a significant reduction in “metabolic jetlag” (or, what I like to call “social jetlag”). In other words, they didn’t disrupt circadian rhythms with a weekend of sleeping in, eating later, etc.
• Perhaps most telling, all of the time-restricted feeding participants voluntarily expressed an interest in continuing with the 10- to 11-hour time-restricted eating regimen after the end of the 16-week study period.

In a subsequent time-restricted feeding study, Italian researchers found men who restricted caloric intake to an 8-hour window each day (eating meals at 1:00 pm, 4:00 pm, and 8:00 pm) for 8 weeks lost significant body fat. In fact, the TRF group lost 16.4% of their initial fat mass while the “normal” diet group (which ate meals at 8:00 am, 1:00 pm, and 8:00 pm daily) lost only 2.8% of their body fat. Interestingly, the researchers reported both groups consumed similar caloric intakes. ³

The participants in this study were young, healthy males who lifted weights throughout the duration of the experiment. And, the researchers found the guys in the time-restricted feeding group maintained their muscle just as well as the normal diet group.

This is cool because many people with an old-school “eat-all-day” mindset are worried about losing muscle when fasting. This study suggests such a concern is not warranted. Another insightful finding was that time-restricted feeding helped reduce markers of inflammation (such as IL-6, TNF-α, IL-1β). These benefits were not realized by the “normal” diet group.

In another recent study headed by Grant Tinsley at Texas Tech University, researchers examined the impact of time-restricted feeding on body composition and muscular strength in a group of healthy, young men. In this study, the guys in the time-restricted feeding group consumed all their calories within a self-selected 4-hour window between 4:00 pm and midnight on non-workout days. On the three days they lifted weights, they were allowed unrestricted food intake (no time restrictions). And on all days, the number of calories and specific foods were not limited. The “normal” diet group simply followed their normal eating patterns. ⁴

There were several important findings from this study. Foremost, the 4-hour time-restricted feeding window reduced caloric intake by about 650 calories per day in the TRF group (compared to non-fasting days). On average, the TRF group consumed fewer calories on a daily basis compared to the normal diet group. Although the results weren’t statistically significant, they also lost body weight and fat mass while the normal diet group increased both.

Plus, the men in the time-restricted feeding group did not lose muscular strength or muscle mass. However, the guys in the normal diet group gained over 5 pounds of lean mass (e.g., muscle), on average. This suggests this degree of time-restricted feeding (i.e., 4-hour feeding window) may hinder at least some of the adaptations to strength training.

However, the researchers found the TRF group consumed considerably less protein compared to the normal diet group. This suggests if you keep your protein intake relatively high (e.g., 0.7 grams per pound per day, on average), you may be able to promote increases in muscle mass when TRF is combined with resistance training.

Plus, the men in the time-restricted feeding group did not lose muscular strength or muscle mass. However, the guys in the normal diet group gained over 5 pounds of lean mass (e.g., muscle), on average. This suggests this degree of time-restricted feeding (i.e., 4-hour feeding window) may hinder at least some of the adaptations to strength training.

However, the researchers found the TRF group consumed considerably less protein compared to the normal diet group. This suggests if you keep your protein intake relatively high (e.g., 0.7 grams per pound per day, on average), you may be able to promote increases in muscle mass when TRF is combined with resistance training.

Finally, the most recent study—led by a pioneering group of intermittent fasting researchers, including Dr. Panda, Kristin Hoddy, and Krista Varady (author of The Every Other Day Diet)—found that obese participants who restricted their feeding to an 8-hour window (between 10:00 am and 6:00 pm) daily for 12 weeks lost a significant amount of weight (about 3% of their starting weight). ⁵

Among the many interesting findings of the study, the participants in the time-restricted feeding group reduced their daily eating duration by about 3 hours (from 11 hours/day to 8 hours/day) compared to their “normal” eating. And the participants were highly compliant, eating within the assigned timeframe an average of 6 days a week.

Just like in the other time-restricted feeding studies mentioned, this study placed no limits on what or how much the participants ate. Yet despite no intentional calorie counting, the participants in the time-restricted feeding group decreased caloric intake by over 300 per day.

The Benefits of Time-Restricted Feeding

This may arguably be the greatest advantage of time-restricted feeding. While most diet plans focus on the what and how much, time-restricted feeding only emphasizes when you eat. Practically speaking, even though time-restricted feeding protocols don’t necessarily restrict calories or what you eat, it almost invariably reduces caloric intake.

Now, I’m not saying time-restricted feeding is a license to eat whatever you want. Yet it does tend to afford a bit more wiggle room for social eating opportunities.

More importantly in my opinion, what makes this aspect of time-restricted feeding so attractive is it can be combined with any dietary protocol you find works for you: Paleo, Keto, Low FODMAP, etc. I suspect when you combine healthier dietary protocols with time-restricted feeding, you’ll put yourself in a position to achieve even better results than either alone. (For example, better weight loss than the modest results reported, better muscle enhancements with a higher-protein version of time-restricted feeding.)

Another attractive benefit of time-restricted feeding is the low barrier to entry. In other words, it’s generally pretty easy to get started. For most people, it’s a matter of pushing back the first meal of the day and stopping eating a bit earlier.

I think this latter point deserves more attention. Like we’ve talked about before, food timing is one of the major variables that impacts circadian alignment. Along those lines, IF regimens (such as TRF) that limit food consumption to daytime/light hours may be particularly effective at promoting metabolic health.

Ironically, most people who implement TRF eat the bulk of their food/calories at nighttime. This schedule may disrupt circadian rhythms and lead to negative health and metabolic outcomes. For instance, even after controlling for diet and lifestyle, results from a 16-year follow-up study of 26,902 men found late-night caloric intake increases heart disease risk by 55%. ⁶

If nothing else, placing a hard stop on when you are done eating for the day typically nips late-night eating in the bud. Considering most people mindlessly eat junk food at those times, this is an exercise in addition by subtraction. There’s no hard and fast rule, but many respected health professionals and researchers suggest putting a stop to eating within 2 – 3 hours before bed.

Okay, Mike, there’s a lot here. I appreciate you bearing with me while I geeked out for a minute. Let’s sum things up with and provide some action steps if you’re ready to give time-restricted feeding a try:

• Time-restricted feeding (TRF) is one form of intermittent fasting (IF). It tends to be the most practical and attractive implementation of IF.
• TRF involves fasting daily for a 14- to 20-hour period. In other words, all food/caloric intake is typically consumed within a 4- to 10-hour window. An 8-hour feeding window (16/8) is the most common.
• TRF does not typically restrict what you eat or how much. In other words, it doesn’t usually involve calorie counting. By restricting the amount of time you eat, however, caloric intake tends to be invariably reduced.
• Despite no caloric restriction, TRF reliably leads to weight loss—albeit a bit more modest compared to more extreme forms of IF.
• If you’re new to TRF, consider a gentle introduction, starting with a 10-hour feeding window. For many people, this means skipping breakfast or pushing back the first meal of the day and stopping eating about 2 – 3 hours before bed.
• If it’s possible to schedule your feeding window to align with daylight hours (i.e., eat most, if not all, your food during daylight), there may be benefits on circadian rhythms.
• If your goal is to maximize weight loss, a more aggressive approach may yield the best results. In other words, a 4-hour feeding window may lead to greater weight loss than an 8-hour window.
• If your goal is to maximally increase muscle mass while losing body fat, then a more modest TRF approach (i.e., 8-hour feeding window) may be the best route. Of course, that assumes you’re lifting weights at least 3 days per week.
• If your goal is to optimize body composition, make sure you’re consuming at least 0.7 grams of protein per pound per day—regardless of your TRF protocol.
• You can combine TRF with any healthy dietary pattern: Paleo, Mediterranean, Keto, Low FODMAP, etc.
• Finally, if you’re from the “eat-all-day” camp, you can let go of the misconception that fasting (or even skipping a meal) reduces metabolic rate. Various IF protocols, including TRF, have no adverse effects on metabolic rate.