Time restricted eating(TRE) is an increasingly popular practice where people restrict when they eat to certain parts of the day. Typically, people restrict their feeding to an 8-12 hour period and fast for the remaining 12-16 hours.

In mice, TRE seems to have beneficial effects on metabolism independent of how much they eat. In other words, you can capture some of the benefits of calorie restriction without restricting calories.

The theory behind how it works centers around circadian rhythms. Circadian rhythms are anticipatory physiological responses created by regular daily habits.

We have a master clock in the brain set by light. Peripheral clocks exist in most organs and tissues in the body(eg., skin & liver), set by various signals. Many of these organs and tissues communicate with one another to regulate things like immune function and blood sugar.

Therefore, synchronizing all the clocks is of central importance to creating physiological synergy. Consequently, desynchrony between the clocks may lead to metabolic dysfunction and poor health outcomes.

A large trove of previous data points to feeding as the dominant environmental signal for peripheral clocks. A recent paper finds that time restricted eating isn’t an adequate signal to synchronize the peripheral clocks.

At least, not on its own.

Time restricted eating as a zeitgeber

Circadian rhythms create anticipatory responses that improve our ability to adapt to our environment. For example, our daily exposure to light sets our sleep/wake cycle. But, in the event our light exposure changes drastically during a single day, our inner clock keeps this cycle consistent.

Circadian rhythms help improve our chance at survival. On top of our sleep/wake cycle, they help regulate alertness, mood, physical and mental performance, and our metabolism.

One important requirement for survival is food. In our current environment, we take for granted many aspects of our food. One thing in particular is finding it.

Before we could simply go the the refrigerator to get whatever we want, we had to hunt and gather for food. In the event we were successful in finding food at a certain time on a regular basis, our physiology would adjust to that time by:

• Making us more alert
• Increasing motivation/appetite
• Optimizing decision making and learning
• Enhancing muscular performance
• Increasing energy levels

So it make sense that scheduling meals at the same time every day will adjust our physiology to optimize these factors at that time. Furthermore, it helps develop a rhythm of use and repair that allows us to limit the accumulation of damage in our body.

There is a good amount of evidence showing health benefits to TRE, some of which we’ve covered here, here, and here.

On its own, TRE may not be sufficient to synchronize peripheral clocks

There is a ton of evidence in mice showing robust metabolic improvements with time restricted eating. For instance, the same type of diet that leads to metabolic dysfunction under unlimited access to food does not under TRE. This, despite consuming the same number of calories.

The problem is, an overnight fast in mice can lead to a 15% reduction in bodyweight. This is equivalent to a 180lbs human losing 12 lbs overnight. Therefore, translating mouse TRE studies to humans is difficult.

With this is mind, a new study looked at TRE as a synchronizing signal for the peripheral clocks. Under normal lighting conditions, TRE was insufficient as a synchronizing signal for the peripheral clocks. In constant darkness, it helped set the liver and kidney clock, but not the clock in the submandibular gland.

Consequently, rest/activity patterns outperformed feeding patterns as a synchronizing signal for peripheral clocks. This isn’t all that surprising.

We covered the importance of physical activity as an important signal in a previous blog. Physical activity is the most effective way to raise core body temperature. Core body temperature synchronizes individual cells within an organ and individual organs with one another.

Eating raises core body temperature as well, but not as strongly as physical activity. Keep in mind, this doesn’t mean that metabolic signals elicited from TRE aren’t important, just that activity patterns are a stronger signal than when you eat.

Insulin signaling is an important signal considered to be the “food entrainable oscillator” that can be strengthened with TRE. But you don’t have to do TRE to make that signal work for you: sleep, physical activity, and total calorie intake also regulate insulin.

Of course, it’s insulin signaling that’s important, not necessarily how much insulin is in your blood.

Building a strong signal

It’s important to keep in mind that, while there may be signals that work for peripheral clocks as a whole, individual clocks may have signals more suited to their function. Additionally, these organ specific signals may have a stronger effect in the target tissue than the feeding/fasting cycle.

For example, it makes sense for feeding to be the strongest signal for the liver or gut. When we eat, it’s our liver and gut that process our food.

But physical activity is likely a stronger signal for muscles and bone. That doesn’t mean that physical activity isn’t important for the liver. Being the central regulator of metabolism, and one of the primary organs that regulates blood glucose, activity patterns are an important signal for the liver as well.

Thus, it’s important that our behaviors take in to account as many signals as possible to build a strong, overall signal. This means, for example, having feeding and activity patterns that are out of sync with one another may lead to some level of disruption.

Skipping breakfast is one way to do this, as is doing most of your physical activity after work. Ultimately, it’s how you layer these things and what your goals are that dictate the best course of action for you.

Time restricted eating works best when incorporating other dietary factors

So, taken on it’s own, time restricted eating may provide some level of benefit to people. Particularly if it leads to calorie restriction and enhanced insulin sensitivity.

However, other dietary factors may ultimately dictate the success of TRE on metabolic health. Factors such as:

• Length of the overnight fast
• Meal composition and size when breaking the fast
• Macronutrient ratios
• Fiber intake
• Consuming certain nutrients
• Clock time when breaking the fast

But that doesn’t mean diet makes up for being sedentary. According to this study, activity patterns are a stronger signal for synchronizing peripheral clocks than the feeding period.

It’s important to pay attention to other dietary and non-dietary signals and develop a total lifestyle that fits your needs. On its own, TRE isn’t sufficient.

Conclusion

Previous research indicates that time restricted eating has metabolic benefits to human health. These benefits largely go through improved circadian rhythms.

A new study, however, finds that the feeding/fasting cycle is not as strong of a signal as previous believed. In fact, rest/activity patterns are better predictors of circadian rhythms in peripheral clocks than the feeding/fasting cycle.

While TRE had a strong influence over clock expression in the liver and kidney during constant darkness, during normal light/dark conditions it did not.

Thus, on its own, time restricted eating is not adequate to synchronize circadian rhythms in peripheral clocks.