Your digestive system is a critical component of your overall health. It consists of 3 components:

• Your digestive tract, the hollow tube that runs from your mouth to your anus
• Accessory organs/glands such as the pancreas and gallbladder
• The microbiome, which essentially functions as an extra organ with a completely different genome

The traditional view of the gut is that it promotes the digestion and absorption of our food. However, it also plays an essential role in immune function, mood, cognitive performance, metabolism, and the stress response. Therefore, poor digestive function can have a negative impact on your overall quality of life, not just your digestion.

Circadian rhythms play an important role in optimizing our physiology for the environment. Most importantly, they regulate involuntary functions outside of our conscious control. This includes all of the above mentioned functions performed by the gut.

Routine environmental exposures are critically important for healthy circadian rhythms. Specific exposures called zeitgebers are an environmental language that our body translates into neural and hormonal signals. As a result, these signals adjust our physiology to better deal with our environment. But consistent routine drives circadian rhythms, you don’t get the benefits without routine.

A new review titled Circadian clocks in the digestive system covers how circadian rhythms regulate the digestive system and, in turn, the digestive system regulates circadian rhythms. As you’ll see, this has a huge impact on your overall health and well-being and points to the importance of routine in our everyday life.

Circadian clocks in the digestive system

As mentioned, we can look at the gut as comprised of the digestive tract, several accessory organs, and the microbiome. These 3 key components require proper communication with one another to function optimally. Moreover, they need to communicate with other organ systems as well.

For digestion and absorption, the synergy between these 3 components regulates most functions of the digestive system including:

• Saliva secretion
• Bile synthesis and output
• Motility
• Gastric acid/enzyme secretion
• Microbiome composition
• Nutrient absorption
• Intestinal permeability

Communication between the gut and several other organ systems is also important for glucose and lipid metabolism. Hormones secreted from the gut called incretins play a role in regulating insulin and glucagon secretion by the pancreas as well as appetite. We covered the importance of gut hormones in regulating blood glucose in our last blog you can review here.

Mood, immune function, cognitive performance, and even seemingly unrelated processes such as bone density all rely on circadian synchronization. This goes for synchronization between the 3 components of the gut, as well as the gut and other systems such as muscles, the brain, and adrenals.

Zeitgebers for the digestive system

We have a master circadian clock in our brain in an area called the suprachiasmatic nucleus(SCN). This master clock primarily responds to light exposure and helps control other clocks known as peripheral clocks. The peripheral clocks respond to various cues including feeding, exercise, stress, and core body temperature.

The master clock exerts its control over the peripheral clocks through neuronal and hormonal means. The SCN uses both the sympathetic and parasympathetic arms of the autonomic nervous system to neurally control peripheral clocks. Melatonin and cortisol(Glucocorticoid) are 2 important hormones that help the master clock relay light information throughout the body.

The peripheral clocks communicate with one another via various hormones and signaling molecules. For example, insulin from the pancreas signals information on feeding, as does ghrelin from the stomach. We’ve covered ghrelin in previous blogs, one you can check out here.

Ghrelin seems to help us “sense” when food is normally available and motivates us to seek it out at those times. Regular meal times create anticipatory ghrelin bumps before scheduled times.

Ghrelin may also affect the master clock, as do other hormones. These hormones generally have a local effect where released, but also affect the function of other organ systems.

It’s important to point out that routine feeding/fasting periods are very important for the digestive system. Routine creates anticipatory responses over time, and this makes digestion more efficient and creates synchronization throughout the body. Erratic feeding times are best avoided.

In addition to our own clocks, the circadian clock of our microbiome is very important, particularly for our metabolism. One of the biggest drivers of the function of the microbial clock is our habitual diet, so consistency there is also important.

Metabolism as the circadian language

As you can tell, effective communication is essential to make this whole thing run properly. In order for different cells, organs and tissues to synchronize with one another, they have to speak the same language. Since all of these components utilize the same energy pathways, metabolism may be this common language.

Many of the pathways regulating circadian rhythms use nutrient sensing pathways to differentiate between use and repair. During use we break things down, and during repair we build them back up. The central anabolic switch in our cells is called mTOR, and many different nutrient sensing pathways converge on it including:

• Insulin/IGF-1 signaling-We’ve covered how insulin/IGF-1 signaling may be the food entrainable cue here
• Sirtuins/NAD-We’ve covered how NAD+ is both a circadian signal AND regulated by circadian rhythms here
• Magnesium regulates mTOR-We covered that in a blog here
• AMPK-Senses energy stress in cells(ATP:ADP ratio). We covered this in a blog you can check here.

All these pathways in one way or another play a role in synchronizing circadian signals throughout the body. Balancing use vs repair is a central component of circadian rhythms, sequestering them away from one another to the appropriate times of day.

This image clearly illustrates the importance of healthy metabolism for building strong, synchronized circadian rhythms.

If metabolism is the common language our circadian clocks use to communicate, you’d expect metabolic dysfunction to couple with circadian disruption. Indeed, this does appear to be the case.

The digestive system as a central hub for circadian rhythms

Since a healthy metabolism is so important for maintaining strong circadian rhythms, you’d expect systems that play a larger role in managing metabolism to be very important. Aside from the liver, no other organ system is as important for maintaining a healthy metabolism as the gut.

Ghrelin secreted by the gut regulates feeding behavior, as does GLP-1, PYY, and oxyntmodulin. The gut-derived incretin hormones GLP-1 and GIP manage blood glucose levels by altering gastric emptying as well as insulin and glucagon secretion by the pancreas.

The microbiome secretes various metabolites such as short chain fatty acids and secondary bile acids that regulate metabolism in various tissues. It also regulates intestinal permeability, plays a role in immune function, may be essential for circadian GLP-1 secretion, and modulates appetite and metabolism via the brain-gut-microbiome axis.

The microbiome itself is a very interesting “organ”, particularly since it’s the only organ that doesn’t share our genome. Furthermore, it’s an organ that can switch genomes when we change our diet, as diet selects for microbial functions. In fact, the microbiome makes us very “plastic”, in a metabolic sense and varies between our feeding & fasting periods. This creates its own circadian rhythm.

How each one of us responds to diet is highly individual; our postprandial(Postmeal) metabolic response to the same meal is vastly different. Genes play a small role, but environmental factors such as circadian rhythms, physical activity patterns, meal patterns, and the microbiome play a much larger role.

Since the microbiome is largely dictated by our habitual diet, and the gut clock is regulated by feeding time, routine feeding times and meal composition are important for establishing synchronization of our circadian rhythms.

Conclusion

Circadian rhythms play an important part in making us healthy. Anticipatory responses to routine environmental exposures help us thrive in our environment.

Circadian rhythms are particularly important for processes outside of our conscious control. The function of the digestive system is heavily regulated by circadian rhythms. On top of that, the digestive system plays an important role in circadian rhythms by regulating metabolism.

Establishing routine in our meal patterns and composition helps us take advantage of the beneficial effects of circadian rhythms for gut health.