Fasting + Gut Health
The Science of Intermittent Fasting
One of the very first changes I noticed after I started intermittent fasting was a clear head and a calm belly. It was like a dense fog lifted from my brain and digestive tract. I wanted to know more about why I was feeling these effects so I started to sift through the research.
Over the next few weeks as I read through the studies and continued with my 16-hour daily fasts, I noticed my energy levels were much more stable throughout the day, my joint stiffness went away, and my abdomen began to flatten out. Other documented benefits include improved insulin resistance and blood lipid profiles; reduced inflammation and autoimmunity; and protection from neurodegenerative disease and mood disorders.
These are pretty astounding effects from literally doing nothing, so I wondered what was the underlying mechanism for all of these health benefits. As I dug further, I realized that the gut microbiome played a very big role.
Gut Circadian Rhythm
Just about every major system of the body works in sync with the brain’s 24-hour circadian clock including our digestive system. Many of our digestive functions follow a sleep-wake cycle, being more or less active depending on the time of day. For example, our metabolic response to glucose and gastric emptying time are more robust during the day than at night.
It turns out our gut microbiome follows a circadian rhythm as well, which is synchronized with our daily hunger signals. Studies show that when we travel across time zones frequently or eat late at night we can disrupt our gut flora and develop glucose intolerance. Animal studies suggest that restricting eating times to when the body’s metabolism is most active, restores normal digestive rhythms and protects against obesity and chronic disease. The results of these studies mirrored what I was experiencing. As I allowed my digestive system to completely rest at night, I experience less bloating and more energy throughout the day.
Microbiologists once thought our gut flora shifted slowly over weeks and months in response to dietary changes, but it turns out that can happen in a matter of hours or days. In a small clinical trial, participants were required to switch the composition of their diet to either plant-based or animal-based foods. Within a matter of days the researchers saw changes in the richness and diversity of the participant’s gut bacteria.
It turns out our gut microbiome is highly flexible and responsive to change, not only to the foods we eat, but also to our patterns of feeding and fasting. A number of animal studies and clinical trials have shown that intermittent fasting can influence the types of bacterial species present in the colon. A key finding in one of the animal studies was that a time-restricted eating pattern altered the makeup of the gut microflora, even when the animals were fed exactly the same type of food.
The capability of the microbiome to rapidly adapt to changes in food and feeding patterns is considered an evolutionary skill that was likely vital to ancient humans. The hunter-gatherer diet was characterized by change, seasonality and regular periods of scarcity. An adaptable microbiome would ensure the maximum absorption of nutrients and improve human survival.
Early humans evolved in an environment where they had to go for regular periods with little or no food, similar to the lives of many wild animals. For example, the wolves of the Northern Rocky Mountains typically kill and eat prey only once every week or two. In order to survive in this manner, they must be able to rapidly shift their metabolism from fat storage to fat mobilization. We have a similar capacity that scientists call ‘metabolic flexibility’.
Metabolic flexibility means our body can readily transition from using glucose for fuel to using fatty acids and ketones, while still maintaining normal energy levels and preserving lean muscle mass. Being able to flip this metabolic switch means we can go from fat synthesis when food is readily available, to fat burning when food is scarce. For hunter-gatherers metabolic flexibility would have given them an important survival advantage.
This metabolic switch typically begins somewhere between hour 12-36 of fasting. It depends on the level of glycogen stores in the body at the beginning of the fast and the amount of energy burned during the fast. Intermittent fasting is thought to mimic the hunter-gatherer lifestyle, helping to maintain metabolic flexibility, compared to those on a standard feeding cycle. When metabolic flexibility is lost from chronically over-eating, insulin resistance and oxidative stress is the result.
A small but exciting body of research has looked at the potential of intermittent fasting for autoimmune conditions like MS, Crohn’s and Colitis. In both laboratory and clinical trials IF has shown to restructure the gut environment by:
· Improving gut barrier function
· Increasing bacterial diversity
· Lowering autoimmune response
· Increasing antioxidant activity
· Reducing inflammation
All of these beneficial outcomes were a result of changes in the gut microbiota in response to time restricted eating, also known as intermittent fasting.
A Whole System Perspective
In 2016, researchers from the Salk Institute published an in vivo study in the Proceedings of the National Academy of Sciences, showing how the brain communicates with the gut during fasting, further expanding on our understanding of the gut-brain axis. They found that a genetic switch turned on in the brain during fasting, telling the gut to strengthen its barrier function, which in turn prevented the gut from ‘leaking’ and causing an auto-inflammatory response. This study is part of an ongoing investigation into how fasting can help people with inflammatory bowel disease.
There is not only a connection between the gut microbiome and the brain. In the same year, another study done by researchers from the National Cancer Institute at the NIH describe a gut-fat axis which links fasting-induced microbial changes to fat beigingwhich is an intermediate step in fat browning, a thermogenic process that increases fat burning. It would appear that the thermogenic effects of fasting are in fact mediated by gut bacteria.
The current research linking fasting with the gut microbiome is nothing short of fascinating. Controlling our feeding window with intermittent fasting is easier than it sounds and has profound benefits for something that is essentially free and involves doing absolutely nothing!