Fasting and time-restricted feeding aren’t just trends—
they’re tools to help your body reset, reduce inflammation, and restore energy naturally.
By Dr. Edwin E. Spencer | Photography by Anna N.
Appeared in Cityview Magazine, Vol. 41, Issue 6 (Nov/Dec 2025)
Dietary trends change as frequently as the weather in East Tennessee. However, to consider fasting or time-restricted eating a trend is really not doing it justice. Not only are there positive health benefits that extend far beyond weight loss, there is spiritual value as well. Many religions have espoused fasting as a spiritual tenet. After all, Jesus himself fasted for 40 days while in the wilderness prior to spiritual warfare with the devil. Is it possible that Jesus had more mental clarity after his fast?
While I have never fasted for more than 18 hours, I have researched the data which is replete with incredible health benefits. My fast, however, was externally imposed during my residency where the workload and lack of available food conspired to make a young cranky doctor. Fasting is becoming more popular and we actually discussed fasting at our men’s group the other night at church. Only one man out of eight had actually fasted for the sake of religion. He fasted for three days and said that after 24 hours, he felt great.
When I started researching fasting years ago, I thought that this was just another means of caloric restriction for the sake of weight loss. I mean it makes sense that if don’t eat, then you should be in a negative caloric balance and should lose weight. However, there is much more to fasting or time restricted eating than just imposing a caloric deficit.
However, don’t get me wrong, caloric restriction is very important and does occur spontaneously in clinical trials studying time restricted eating. Understand that 70% of our standard American diet is ultra processed food that is loaded with high fructose corn syrup that causes us to eat on average 25% more calories. Therefore, reducing calories is critical in America and will help with all health metrics.
Intermittent fasting or time restricted eating (TRE) has been studied in humans and animal models. Interestingly not only does the length of the fasting matter but also the timing and consistency of the eating pattern. For instance, a mouse study found that mice fed “out of phase” (this is during the daytime for mice as they usually feed at night) caused them to live 10% shorter lives with an increase incidence of obesity and diabetes. This is because we have a genetically encoded 24-hour circadian rhythm as do mice. This circadian rhythm is set by light exposure to our suprachiasmatic nucleus in the brain which then controls when genes are turned off and on. Approximately 80% of our genes are controlled by this circadian rhythm to include those employed in digestion as well as our microbiome. When humans eat “out of phase” (at night is out of phase for us), we too gain weight and the incidence of diabetes increases.
In that same mouse study, the “in phase” (eating at night) mice were either allowed to eat ad libitum (whenever they want) or they were restricted to an 8-hour feeding window. The two groups were isocaloric, meaning they each had the same number of calories, just a different feeding pattern. It was found that both eating in phase and an 8-hour time restricted window led to a 35% increase in lifespan. That is an amazing increase in lifespan just based on eating patterns. No fancy supplements like NAD or methylene blue or rapamycin but simply adopting an eating pattern that is consistent with our circadian rhythm and restricted to a certain time. Although we cannot replicate “lifespan” studies in humans, human clinical trials of time-restricted feeding have demonstrated similar decreases in weight, diabetes, oxidative stress and chronic inflammation and better insulin sensitivity.
There are many studies evaluating different time-restricted eating patterns from eating only over a 4-hour period to 10-hour period to alternate day fasting (eating one day and not the next) to longer fasts of several days. All have shown similar benefits in the same health metrics except the 4-hour window as participants tended to overeat during that window and actually gained weight. There is something to time-restricted eating that improves glucose and insulin levels even in diabetics.
“When we align our eating with our circadian rhythm and allow time for fasting, the body shifts from growth mode to repair mode—recycling damaged cells and improving metabolic health.”
Why might time-restricted eating have such health benefits? The magic really happens in the fasting state. For the purpose of this article, we will reduce the science and biochemistry to two major levers. These are mTOR (mammalian target of rapamycin) and AMPK (AMP activated protein kinase). AMPK is activated when AMP (adenosine monophosphate) levels rise. AMP is the end product of ATP (adenosine triphosphate – the energy currency of the body) breakdown that occurs as cells in the body use their ATP stores for various processes. As the cells use up ATP with exercise or nutrient depletion (fasting), the AMP rises and activates AMPK. AMPK then stimulates cells to use more fat as fuel, instead of glucose, and puts the cell into a state of recycling old or defective parts. It also encourages the cells to make more mitochondria which are the energy producers of the cell. Instead of growth, AMPK can be thought of as a braking mechanism that allows the cells to repair and recycle.
On the other hand, mTOR can be thought of as the opposite of AMPK. It causes cells to be in a growth mode building and multiplying. Whenever we are in a fed state, mTOR is more highly expressed and when we are in a fasted state, AMPK is expressed preferentially. Just as with your car, you need an accelerator but also brakes. Having one system on all the time is not good for your car or body.
The average American typically eats over a 15-hour window and we tend to have our largest meal later in the evening. This puts us in an almost constant growth mode with very little time for repair and recycle. We tend to have 7-8 eating periods throughout the day as we have access to snacks everywhere. This was not always the case. Just 100 years ago, there were very few snacks and people ate basically 3 meals per day. Prior to Edison’s development of the lightbulb in 1879, people ate their last meal fairly early and went to bed not long after the sun went down.
This constant growth mode and caloric toxicity has created an America that looks far different than it did when I was a kid. Now 74% of Americans are overweight or obese and 50% are on the diabetic spectrum. Through time restricted eating we can change this trend. We can put ourselves into a repair/recycle mode where cells undergo “autophagy” or self-eating or self-cleanup. To further highlight this growth problem, just look at the rates of cancer in diabetics. Reports vary but most state that diabetics have a two-to four-fold increase in the top five cancers. These cancers are fueled by a constant supply of glucose.
Moreover, it is well known that cancer cells prefer glucose as a fuel source and make energy through anaerobic (without oxygen) respiration. This is called the Warburg Effect. This is in contrast to most of our normal cells which at rest will produce energy by using oxygen and can use fat for fuel. One of my friends who is fighting cancer right now is using this information to slow down his cancer by eating straight ketogenic. This means eating mainly fat which the body turns into ketones which our normal cells can easily use for fuel and in a sense, starving out the cancer cells. This can be accomplished through fasting as well. Although, most of us take about 72 hours or so to get into full ketosis with a fast, some people enter it sooner.
Through intermittent fasting or time-restricted eating, we can teach our cells to be more metabolically flexible. They can use glucose when available after a meal, but can also use fats and ketones when in a fasted state. This metabolic flexibility allows your body to burn more fat when at rest. There is good data that demonstrates that when a pattern of time-restricted eating is adopted for a couple months, you will actually preferentially burn more fat.
Most of the data shows that an eating window of 8-10 hours is optimal. Human clinical trials have demonstrated spontaneous reduction in caloric intake, better glucose control, improved insulin sensitivity, and reduction in blood pressure. I kind of fell into this window just based on lifestyle and training preferences and it is still consistent with social activities. If you are interested in trying intermittent fasting, I would suggest cleaving some time at both ends of your current eating window, meaning trying not to eat within the first hour or two upon wakening allowing yourself to stay in your overnight fast a little longer, and then trying not to eat within 3 hours of going to bed. I’ll admit the later one is a little harder for me especially in the summer when the days are nice and long. Longer fasts like alternate-day fasting has been shown to yield faster results as far as glucose control and weight loss and are safe. Most humans can go 30 days without eating but one would probably need to do this with some medical supervision to assess proper micronutrient levels. Facilities that assist with these longer fasts are gaining popularity especially in Europe.
“Fasting isn’t just about restricting calories—it’s about restoring balance. It’s a return to the way humans were designed to live, with periods of nourishment and periods of rest.”
Fasting is not new and was just a function of the normal human condition back in the day. Avoiding ultra processed foods was easy back in the day because they didn’t exist. Getting eight hours of sleep and getting outside used to be the norm. I’m not suggesting that our modern conveniences like indoor lighting and food access are bad, but they have lulled us into metabolic complacency. The apogee of American prowess has come with a desultory approach to health and nutrition. We now have to invent medications and strategies to combat the overabundance of unhealthy foods.
I think you will find fasting is a salutary endeavor with manifold health benefits. It really is the essential condition that allows our bodies time to recycle and repair. With all that said, my first 24-hour fast is coming up this week…wish me luck.
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