“If we cannot give the body what it needs, we are unlikely to have any success in caring for the soul." - Marc David
Intermittent fasting has become the latest popular weight loss strategy, with a proliferation of “experts” who are more than willing to tell you how to do what our early ancestors did out of necessity. As hunter-gatherers, humans were required to go for long periods without eating; locating, stalking, killing, and processing our food was a complicated, physically demanding, and time-consuming process. Early humans frequently went for days without eating or eating very few calories, all while expending energy gathering food. John Hopkins neuroscientist Mark Mattson, who has studied fasting for 25 years, tells us that our current physiology reflects this ancient adaptation; we are still able to go for long periods without food, even in our era of abundant and easily accessible calories.
What is commonly referred to as intermittent fasting is misleading as it reflects a variety of protocols that do not always include fasting but feature long periods of 14 to 20 hours without eating. Fasting is the act of going without food for a 24-hour period or longer during which only black coffee, tea, or water is consumed. A full day of not eating is difficult for many, so in the past decade various schedules have been designed as an alternative to a fast. These protocols vary periods of strict calorie restriction with shorter periods of eating. The protocol that is closest to more extreme fasting protocols is one that alternates between a normal eating pattern for one day, followed by a fast the following day. Less severe protocols include intermittent fasting during which people eat normally for five days of the week and fast for two non-consecutive days. The protocol that has gained the most popularity is time-restricted eating, or TRE, which isn’t really a fast, but a way of scheduling when you can eat. All TRE protocols feature a “feeding window” in which meals and calories can be consumed. There are a variety of TRE schedules, each characterized by a period of calorie restriction that is longer than the time allowed to eat. Popular protocols include 20:4, 18:6, and 14:10, all of which include overnight hours.
Fasting has been around for a long time, practiced by ascetics, yogis, and religious figures, and is a mainstay of so-called “cleanses” promising to detoxify the body as much as it is a part of serious political protests. When did it become such a popular weight loss strategy? For decades researchers have known that lab animals who consumed 30% fewer calories lived longer, provided their diets were nutritionally balanced. This research gained attention starting around 2014 when it was reported that mice who were fed following schedules of intermittent fasting showed improvements in metabolic health as well as numerous positive neurological changes, including memory and learning. They also had reduced heart disease and increased insulin sensitivity. Even though we are decades from knowing whether humans will experience these benefits, the public decided to conduct their own experiments, starting the now popular Intermittent Fasting and TRE eating plans. The thinking is that, if it works for lab mice, why not humans?
The allure of fasting and TRE is their ease--you can eat whatever you want, provided you do so within the hours of the established feeding time. No counting calories, keeping a food diary, weighing food, or eliminating an entire food group—what could be easier? Unfortunately, research related to intermittent fasting and TRE in humans has failed to show any enhanced weight loss benefit from eating all the day’s calories within a narrow window. When compared to those who simply ate a restricted number of calories without regard to timing, those who followed a fasting or TRE protocol lost just as much weight and showed the same improvements in blood glucose levels, insulin sensitivity, blood lipids, and blood pressure as those who simply ate fewer calories. Restrictions in caloric intake explain most of the beneficial changes found in these studies rather than any independent benefit from fasting.
Although weight loss may not vary much from total calories consumed and fasting protocols, TRE may have an inherent benefit in limiting the number of calories a person can consume each day. For people who tend to snack, limiting one’s feeding window can be a way of controlling daily food consumption. This is especially true for those who tend to stay up later and snack after dinner. Eliminating nighttime noshing can significantly reduce daily caloric consumption, potentially adding up to a substantial weight loss over time. Not eating after 7 or 8:00 pm may also improve one’s sleep, which is known to play an important role in appetite control and can impact the regulation of blood glucose, blood pressure, and our response to stress.
An important variable that impacts fasting’s ability to help us lose weight lies in the length of time in which we do not eat. Mark Mattson’s research at John’s Hopkins has shown that the fasting period must be long enough to deplete the liver of its glucose stores, which triggers a shift to other fuel sources, primarily fat. This is a process referred to by Dr. Mattson as metabolic switching. Provided the total time spent fasting is long enough and, since it should mimic conditions of our hunter gatherer ancestors, is accompanied by some physical activity or exercise, glucose levels will be lowered enough to trigger the switch to fat as fuel. Metabolic switching is what creates weight loss and triggers changes in blood chemistry, blood pressure, and one’s metabolic profile. There is not enough research using human subjects to know for sure how fasting stimulates metabolic switching in a wide range of people. However, not eating for 24 hours while engaging in physical activity is a hard protocol for most to follow. Even if this protocol is beneficial, it would be a tough regimen for most to follow.
Just because intermittent fasting does not show any superpower over simple calorie restriction, there is evidence that the switch from fast to refeeding may trigger some interesting, as well as life extending, changes in the body as well as the brain. Several clinical trials have found that fasting increases levels of a substance called brain derived neurotrophic factor (BDNF), which plays a role in learning and memory, as well as the maintenance and repair of neurons. Fasting stimulates the production of ghrelin, the so-called “hunger hormone”, which does much more than stimulates our appetite. Ghrelin can cross the blood-brain barrier and increase neurogenesis in the hippocampus, resulting in improved memory function and possible elevation in mood. Fasting also seems to stimulate the regulation of groups of proteins called sirtuins, which play a variety of roles in our health. Some sirtuins help to break down the amyloid proteins that are characteristic of Alzheimer’s, while others improve insulin sensitivity, energy production, and provide protection from stress.
It is important to note that much of the evidence linking fasting to brain health is still theoretical or in the animal model stage. There is limited research about fasting in human subjects, so we are a long way from knowing how fasting and TRE impact the brain. Some discoveries about fasting and brain health from 2014 conducted by Dr. Valter Longo suggest that a fast of more than three days stimulated a process of rejuvenation and regeneration of immune cells. Further research may hold promise for helping people with neurodegenerative diseases such as multiple sclerosis and is being reviewed as a possible form of treatment.
It is important to note that it is the intermittent nature of fasting that is beneficial; prolonged periods of simply not eating without careful scheduling of refeeding puts our body under tremendous stress. Short periods between 12 to 24 hours of reduced caloric intake can be rejuvenating and good for our health, but when extensive, can cause depression, destructive changes in brain chemistry, and can stimulate a loss of muscle and bone tissue.
Certain aspects of human physiology may be similar to that of lab mice but our eating habits, energy demands, relationships with food, microbiomes, and individual differences make any study involving food and eating a daunting task when humans are the subjects. Until we have more clarity on how fasting impacts human health, we may want to approach fasting and TRE from a more qualitative angle rather than as a means of losing weight or extending longevity. Dr. Longo’s 2014 study showed that fasting and periods of reduced caloric intake stimulated a type of rejuvenation of a participant’s immune system. During the fasting period, participants’ bodies started breaking down damaged or older white blood cells for energy--the body was essentially taking out the biological trash. Upon refeeding there was an increase in the production of fresh cells to replace those that were broken down, essentially refreshing the immune system. Similar processes were found in the rejuvenation of nerve cells in mice, so it is possible that the fast to refeeding cycle may stimulate similar biological renewal in similar systems in humans.
There was a time in our history when a 12 hour fast was a common daily occurrence. As late as the 70’s and early 80’s, before the arrival of cable TV and FM radio, there was very little to do after midnight. Most people were in bed when television stations signed off for the evening. It is interesting to note that obesity rates in the U.S. started to rise in the 1980’s, coinciding with the rise in snack foods and 24/7 electronic entertainment. Fast forward to 2023, with the proliferation of screens that engage us at all hours as well as the cheap and massive amount of processed food, and it is no wonder obesity and chronic diseases are skyrocketing.
All our biological systems follow their own individual circadian rhythm; perhaps TRE and fasting is a way to tune into the rhythms of our digestion, helping us to refresh and renew a system that, for many, works overtime every day, well into the night. Long periods of restricted calories are not appropriate for everyone, but perhaps we can find time throughout the day when we give our taste buds, stomach, and guts a chance to pause—a time without sipping, snacking, or picking. Doing so may help us become familiar once again with the sensation of hunger as well as satiety, of savoring flavors and sharing a meal rather than eating mindlessly while sitting at a traffic light. TRE and various fasting protocols may not lead to weight loss, but they do hold promise for their ability to help us be renewed and rejuvenated.
I invite you to take a stretch of time this week to take a pause, put your fork down, and find a longer than usual period of time to give your body a rest from eating. You most likely will not starve, but you may feel uncomfortable. Listen carefully to hear what your body is telling you; don’t go to extremes but find your edge. Can you reconnect to your feelings of hunger and satiety, of the body’s true need for food and its need to rest?