Intermittent fasting is the idea that we can selectively use fasting as a tool for health. In addition to potentially being a more effective approach to weight control, it may also protect against chronic degenerative diseases such as cancer. "Emerging findings from human studies indicate that different forms of fasting may provide effective strategies to reduce weight, delay aging, and optimize health." (1)
Unfortunately, nutrition is one of those topics where there are a lot of people who spout opinions which don't seem to be backed up with scientific evidence. Often information sources on the internet seem more interested in selling diet-related products than exploring the truth of the science as we currently understand it. I decided to look into the evidence myself and here's what I learned.
Comparing Fasting to Traditional Diets
As I dove into the medical literature, I discovered a number of technical terms around dieting and fasting. I noticed that people seemed to commonly confuse the different approaches, so let's get clear on the definitions before we proceed.
A traditional diet, consisting of simply eating less food all the time, is called a caloric restriction (CR) diet (also known as a continuous energy restriction diet). These diets are psychologically unpleasant to follow because you end up being hungry and cranky most of the time. Ultimately this results in most people quitting the diet and regaining any weight they may have lost during the unpleasantness. It also turns out that people are bad at guessing how many calories they are eating, even if they are diligent in trying to keep a food journal. The result of these factors together means that out of all the people trying caloric restriction, "only 20% of dieters are thought to be able to successfully sustain a 10% weight-loss for more than 12 months." (2) I think we can agree that 20% is an unacceptably low success percentage if we are looking to transform our own lives.
In contrast, intermittent fasting involves controlling when you eat rather than what you eat. Another name for intermittent fasting is periodic fasting because you are fasting for recurring periods of time. Interestingly, this approach still results in less overall food consumption even though participants are allowed eat as much as they want during the non-fasting periods. In an example from one of the studies I reviewed, "even a 1-day fast or 75% calorie restriction was shown to reduce caloric intake by approximately 30% during the subsequent 3 days." (3) So, in general, fasting decreases your overall appetite after the fast is over.
Does intermittent fasting work for weight loss? Yes. The literature is fairly clear on this point. "Statistically significant weight reduction was observed in 73% of trials of intermittent fasting." (4)
Major Types of Intermittent Fasting
Inside of the general umbrella of intermittent fasting, there are a variety of specific approaches. A lot of the scientific studies used alternate day fasting, which is when you eat whatever you want for one day and then eat nothing at all the next day. Each subsequent day then repeats this "one day on, one day off" cycle. While alternate day fasting was effective for weight loss, participants experienced similar problems to those on caloric restriction diets, namely that going for a full day without any food at all is completely miserable. In response, modified alternate day fasting regimes were developed which allowed participants to eat 25% of the usual amount on fast days. These were more effective but still quite unpleasant. "Reports of extreme hunger while (alternate day) fasting indicate that this may not be a feasible public health intervention." (5) Similar problems occur in the 5:2 diet, which is another variant of modified alternate day fasting.
The other major type of intermittent fasting is called time restricted eating (also known as time restricted feeding). In time restricted eating, you only eat during an 8 to 11 hour window during the daytime. The rest of each day and all of each night is a fasting period. In other words, every day is an eating day and also a fast day. This appeals to my desire to craft an optimal routine for myself which remains roughly constant on a day to day basis. When I tried time restricted eating out for myself, I didn't have that unpleasant starvation feeling.
Time Restricted Eating and Circadian Rhythms
The data strongly suggests that "the timing of food intake is an important determinant of human health and disease risk" (6) because of the importance of the circadian rhythm. Many cycles in nature are timed directly or indirectly from the day/night cycle. The day/night cycle influences plant growth, humidity, temperature, and a variety of other factors which in turn influence the animals. To stay in touch with these natural rhythms, animals have developed the circadian rhythm, which is an internal clock tracking and mimicking the day/night cycle. This internal clock then drives various internal physiological processes.
Your main internal body clock is set based on exposure to light. This was highly adaptive in the time before artificial lighting, but in modern society the abundance of light after dark is confusing our main internal clocks. In addition to the main clock, there are secondary clocks located in the stomach and liver. These clocks are timed based on when you eat. If you eat during the day-time, as would have been typical for ancient people, your main internal clock is sync'd up with your other internal clocks and all of those are synchronized with the cycles of nature. If instead you eat in the middle of the night, your clock in your stomach and liver gets out of sync with your main clock and with nature's clock. The resulting desychronization is akin having jet lag all the time, which is very unhealthy. "Feeding signals appear to be the dominant timing cue for the rhythms of peripheral clocks... Late night eating in humans may reset some peripheral clocks and disrupt energy balance." (7)
Disrupted circadian rhythms cause issues with your gut microbiome, the bacteria in your gut which is crucial to health. In a non-obese person with synchronized internal clocks, the healthy gut bacteria fluctuate daily in a cyclical pattern. If becomes obese, these cyclical fluctuations in gut bacteria dampen. "Time-restricted feeding... partially restores these cyclical fluctuations. Thus, cyclical changes in the gut microbiome resulting from diurnal feeding and fasting rhythms contribute to the diversity of gut microflora and represent a mechanism by which the gut microbiome affects host metabolism." (8) Remember that it's the bacteria in your gut that dictate which food you crave and also dictate the efficiency of your metabolism. "Fasting regimes appear to have positive impacts on the gut microbiota." (9)
Time restricted eating is consistent with your circadian rhythms because the 8 to 11 hour eating window is during daylight hours. Because the night time is always during the fast period, time restricted eating encourages eating consistent with the natural cycles. "Time restricted feeding regimens ... actively impose a ... rhythm of food intake aligned with the 24-hour light-dark cycle (which leads) to improved ... energy metabolism and improved body weight regulation." (10)
Interestingly, time restricted feeding also seems to have a beneficial effect on your sleep, which makes sense since your sleep is timed by your circadian rhythm. In contrast, eating late at night seems to disrupt sleep quality, which can in turn cascade into a host of other possible medical issues. "Numerous observational studies have reported that nighttime eating is associated with reduced sleep duration and poor sleep quality, which can lead to insulin resistance and increased risks of obesity, diabetes, cardiovascular disease, and cancer. Specifically, eating meals at abnormal circadian times (i.e., late at night) is hypothesized to lead to circadian desynchronization and subsequent disruption of normal sleep patterns." (11)
A Promising Approach to Nutrition
Time restricted eating seems to be a particularly promising approach to keeping your body in a healthy balance. It seems to directly reduce the total amount of food consumed, which is great for weight loss, while also regulating your body clock in a way that may produce numerous health benefits. Because fasting effects the physiology of the entire body, what's exciting is the wide range of possible positive consequences from this sort of eating system. Intermittent fasting in general and time restricted eating in particular "have emerged as potential strategies for avoiding major dietary changes while achieving strong effects not just for one disease risk factor but for an array of factors that constitutes the foundation for metabolic syndrome, cardiovascular disease, cancer, and possible neurodegenerative diseases." (12) There also seem to be benefits related to slowing down the aging process in general. (13)
If you want to lose weight and/or feel great, perhaps give time restricted eating a try. If you want to lose weight, choose an eating window around 8 to 9 hours during the daytime. If you want to maintain your existing weight but still reap the health benefits, choose an eating window of 11 hours during daytime. Outside of your eating window, you may drink water but no tea, coffee, supplements, or anything else that your stomach has to digest. You will probably notice increased mental clarity as well as better sleep. Note - I am not a medical doctor. See this legal disclaimer. If you do try time restricted feeding for yourself, please let us know about your experience. If you enjoyed the article, please considering sharing it on your social media with your friends.
(1) Longo VD and Mattson MP, "Fasting: Molecular Mechanisms and Clinical Applications," Cell Metabolism, 2014, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3946160/#
(2) Rona Antoni, Kelly L. Johnston, Adam L. Collins and M. Denise Robertson, "The Effects of Intermittent Energy Restriction on Indices of Cardiometabolic Health," Research in Endocrinology, 2014, http://ibimapublishing.com/articles/ENDO/2014/459119/
(3) Ruth E. Patterson and Dorothy D. Sears, "Metabolic Effects of Intermittent Fasting," The Annual Review of Nutrition, 2017, http://www.annualreviews.org/doi/10.1146/annurev-nutr-071816-064634
(4) through (11) Ibid.
(12) Longo VD and Mattson MP, "Fasting: Molecular Mechanisms and Clinical Applications," Cell Metabolism, 2014, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3946160/#
(13) Mattson MP, Longo VD, and Harvie M, "Impact of Intermittent Fasting on Health and Disease Processes," 2017, https://www.ncbi.nlm.nih.gov/pubmed/27810402?report=abstract