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Different intermittent fasting strategies, and how eating outside of “normal” eating hours can adversely affect health
Dr. Michael Brown, ND, takes a comprehensive look at this popular and trending approach to a healthier lifestyle. To view the original article
Traditionally, fasting has predominantly been associated with religious and spiritual practices,[1],[2],leading many to assume fasting inherently entails very restricted eating or no food whatsoever for days or weeks at a time. What most don’t realize is that fasting is actually something that the majority of us partake in every night when going to bed. “Breakfast”, being the first meal of the day, actually refers to breaking the fasting period of the previous night. Fasting during this time, which usually ranges from about six to 12 hours, falls into line with our natural circadian rhythms and the rising and setting sun, which we know very much impacts our physiology and metabolism.[3]
Intermittent fasting: a modern version of an age-old tradition
Although there are a variety of ways one can fast, a newer phenomenon trending in the health and fitness world is referred to as intermittent fasting (IF), piquing the interest of not only health enthusiasts, but the scientific community as well. A quick Google search using the term “intermittent fasting” produces an astounding 55,200,000 results! This form of fasting is achieved by ingesting minimal or zero amounts of food and caloric beverages for periods of time typically ranging from 12 hours to several days.[2] IF is distinct from caloric restriction diets in which the daily caloric intake is reduced chronically by 20 to 40%, but meal frequency is maintained.
Although there are a variety of ways one can fast, a newer phenomenon trending in the health and fitness world is referred to as intermittent fasting (IF).
Of the various forms of IF, three of the more popular and studied variations are as follows:
- Alternate-day fasting: This form of fasting involves modified fasting every other day. For example, limiting your calories on fasting days from zero up to 500 calories (or a maximum caloric consumption of approximately 25% of your normal intake). On non-fasting days, you would resume a regular, healthy diet.[1],[4]
- Time-restricted feeding: In this option, you have set fasting and eating time periods. A very popular approach is the 16/8 method where you only eat between 11 am and 7 pm or noon and 8 pm. This method can be repeated several times per week and is popular due to easier compliance given a majority of your fasting occurs overnight.[1],[2],[3]
- Twice-a-week method: This is also called the 5:2 diet because five days of the week are normal eating days, while the other two restrict calories to 500 to 600 calories per day. For five days per week, you eat a regular, healthy diet. Then, on the other two days, you reduce your calorie intake to a quarter of your daily needs. You can choose whichever two days of the week you prefer, as long as there is at least one non-fasting day in between them.[5],[6],[7]
Circadian rhythms, eating patterns, and their impact on health
A circadian rhythm is a natural, internal process that regulates the sleep-wake cycle and many other physiological processes.[3],[8] It repeats roughly every 24 hours, being sensitive not only to light and darkness but also other cues, known as zeitgebers, such as temperature and social interactions. Most living organisms, including humans, have evolved internal metabolic mechanisms that allow them to anticipate and prepare for activity, sleep, and food intake at a specific time of day.
Both food ingestion and fasting can alter our metabolic processes. Consuming calories outside the normal eating hours (i.e., late night eating in humans) may negatively impact circadian rhythms and associated cardiometabolic parameters.[3],[8]It has been demonstrated that shift-work is associated with nighttime eating and increased risks of obesity,[9] diabetes,[10] cardiovascular disease[11], and cancer (particularly breast cancer).[12],[13],[14],[15],[16] Data from trials and prospective cohort studies also support the idea that consuming the majority of calories earlier in the day, thus prolonging the night-time fasting state (with little or no food), is associated with lower weight and improved health.[17],[18],[19],[20],[21]
Data from trials and prospective cohort studies also support the idea that consuming the majority of calories earlier in the day, thus prolonging the night-time fasting state (with little or no food), is associated with lower weight and improved health.
This data is in line with animal studies demonstrating that restricting caloric intake to an eight to 12 hour window helps support natural circadian rhythm and is associated with reduced adiposity, elevated lean muscle mass, longer sleep duration, increased endurance, reduced systemic inflammation, gut homeostasis, and improvement in other clinical biomarkers.[22],[23],[24],[25],[26]
Clearly, these studies suggest that the time during a 24-hour period in which we are eating has an impact on our health.
The clinical data on intermittent fasting, and comparisons with caloric-restriction diets
A substantial number of published studies suggest that intermittent fasting (IF) promotes weight loss and may improve metabolic and cardiovascular health parameters. Unfortunately, the majority of these findings are from preclinical animal studies. There is, however, emerging data from a growing number of human clinical trials which we will primarily focus on in this article. Interestingly, the growing body of research based on trials involving humans is suggesting that the timing of the fast is also a key component to the potential health benefits of IF.[1]
Interestingly, the growing body of research based on trials involving humans is suggesting that the timing of the fast is also a key component to the potential health benefits of IF.
Clinical studies on time-restricted feeding
A published review that examined 16 human intervention studies concluded that eating patterns which reduce or eliminate nighttime eating and prolong nightly fasting intervals may result in weight loss and improved metabolic health.[2] These same authors also used data from the National Health and Nutrition Examination Surveys (known as NHANES) to show, for each three-hour increase in nighttime fasting duration, there was a significant reduction in HbA1c (long-term marker of blood sugar control) and CRP levels (marker of inflammation) in women who consumed less than 30% of their daily calories after 5:00 pm.
A study using a smartphone application to monitor the natural daily eating pattern and caloric intake of healthy adults found similar results.[3] Data from a survey of 156 individuals demonstrated that people tend towards eating frequently and erratically throughout wakeful hours with a bias towards eating late. The average window of caloric intake exceeded 14.75 hours for half of the participants with less than 25% of calories being consumed before noon and greater than 35% consumed after 6 pm.
When a group of eight subjects having a BMI of greater than 25 kg/m2 and a normal eating duration of >14 hours shortened their eating time to only 10 to 11 hours daily for a 16-week period (monitored by the same smartphone application and protocol), they reduced their body weight by an average of 7.2 lbs.
The same group of researchers found that when a group of eight subjects having a BMI of greater than 25 kg/m2and a normal eating duration of >14 hours shortened their eating time to only 10 to 11 hours daily for a 16-week period (monitored by the same smartphone application and protocol), they reduced their body weight by an average of 7.2 lbs. Participants also reported feeling more energetic and experienced improved sleep quality. Interestingly, everyone enrolled in the study voluntarily expressed an interest in continuing unsupervised with the 10 to 11-hour IF regimen after the conclusion of the 16-week trial. Follow-up showed that the improvements reported at 16 weeks persisted at one year.
Intermittent fasting versus calorie-restricted dieting
In 2016 a systematic review and meta-analysis of the literature examined the results of human IF trials lasting a minimum of six months.[4] Each of the various forms of IF employed across the studies were successful in achieving significant weight loss from baseline. Of the studies that lasted for 12 months or longer, weight loss average ranged from four pounds up to 23 pounds.[5],[6],[7],[8],[9],[10] The authors concluded that IF is a successful strategy for weight loss but there was no evidence that it was superior to caloric restriction (CR) dietary approaches. Dropout rates were similar in the IF and CR arms of the included studies, suggesting that long term adherence to IF may be similar to CR diets and therefore presents a successful alternative for individuals who find CR too restrictive.
Another meta-analysis of randomized controlled trials (RCT) comparing IF to CR diets on the outcome of weight loss was published in 2018.[11] Results were strikingly similar: of the 11 RCTs that were included in this review, comprising 630 participants, they all reported weight loss in the IF arms ranging from 5.2% of baseline weight up to 12.9%. In the CR arms, weight loss ranged from 4.3% to 12.1%.[12],[13],[14] Assessment of fasting blood sugar, insulin, and HbA1c showed no statistical difference between IF and CR diets.
Addition studies available comparing IF to CR show similar benefits on various parameters related to glucose metabolism and fat mass.[15],[16],[17] Again, authors concluded that IF is as effective as CR diets in overweight/obese adults for promoting weight loss and metabolic improvement in the short-term. However, further long-term investigations are warranted to draw definitive conclusions.
Highlights from additional clinical studies on intermittent fasting
The highlights of recent clinical studies, presented below, shed light on other potential benefits IF may hold.
Study 1. Thirty-four resistance-trained males were randomly assigned to IF group or normal diet group (ND).[18] IF subjects consumed 100% of their caloric intake in an eight-hour time period, and fasted the remaining 16 hours. Those following the ND consumed 100% of their energy needs from 8 am to 8 pm. Groups were matched for macronutrients and calories consumed. Subjects were tested before and after eight weeks of diet and standardized resistance training.
Conclusion: IF program improved some health-related biomarkers, decreased fat mass, and maintained muscle mass.
Study 2. In a five-week, randomized, crossover, isocaloric and eucaloric (a diet that contains about the same number of calories an individual uses [or “burns”] each day) controlled feeding trial in men with prediabetes, participants adopted a six-hour daily, IF schedule, with dinner before 3 pm.[19]
Conclusion: IF improved insulin levels, insulin sensitivity, blood pressure, and oxidative stress levels even though food intake was matched to the control arm and no weight loss occurred.
Significance: This was the first RCT to show that IF has benefits independent of food intake and weight loss in humans.
Study 3. An interesting pilot RCT published in 2019 evaluated the effects of combining IF and a nutrition program on healthy, overweight adults during the six-week winter holiday period between Thanksgiving and New Year.[20]Participants were assigned to a nutritional program that included IF or a control group. On two consecutive days of each week, participants in the nutritional program group decreased their caloric intake by consuming a commercially available shake four times per day (170 calories per serving) along with a dietary supplement to ensure adequate intake of essential nutrients. On the remaining five days, they consumed their normal, habitual diets with no specific dietary recommendations.
Conclusion: In the nutrition program group, a significant weight loss from baseline was observed along with an increase in HDL cholesterol and decrease in triglyceride scores. Increases in insulin, LDL cholesterol, and total cholesterol was noted in the control group. Overall compliance rate was 98%. Results suggest that IF might be a useful tool for the maintenance of a healthy weight and metabolic health during the winter holiday season, when weight gain is typically observed.
Results suggest that IF might be a useful tool for the maintenance of a healthy weight and metabolic health during the winter holiday season, when weight gain is typically observed.
Where do we go from here?
The studies reviewed here suggest that reducing our window of calorie consumption (especially nighttime eating) and prolonging our nightly fasting intervals may be a simple, viable, and effective approach for weight loss, potentially improving cardiovascular and metabolic health parameters as well. It is uncertain at this point whether IF is any more effective than CR diets for obtaining these health benefits, and more large-scale randomized trials of IF regimens are needed. That said, as someone who personally uses IF and recommends it clinically, I am excited to see what the data has to say from larger, more comprehensive clinical trials.