The clinical data on intermittent fasting, and comparisons with caloric-restriction diets
In this two-part series, Dr. Michael Brown, ND, takes a comprehensive look at this popular and trending approach to a healthier lifestyle. In Part I, a look at the different approaches to intermittent fasting and how nighttime eating can adversely affect health. In Part 2, Dr. Brown takes a closer look at the clinical studies surrounding intermittent fasting, as well as those comparing this dietary strategy 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/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. 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.
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