Introduction
Intermittent fasting (IF) has gained popularity as a weight loss strategy. IF involves regular periods of reduced or no calorie intake. This article examines the effectiveness of IF for weight loss, its impact on glycemic control, and potential risks and benefits, based on available research.
What is Intermittent Fasting?
Intermittent fasting (IF) refers to regular periods with no or very limited caloric intake. Various terms are used to describe regular intermittent calorie abstention, including intermittent fasting, alternate-day fasting, reduced meal frequency, and time-restricted feeding. It commonly consists of a daily fast for 16 hours, a 24-hour fast on alternate days, or a fast 2 days per week on non-consecutive days. During fasting, caloric consumption often ranges from zero to 25% of caloric needs. Consumption on nonfasting days might be ad libitum, restricted to a certain diet composition, or aimed to reach a specific caloric intake of up to 125% of regular caloric needs. Intermittent fasting can be used with unrestricted consumption when not fasting or in conjunction with other dietary interventions.
Research Methodology
An EMBASE and MEDLINE search of articles from January 1, 2000, to July 1, 2019, returned 1200 unique results using the key words alternate day fasting, intermittent fasting, fasting, time restricted feeding, meal skipping, and reduced meal frequency. Studies were included if they were in English and focused on weight loss for overweight and obese participants (body mass index [BMI] of ≥ 25 kg/m2). Studies of very short duration (< 2 weeks), studies of those requiring inpatient treatment, or studies focused on stroke, seizures, or other specific medical conditions were excluded. Study interventions incorporated IF in a variety of ways, from a 24-hour fast several days per week (eg, the “5 and 2” protocol) to a daily 16-hour fast. The most common study design was to alternate 24-hour periods of fasting with unrestricted consumption (alternating fast and feast days). Study protocols also varied in their recommendations on caloric intake, enrolment of patients with diabetes, presence of a control group, and study duration. Some studies restricted calories while others allowed ad libitum consumption when not fasting. The rigour of fasting also varied, with several studies allowing 25% of regular caloric consumption during fasting periods.
Weight Loss Outcomes
Forty-one articles describing 27 trials addressed weight loss in overweight and obese patients: 18 small randomized controlled trials (level I evidence) and 9 trials comparing weight after IF to baseline weight with no control group (level II evidence). All 27 IF trials found weight loss of 0.8% to 13.0% of baseline weight with no serious adverse events. In all 27 trials examined, intermittent fasting (IF) resulted in weight loss, ranging from 0.8% to 13.0% of baseline body weight. Weight loss occurred regardless of changes in overall caloric intake. In the studies of 2 to 12 weeks’ duration, body mass index decreased, on average, by 4.3% to a median of 33.2 kg/m2.
Studies were often of short duration (2 to 26 weeks) with low enrolment (10 to 244 participants); 2 were of 1-year duration. Twelve studies comparing IF to calorie restriction found equivalent results. Study duration was 8 weeks to 1 year, with a combined total of 1206 participants (527 undergoing IF, 572 using calorie restriction, and 107 control participants) and demonstrated weight loss of 4.6% to 13.0%. All of the 11 other comparisons of IF and calorie-restriction diets also found similar results between both groups. In several of these studies, those in the IF group consumed the same amount of calories or less than those in the calorie-restriction group.
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Fat Loss
Most of the weight loss with IF is fat loss. A 2011 study by Harvie et al calculated that 79% of weight loss was owing to loss of fat specifically (level I evidence).
Weight Regain
Participants regained some weight during follow-up after intervention, although average body weight remained statistically significantly lower than baseline levels. Weight regain did occur after 6 months. Five studies followed participants for 6 months or longer after completing IF interventions of 8 weeks to 1 year and most studies saw body weight increase by 1% to 2% of their weight nadir. In 6 comparisons of IF and calorie restriction, the amount of weight regained after IF and calorie restriction was similar.
Practical Considerations
The practical length of a fast to effect changes in weight appears to be 16 hours. In IF studies with a daily fasting intervention, a total of 120 participants were able to maintain a minimum daily fast of about 16 hours (15.8 to 16.8 hours), with an 8-hour eating window each day. Combining exercise with IF improved weight loss in a 2013 study by Bhutani et al of 64 obese patients.
Impact on Glycemic Control
While IF is a moderately successful strategy for weight loss, it shows promise for improving glycemic control. Five studies exclusively enrolled individuals with type 2 diabetes. In both diabetic and non-diabetic populations, cardiovascular risk factors were reduced.
Kahleova et al compared a daily fast of at least 16 hours to caloric restriction (n = 54). Both groups experienced decreases in insulin levels but IF participants had significantly lower fasting glucose levels (−0.78 mmol/L vs −0.47 mmol/L, P < .05). Increased oral glucose insulin sensitivity, decreased C-peptide levels, and decreased glucagon levels were also statistically significantly greater in the IF group. In a 2016 pilot study, Carter et al implemented a fast 2 days per week with an otherwise usual diet versus caloric restriction every day in participants with diabetes (n = 51). Medication use was reduced and hemoglobin A1c levels decreased significantly (by 0.7%) during the 12-week study (P < .001), but the effect of IF on weight did not differ from that of caloric restriction (level I evidence). The 2018 trial that followed (n = 137) saw the same result over 12 months of IF or calorie restriction (level I evidence). The improvements in hemoglobin A1c level were lost during the 12 months after IF, although weight losses and medication reductions remained. In the 2017 Saskatchewan study by Arnason et al, 10 participants with type 2 diabetes fasted an average of 16.8 hours per day for 2 weeks. They found improved glycemic control with lower morning, postprandial, and average mean daily glucose levels (level II evidence). These improvements regressed once participants returned to their usual diets.
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Use of IF in patients with diabetes poses a risk of hypoglycemia.
Ramadan Fasting
Ramadan is a culturally determined example of IF for many Muslims. Those who fast often do so for approximately 14 hours per day for 30 days, presenting a real-world opportunity for examining effects of fasting. Eight Ramadan studies examined weight loss in obese adults (n = 856). Weight losses ranged from 0.1 kg to 1.8 kg (level II evidence).
Safety and Adverse Events
No serious adverse events were reported in the 27 IF trials. Fasting-related safety concerns include mood-related side effects and binge eating, among other symptoms.
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