Obesity is a widespread health concern with significant comorbidities. As such, effective weight loss strategies are of paramount importance. Intermittent fasting (IF) has emerged as a popular dietary approach, characterized by regular periods of reduced or no caloric intake. This article aims to provide a comprehensive overview of intermittent fasting, its various forms, its effectiveness in weight loss, and its potential benefits and risks based on available scientific evidence.
What is Intermittent Fasting?
Intermittent fasting (IF) involves structuring eating patterns around specific periods of fasting and non-fasting. Instead of focusing on what to eat, IF emphasizes when to eat. Various terms are used to describe regular intermittent calorie abstention, including intermittent fasting, alternate-day fasting, reduced meal frequency, and time-restricted feeding.
Common IF Methods
Several intermittent fasting protocols exist, each with its own unique approach:
- 16/8 Method: This involves a daily fast of 16 hours, followed by an 8-hour eating window.
- Alternate-Day Fasting: This protocol consists of a 24-hour fast on alternate days.
- 5:2 Diet: This involves eating normally for five days of the week and restricting caloric intake to 25% of caloric needs for two non-consecutive days.
During fasting periods, 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.
Intermittent Fasting and Weight Loss: What the Research Says
Numerous studies have investigated the effectiveness of intermittent fasting for weight loss in overweight and obese individuals.
Read also: Flexible Diet for Women Over 50
Overview of Studies
A comprehensive review of articles from January 1, 2000, to July 1, 2019, identified numerous studies focused on weight loss for overweight and obese participants (body mass index [BMI] of ≥ 25 kg/m2). 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). Studies were often of short duration (2 to 26 weeks) with low enrolment (10 to 244 participants); 2 were of 1-year duration. All 27 IF trials found weight loss of 0.8% to 13.0% of baseline weight with no serious adverse events.
Weight Loss Outcomes
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.
IF vs. Calorie Restriction
Twelve studies compared IF to calorie-restricted diets and found equivalent weight loss in both groups. 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%. Adherence appears similar for both weight loss strategies.
The largest study comparing IF with calorie restriction was by Headland et al in 2019 of 244 obese adults who achieved a mean 4.97-kg weight loss over 52 weeks versus a mean weight loss of 6.65 kg with calorie-restricted diets (P = .24). 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.
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).
Read also: Borino-Quinn: "Sopranos" and Beyond
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. 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. Use of IF in patients with diabetes poses a risk of hypoglycemia.
Read also: Weight Loss Guide Andalusia, AL
Safety and Potential Side Effects
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.
Cultural Practices: 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).