Sudden rise in popularity
INTERMITTENT fasting (IF) is a dietary practice in which periods of regular consumption of foods and beverages are interspersed with periods of severe energy restriction or by fasting.
The excessive intake of energy is associated with the worldwide increase in the incidence of chronic diseases, including obesity, type II diabetes mellitus (T2DM) and metabolic syndrome.
Caloric restriction by IF increases longevity and reduces the incidence of chronic noncommunicable diseases (NCDs) associated with aging such as obesity, cardiovascular diseases, cancer, renal disease, and diabetes mellitus.
IF has grown in popularity in the past few years as they can produce clinically substantial weight loss (>5 per cent loss from baseline) in many people and are often considered to be less complicated than traditional forms of dieting such as calorie restriction.
IF requires fasting for a period of time, typically a minimum of 12h, followed by a period of ad libitum eating. One of the reasons for the sudden rise in popularity of IF is its simplicity.
Previous popular diets have generally required individuals to vigilantly monitor energy intake or meticulously count carbohydrates.
IF can often be a refreshing alternative for many individuals, in that it does not require people to track calories every day, nor does it forbid individuals from eating certain food groups.
Types of IF
Three types of IF are in practice largely: Alternate Day Fasting (ADF), the 5:2 diet and Time-Restricted Eating (TRE) also known as Time Restricted Feeding (TRF).
1. Alternate Day Fasting (ADF)
ADF involves a ‘fast day’ alternating with a ‘feast day’. On fast days, individuals can choose to consume only water, which is termed ‘zerocalorie alternate day fasting or alternatively, individuals can consume 25 per cent of their energy needs (about 500 kcal per day), which is called ‘modified alternate day fasting’.
The fast day meal can be consumed all at once or spread throughout the day, without affecting the degree of weight loss achieved. The timing of the fast day meal is optional.
However, participants generally prefer to consume the meal at dinner time so they can engage in their habitual social eating patterns, such as eating dinner with their family or friends. On feast days, individuals can eat freely, with no restrictions on types or quantities of foods consumed.
2. 5:2 Diet
The 5:2 diet is a modified version of ADF, which involves two fast days (500–1000kcal per day), and five feast days per week. The fast days can occur on consecutive or non-consecutive days in the week
3. Time-Restricted Eating (TRE)
TRE differs from ADF and the 5:2 diet in that it necessitates individuals to fast for a certain period of time every day. TRE involves confining the eating window to a specified number of hours per day (typically four to eight hours), and fasting with water or zero-calorie beverages for the remainder of the day.
TRE is a unique form of IF in that it does not require individuals to monitor their energy intake or count calories during the eating window.
In contrast to traditional IF, TRE is usually performed on a daily basis and does not need prescribed restrictions.
The TRE diet is of special interest among physically active people because of reports on its effect on weight reduction while maintaining muscle mass.
Additionally, the fasting window may be planned during night time. Thus, it can help some individuals to avoid night eating and follow a circadian rhythm. Protocol may vary according to individual preferences and lifestyle.
Timing of food intake during IF (ADF, 5:2 diet and TRE)
For TRE, in the majority of trials the suggested eating window is 8h.
For ADF and the 5:2 diet, the suggested food intake window is 17:00h to 19:00h.
However, the fast day meal can be consumed all at once or spread throughout the day, based on individual preference.
The effects of IF on body weight
Findings to date reveal that ADF and the 5:2 diet produce similar degrees of weight loss (4–8 per cent loss from baseline) over short intervention periods (eight–12 weeks) in men and women with obesity.
Longer-term studies have found that ADF and the 5:2 diet (24–52 weeks) do not result in greater body weight reductions than found in short-term studies, suggesting that the weight loss efficacy of these diets might peak at 12 weeks.
The degree of weight loss achieved with TRE seems to be less pronounced (3–4 per cent loss from baseline) over 8–12 weeks than with ADF and the 5:2 diet. Worth mentioning, no study to date has demonstrated clinically significant weight loss (>5 per cent loss from baseline) with TRE.
The TRE literature is limited, however, in that no study longer than 12 weeks has been performed. It is possible that clinically significant weight loss would occur with longer durations of TRE.
The efficacy of IF in maintaining weight has only been tested in four trials in humans. The results of these trials revealed that both ADF and the 5:2 diet are effective in preventing weight regain during 12–24 weeks of follow-up.
The effects of IF compared with traditional dieting (daily calorie restriction) on body weight have been investigated in several trials and meta-analyses.
In a 2017 study, ADF and calorie restriction (25 per cent daily calorie restriction, about 1500 kcal per day) produced similar degrees of weight loss (5–6 per cent loss from baseline) over 52 weeks in adults with overweight or obesity.
Short-term studies (less than 12 weeks duration) of ADF have also shown similar body weight reductions to calorie restriction.
As for the 5:2 diet, similar degrees of weight loss have been reported when this diet is compared with calorie restriction in short-term and long-term trials.
Although the data are limited, preliminary findings indicate that IF and traditional dieting produce similar reductions in body weight in individuals with overweight, obesity, type one diabetes mellitus and (T1DM) and type two diabetes mellitus