Intermittent fasting (IF), or time-restricted feeding, is an emerging dietary intervention
that restricts intake of food and energy for a given period. Not only does this method
restrict total caloric intake, it also promotes metabolic homeostasis by supporting
circadian feeding rhythms.1,2 Only in the most recent 200 years have humans been able
to access supplies of vast amounts of food and resources, which has caused a shift
in disease patterns, particularly for metabolic syndromes and obesity. The idea that
reducing caloric intake can result in evolutionary cellular responses for survival
has been an active research topic,3 but with the recent public awareness and interest
in IF, many clinical studies have been published more recently. On December 2019,
de Cabo and Mattson4 published a review article that discusses the mechanisms and
current clinical evidence for IF. This review article discusses various broad-spectrum
benefits of IF, with a positive outlook for more clinical evidence in the future.
The most widely accepted theory behind the primary physiologic response after IF is
the switch of energy source from glucose to triglycerides, which has also been termed
as the “ketogenic diet.”5 This metabolic switch increases mitochondrial stress resistance,
antioxidative defenses, and autophagy while reducing the amount of blood insulin.4,6
de Cabo and Mattson4 illustrated in their review that stimulating autophagy while
inhibiting the mammalian target of rapamycin protein synthesis pathway can lead to
removal of oxidatively damaged cells. In a review by Wahl et al.,7 IF was shown to
have positive effects on memory acquisition and cognitive behavior in rodent models.
One possible hypothesis for this effect is production of proinflammatory factors with
glucose-based diets.8 The reduction of these proinflammatory factors may be beneficial
for reducing systemic inflammation and oxidative stress factors that play a role in
development of atherosclerosis.9
Several clinical studies, including some randomized control trials, have been performed
to analyze the effects of IF and caloric restriction in a wide range of applications.
Although preclinical studies have demonstrated reduction of insulin sensitivity after
caloric restriction and periodic fasting,4,10 clinical studies have showed inconsistent
results. The calorie restriction and cardiometabolic risk (CALERIE) study11 is a phase
2, multicenter, randomized control trial where 218 patients were allocated to either
the 25% caloric restricting group (n=143) or an ad libitum control group (n=75). In
the restricting group, there was significant loss in body weight and reduction in
other cardiometabolic factors such as low-density lipoprotein (LDL) cholesterol and
blood pressure. Fasting glucose was significantly reduced at the first year, but there
was no significant reduction at the second year. However, there was a significant
decrease in a surrogate marker for insulin resistance, which was estimated by the
homeostatic model assessment for insulin resistance (HOMA-IR). Another study by Marinac
et al.8 evaluated the frequency and circadian timing of feeding with metabolic syndrome
and breast cancer risk in women and showed no association of HOMA-IR with evening
calorie intake, eating frequency, or nighttime fasting duration. Recently, Wilkinson
et al.2 published a singlearm study of 19 patients with metabolic syndrome who were
mostly on statin and/or antihypertensive therapy. Patients were restricted to a 10-hour
period of feeding and were examined for body composition and other health metrics
after 12 weeks. There was significant decrease in body weight, body fat, systolic/diastolic
blood pressure, total cholesterol, and LDL-cholesterol. Although there was a trend
toward decrease of blood glucose and glycosylated hemoglobin (HbA1c) levels, there
was no statistically significant benefit. However, in a subgroup analysis of patients
with either elevated fasting glucose ≥100 mg/dL and/or HbA1c ≥5.7%, there was a significant
reduction in HbA1c level (–0.22%±0.32%, P=0.04).
IF acts as a stress signal stimulator, preconditioning cells before ischemic tissue
injury.4 Mauro et al.12 compared rodent models with a 3-day water-only fasting, 1
week protein-free fasting, and overnutrition (high-fat diet) diet protocol prior to
vascular surgery. Short-term, 3-day fasting before vascular surgery significantly
attenuated intimal hyperplasia and reduced ischemia-reperfusion outcomes. A randomized
control trial examined bariatric patients who were scheduled to undergo gastric bypass
surgery and compared a 14-day very low-calorie diet (VLCD) group with the ad libitum
control group.13 Although there was no difference in operation time, the number of
30-day complications was higher in the control group than the VLCD group. Abdominal
surgery increases in difficulty with increasing body fat composition, and high amounts
of visceral fat lead to unclean dissection of the body planes, resulting in an operative
field prone to inflammation and fluid collection. In addition, high subcutaneous fat
complicates wound closure, resulting in more frequent wound complications.
The possible benefits of IF before elective surgery are currently controversial, especially
compared with the Enhanced Recovery After Surgery (ERAS) protocol, which is now being
implemented with good outcomes in various types of surgery.14 One of the popular components
of the ERAS program is reducing preoperative fasting time to 6 hours and providing
oral carbohydrate solutions for up to 2 hours before surgery—encouraging preoperative
oral nutrition. This theoretically reduces preoperative anxiety, and patients transition
into an anabolic state to benefit from postoperative nutrition.15 The ERAS protocol
is a comprehensive idea for reducing surgical stress to promote faster recovery; however,
it does not explore the idea of fasting alone. Thus, the current success of the ERAS
protocols may not necessarily be attributable to reduced preoperative fasting, and
additional well-designed control studies are needed to elucidate how preoperative
fasting affects patient outcome.
Despite all the emerging evidence, there are still some pitfalls and practical difficulties
to IF and caloric restriction that are continuously being studied. First, physiologic
studies have not yet achieved consensus on the optimal timing for IF. Some studies
have used alternative day fasting, and some have used a daily timerestricted schedule.
Although studies have shown that IF reduces patient stress4 in the long-term, most
patients find it stressful and are reluctant to start restricting intake, reducing
patient compliance. Worldwide, especially in Korea, many people believe that skipping
one of the three meals in a day will result in deterioration of health and nutritional
balance. To address such underlying concerns, de Cabo and Mattson4 suggested that
physicians provide adequate information and continual support to applicable patients.
In this era where food is abundant, scientists must re-evaluate the notion that “more
is better” when it comes to nutrition. Despite the amount exploration regarding IF
in clinical practice, there are many areas that have not yet been explored in well-designed
clinical trials. Therefore, additional research and consideration are needed to optimize
patient outcomes.