The Science Behind Fasting and Losing Weight
The current obesity epidemic is staggering in terms of its magnitude and public health impact. Current guidelines recommend continuous energy restriction (CER) along with a comprehensive lifestyle intervention as the cornerstone of obesity treatment, yet this approach produces modest weight loss on average. This article summarizes newer research on fasting, and specifically, intermittent fasting.
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Common forms of IF (intermittent fasting) include fasting for:
- up to 24 hours once or twice a week with ad libitum (ad lib) food intake for the remaining days, which is known as periodic prolonged fasting (PF) or intermittent calorie restriction (ICR)
- time-restricted feeding (TRF), such as eating for only 8 hours then fasting for the other 16 hours of the day; and alternate-day fasting (ADF).
- Most ADF programs involve alternating feast (ad lib intake) and fast days (≤25% of energy needs) with some protocols allowing no caloric intake on fast days
Thus, the degree of fasting varies in ADF based on the specific protocol.
The literature distinguishes between ADF (alternate day fasting) and ICR (intermittent calorie restriction).
Heilbronn et al. evaluated 22 days of ADF (0% intake on fast days, ad lib feast days) in 16 healthy subjects with normal BMI.
- ADF resulted in minor weight loss (2.5%), fat loss (4%), and increased fat oxidation .
- In contrast, scientists evaluated 6 weeks of ADF (very low-calorie diet (VLCD) on fast days, ad lib feast days) in women with overweight or obesity .
- ADF led to 7.1% weight loss and visceral fat mass loss (5.7%).
Alternate-day fasting was feasible in non-obese subjects, and fat oxidation increased. However, hunger on fasting days did not decrease, perhaps indicating the unlikelihood of continuing this diet for extended periods of time. Adding one small meal on a fasting day may make this approach to dietary restriction more acceptable.
Table 1 Comparison of different types of intermittent fasting
Alternate day fasting (ADF) with a high-fat diet produces similar weight loss and cardio-protection as ADF with a low-fat diet. (source)
Recently, there has been increased interest in identifying alternative dietary weight loss strategies that involve restricting energy intake to certain periods of the day or prolonging the fasting interval between meals (i.e., intermittent energy restriction, IER).
These strategies include:
- intermittent fasting (IMF; >60% energy restriction on 2–3 days per week, or on alternate days), and
- time-restricted feeding (TRF; limiting the daily period of food intake to 8–10 h or less on most days of the week).
Conclusion — Fasting or Calorie Reduction May Work
Because of the relative ineffectiveness of traditional CER approaches for achieving and sustaining weight loss, there has been increased interest in identifying alternative dietary weight loss strategies.
One such approach is prolonging the fasting interval between meals (i.e., intermittent energy restriction, IER). The premise of this approach is that individuals do not fully compensate during fed periods for the energy deficit produced during extended periods of fasting between eating bouts.
Furthermore, these regimens may be easier to adhere to and maintain over time than CER. Finally, IER may lead to metabolic adaptations that favor a greater loss of fat mass, the preservation of lean mass, and a greater ability to sustain weight loss .
Overall, the available evidence suggests that IER paradigms produce equivalent weight loss when compared to CER.
IF: intermittent fasting
CR: calorie restriction
ICR: intermittent calorie restriction
ER: energy restriction
TRF: time-restricted feeding
PF: prolonged fasting
ADER: alternate day energy restriction
CER: continuous energy restriction
RCT: randomized controlled trial
GIR: glucose infusion rate
Si: insulin sensitivity
ROS: reactive oxygen species
ad: libad libitum