Fasting or intermitting fasting (IF) is currently a popular dietary approach for those looking to lose weight. It can be defined as specific time periods of eating alternating with periods of energy restriction (fasting). There are several protocols of IF that determines when food can or cannot be consumed. Some examples are:
Alternate Day Fasting. Within this type of IF, there are multiple subcategories:
- 24-hour eating window followed by a 24-hour period of fasting
- 12-hour eating window followed by a 36-hour period of fasting
- Modified alternate day fast: Limiting intake to < 500 calories for 24 hours alternating with 24 hours of normal eating
Whole Day Fasting. This is sometimes called the 5:2 Diet and is defined as fasting for 2 days out of the week, while resuming normal caloric intake on the remaining 5 days.
Time-Restricted Feeding. This is typically defined as 16–20 hour fast followed by a feeding window between 4–8 hours.
Has IF shown to reduce body weight? Yes. The largest systematic review on IF published in 2015 found that IF was effective for weight loss, but to varying degrees (3–9% decrease in body weight). Important details about this review include the population (normal, overweight, and obese subjects), duration (2 to 104 weeks) and size (4 to 334 subjects). Notably, this review also compared IF to traditional continuous energy restriction (CER) for weight loss i.e. removing 500 calories/day. Results showed there was no significant difference in the amount of weight loss between these two dietary approaches. The trials within the review that measured body composition, BMI, waist circumference and hip circumference, also showed equivalent reductions comparing IF to CER.
When examining IF for weight loss, it could be a viable option for individuals who have seen little success with previous attempts to lose weight using other methods. IF provides relatively easy guidelines for an individual to follow that can result in an overall calorie reduction. IF is likely easier for individuals whose lifestyle is not conducive with eating more frequently throughout the day; however, IF’s strict protocol could potentially backfire due to becoming over obsessed about feeding/fasting times that provide no wiggle room. Side effects reported with IF include weakness, headaches, lack of energy and preoccupation with food.
IF would not be an appropriate dietary approach for individuals with a history of disordered eating. Pregnant females, adolescents, individuals with diabetes, and individuals on a medication regimen that requires food should also refrain from IF.