By: The Dr. Perlmutter Team
Time-restricted eating is likely a familiar concept to those of you who participated in our Summer Fasting Challenge. Time-restricted eating, often termed time-restricted feeding (TRF) in scientific literature, is a form of intermittent fasting that restricts consumption of foods and beverages—“energy intake”—to a specific window of time. For example, as we did together in the Summer Fasting Challenge, 18:6 TRF signifies a 6-hour eating window and 18 hours of fasting.
As we discussed during the Summer Fasting Challenge, and as I have written about in recent blogs, it’s clear that there are health benefits to time-restricting our food consumption. In the realm of scientific literature that supports this notion, a study published in the journal Nutrition Reviews is no exception. This study offered a summary of evidence on the effects of time-restricted feeding on both body weight and markers of metabolic disease risk. The authors of this review looked at 11 human studies and 12 animal studies. Both categories of studies included various TRF eating window durations, ranging from 3-4 hours to 12 hours.
Results of animal studies show that TRF was associated with the following outcomes:
- Lower body weight
- Lower total cholesterol and triglycerides
- Lower glucose and insulin
- Lower inflammatory markers, specifically interleukin- 6 (IL-6) and tumor necrosis factor-α (TNF-α)
- Improvements in insulin sensitivity
Data from human studies support the findings of animal studies and show that TRF was associated with the following outcomes:
- Lower body weight (in some studies)
- Lower triglycerides, LDL cholesterol, and glucose
- Higher HDL cholesterol
While there is similarity between human and animal studies on impacts of TRF on metabolic disease risk factors, effects on body weight differ. While the reason behind this difference is unknown, it is important to note that the majority of studies reviewed did not report energy intake or energy expenditure. Because of this, we cannot determine from this review how TRF affects energy intake and expenditure and how each of these variables results in changes in body weight.
The authors also discussed how TRF may compare with intermittent fasting (IF), which involves a partial or complete restriction of energy intake on 1-3 days per week. After reporting a collection of statistics on outcomes from IF and TRF studies, they conclude:
Thus, compared with IF, TRF may yield greater improvements in these metabolic disease risk parameters in a shorter period of time. … A major difference between TRF and IF protocols is the frequency of fasting. TRF regimens require individuals to fast for a certain duration of time every day. In contrast, IF regimens generally only require subjects to fast for 1-3 days per week. Thus, it is possible that the greater frequency of fasting with TRF versus IF may contribute to the superior improvements in metabolic disease risk.
According to their statement, the regularity of a daily fasting period may, in fact, confer greater benefits that are protective in terms of metabolic disease. As I explored in an earlier blog, it is possible that this relates to circadian rhythm and fasting physiology. In addition, some studies have shown that time-restricted eating may be more sustainable in the longer term. In other words, it may be easier for people to integrate a time-restricted eating practice into their lives than to go without food for a few days out of the week (as in intermittent fasting).
Finally, as the researchers of this study highlighted, the animal studies evaluated in this review offer stronger evidence than the human studies because of the types of trials conducted. Therefore, larger and longer-term trials to study the specific effects of TRF in humans are needed. Still, evidence from this review suggests that time-restricted eating may improve a number of really important risk factors for metabolic disease, namely blood sugar and insulin levels, lipids such as cholesterol markers, and some inflammatory factors. Science certainly seems to be pointing to the fact that time-restricted eating is worth trying.