by Eric Trexler, Ph.D.

Study Reviewed: The Effect of Intermittent Fasting on Appetite: A Systematic Review and Meta-Analysis. Elsworth et al. (2023)

A few common strategies fall under the umbrella of “intermittent fasting.” Alternate-day fasting involves alternating between days of total food abstinence (complete fasting) and unrestricted eating. In theory, the dieter creates a net weekly caloric deficit because the energy deficit on fasting days is ideally larger than the energy surplus on feeding days. The 5:2 diet refers to a modified strategy in which dieters limit themselves to very low calorie targets (600kcal/day for men and 500kcal/day for women) twice per week, usually on non-consecutive days. So, they have five days of “normal,” unrestricted eating each week, and two days of very low intake. In contrast, time-restricted eating (also known as time-restricted feeding) involves following the same dietary routine every day of the week, but focuses on when energy is consumed. In time-restricted feeding, dieters restrict all of their caloric intake to a predefined window of time, usually lasting only 4-8 hours per day. But here’s a good question – why bother with any of these?

Intermittent fasting was initially introduced to the nutrition literature as a weight management strategy. In theory, intermittent fasting is intended to offer an alternative to “traditional” calorie restricted diets by offering two advantages: simplicity and tolerability. Rather than following a complicated list of guidelines about food selection, calorie intake, or macro targets, the “rules” for an intermittent fasting strategy simply refer to when a person should (or shouldn’t) consume calories. All you need is a single sentence of instructions, a watch or calendar, and you’re good to go. When it comes to tolerability, there are several factors to consider. One challenge that dieters face is a near-constant focus on food; intermittent fasting helps a lot of people switch their focus away from food during fasting periods. Another challenge is restrictive meals that are unsatisfactory; intermittent fasting often counters this challenge by allowing for relatively larger meals during feeding periods. Obviously hunger presents yet another challenge in traditional calorie-restricted diets. There is some evidence and plenty of anecdotes to suggest that intermittent fasting strategies may impact appetite in a manner that supports weight loss (2). If true, this could explain why intermittent fasting protocols often induce weight loss in the absence of complicated meal plans or daily calorie targets during feeding periods (3).

The presently reviewed meta-analysis (1) sought to quantify the impact of intermittent fasting protocols on various outcomes related to appetite regulation. The authors conducted a systematic search of the scientific literature to round up all of the randomized controlled trials assessing the impact of intermittent fasting protocols (including alternate day fasting, time-restricted eating, and the 5:2 diet) on appetite outcomes compared to continuous energy-restricted diets. They did not limit their search to any particular age range or BMI range. In terms of outcomes of interest, the researchers describe their approach as follows: “To be included in the review, the [randomized controlled trial] must have measured the primary outcome of appetite, e.g., visual-analogue scales of hunger, fullness, desire to eat, and prospective food consumption (PFC). Where measured, secondary outcomes were also included in the review: body weight (kg), energy intake (kcal/day), eating behavior questionnaire scores (e.g., Three-Factor Eating Questionnaire), physical activity, adherence to interventions (%), and dropout.”

After conducting the search and applying exclusion criteria, the researchers were left with 31 published reports from a total of 17 studies (it’s not uncommon for multiple papers and abstracts to come from the same study). There were six alternate-day fasting studies, six 5:2 diet studies, four time-restricted eating studies, and one study that included alternate-day fasting and time-restricted eating. Study durations ranged from 2-12 weeks, with 12 weeks being the most common study duration. In the meta-analyses for hunger, fullness, desire to eat, and prospective food consumption, the researchers wisely kept the data in their original units (millimeters on a visual scale ranging from 0-100mm) instead of transforming them into a standardized effect size unit like Cohen’s d or Hedges’ g. None of the pooled effect estimates were significantly different from zero, indicating no significant differences between groups (intermittent fasting versus continuous energy restriction). Pooled effect estimates and their 95% confidence intervals (in brackets) were as follows:

• Lower hunger in intermittent fasting groups (-3.03 [-8.13, 2.08])
• Higher fullness in intermittent fasting groups (3.11 [-1.46, 7.67])
• Lower desire to eat in intermittent fasting groups (-3.89 [-12.62, 4.83])
• Lower prospective food consumption in intermittent fasting groups (-2.82 [-13.52, 7.88])

The researchers reported that they did subgroup analyses to independently explore the effects of each type of intermittent fasting (alternate day fasting, time-restricted eating, and the 5:2 diet), which did not reveal any statistically significant findings. For secondary outcomes, the researchers also looked at differences in weight loss and energy intake. Intermittent fasting led to larger reductions in body weight (-0.48 [-1.03, 0.07] kg) and lower energy intake (-78.32 [-178.51, 21.88] kcal/day). Neither effect was statistically significant, but both were reasonably close to the <0.05 threshold for significance (p = 0.09 and p = 0.13, respectively).

Before concluding that this meta-analysis “debunks” the idea that intermittent fasting facilitates better appetite management, there are a couple considerations to keep in mind. First…

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References

1. Elsworth RL, Monge A, Perry R, Hinton EC, Flynn AN, Whitmarsh A, et al. The Effect of Intermittent Fasting on Appetite: A Systematic Review and Meta-Analysis. Nutrients. 2023 Jun 1;15(11):2604.
2. Varady KA, Cienfuegos S, Ezpeleta M, Gabel K. Cardiometabolic Benefits of Intermittent Fasting. Annu Rev Nutr. 2021 Oct 11;41:333–61.
3. Welton S, Minty R, O’Driscoll T, Willms H, Poirier D, Madden S, et al. Intermittent Fasting And Weight Loss: Systematic Review. Can Fam Physician. 2020 Feb;66(2):117–25.
4. Moro T, Tinsley G, Pacelli FQ, Marcolin G, Bianco A, Paoli A. Twelve Months of Time-restricted Eating and Resistance Training Improves Inflammatory Markers and Cardiometabolic Risk Factors. Med Sci Sports Exerc. 2021 Dec 1;53(12):2577–85.