The simplicity of intermittent fasting has made it popular, but only a handful of human trials have evaluated the purported health benefits.
This narrative review evaluated the evidence for health benefits from intermittent fasting in human trials.
Intermittent fasting is often split into three categories:
Alternate day fasting (ADF), in which either only water, or one meal of about 25% of daily energy needs (modified ADF), is consumed every other day.
The 5:2 diet, in which five days of each week are feasting days and two are fasting days
Time-restricted eating (TRE), in which eating is restricted to a 4–10 hour window (e.g., noon to 8 p.m.)
All categories result in reduced body weight (1–8% from baseline, comparable to continuous energy restriction), but TRE does not produce what is considered to be “clinically significant” weight loss (more than 5% from baseline).
All categories also result in modestly reduced blood pressure, fasting insulin, and insulin resistance. ADF and the 5:2 diet more consistently reduce low-density lipoprotein cholesterol and triglyceride levels than TRE, but still demonstrate rather variable results that may depend on the extent of weight loss. TRE and ADF studies suggest reductions in oxidative stress, but not inflammatory markers.
Some researchers suggest fasting can improve appetite regulation, sleep quality, and gut microbial composition, but because some categories of intermittent fasting have either no data or inconsistent results for these outcomes, more research is needed.
Intermittent fasting safety concerns include gastrointestinal issues, energy level disturbances, eating disorder symptoms, and hormone and diet quality disruptions.
The authors also note that there may be an adjustment period (i.e., dizziness or constipation from fasting) and suggest dieters should prioritize nutrient dense foods and consume any “fast day meals” after an exercise session.
The big picture
One of the bigger questions of intermittent fasting research revolves around comparing it to standard caloric restriction. Most human studies suggest these two approaches are very similar, and that most benefits are associated with decreases in weight and fat mass. This means that otherwise healthy people may not benefit from fasting, compared to people with overweight or obesity. However, some researchers suggest that intermittent fasting improves health and longevity through small changes in metabolism, stress resistance, cognition, and reductions in reactive oxygen species and inflammation over a long time span. A meta-analysis from 2020 suggests that most kinds of intermittent fasting can improve specific exercise performance outcomes, such as aerobic capacity. Researchers still have plenty of work to do to understand underlying mechanisms and establish causality between intermittent fasting and health outcomes, separate from decreases in weight and fat mass.
A secondary analysis from 2021 of a 2019 8-week randomized controlled trial reported that women who consumed 70% of their energy requirements through intermittent fasting lost more weight and fat, and reduced nonesterified fatty acids (an indicator of excessive fat metabolism and circulating ketone bodies), compared to the same level of caloric restriction or intermittent fasting with no energy deficit. The study data also showed that fasting reduced expression of antioxidant enzymes, suggesting a reduction in reactive oxygen species production, when compared to caloric restriction, but the control group was not included in these analyses. Other measures suggest there are potential changes in lipid turnover (e.g., perilipin protein), but it’s too early in terms of research to contextualize these kinds of measures.
Despite the claims of potential cognitive benefit from intermittent fasting, a 2021 review of the sparse amount of available studies suggests that intermittent fasting does not lead to any short-term cognitive benefits in healthy people, but may protect against the development of neurological disorders.
Some aspects of intermittent fasting that should be considered and may influence health outcomes include adherence and management of hunger, changes in stress response, and quality of food consumed during feast times.
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