Fasting describes any dietary approach that involves cycling between periods of eating and voluntary non-eating. To understand the large volume of scientific data on this topic, researchers use a method called a meta-analysis. This type of study statistically combines the results from many separate but similar clinical trials to provide a more powerful and comprehensive conclusion than any single study can offer. By pooling data, these analyses help clarify conflicting findings from individual studies, offering a clearer picture of what the total body of evidence shows.
The Scope of Fasting Meta-Analyses
The research synthesized in meta-analyses examines a few common fasting protocols. One of the most studied is time-restricted eating (TRE), where all meals are consumed within a specific window each day, often ranging from 4 to 12 hours. A popular version of this is the 16:8 method, involving an 8-hour eating window and a 16-hour fast.
Another frequently analyzed protocol is alternate-day fasting (ADF). This approach involves alternating between days of normal eating and days where calorie intake is substantially reduced. A variation of this is the 5:2 diet, where an individual eats normally for five days of the week and severely restricts calories on two non-consecutive days.
These large-scale reviews primarily draw data from studies conducted on adults with overweight or obesity. Many analyses also include data from trials involving individuals with existing metabolic conditions or those at risk for type 2 diabetes. A smaller subset of the research includes healthy adults to understand the effects of fasting outside the context of disease.
Primary Outcomes for Weight Management
When data from numerous studies are combined, meta-analyses show that various intermittent fasting strategies reliably lead to weight loss. The amount of weight lost can range from trivial to small reductions, with outcomes observed across different fasting types, including time-restricted eating (TRE) and alternate-day fasting (ADF).
Researchers often compare fasting to traditional continuous energy restriction (CER), where calorie intake is reduced moderately every day. The collective evidence indicates that fasting is generally not superior to CER for weight loss over the long term. While fasting may lead to slightly greater short-term weight loss, these differences are not considered clinically significant.
These large-scale reviews also examine changes in body composition, specifically the loss of fat mass versus lean muscle mass. The findings show that intermittent fasting is effective at reducing fat mass. Most combined analyses report that fasting preserves lean muscle mass as effectively as continuous calorie reduction.
Metabolic and Cardiovascular Health Findings
The aggregated findings from meta-analyses reveal notable effects on markers of metabolic health. One of the most consistent outcomes is an improvement in glucose regulation. Evidence points to significant decreases in fasting insulin levels and improved insulin sensitivity, suggesting that fasting can help the body manage blood sugar more effectively.
Further examination shows impacts on lipid profiles, which are indicators of cardiovascular health. Some analyses report that methods like the 5:2 diet are associated with reductions in low-density lipoprotein (LDL) cholesterol. Other reviews have found that alternate-day fasting can reduce triglycerides, though results for lipids can be inconsistent across all fasting types and may not differ from continuous calorie restriction.
The effects of fasting on blood pressure and inflammation are also a focus of these reviews. Moderate-quality evidence suggests some forms of fasting can improve blood pressure. In direct comparisons, however, some analyses found that continuous energy restriction resulted in a greater reduction in systolic blood pressure. Findings for inflammation markers, such as C-reactive protein (CRP), have also been assessed but are not always consistent.
Limitations and Nuances in the Research
The evidence has limitations, with a primary issue being the short duration of most studies. Since clinical trials on fasting often last only a few months, the long-term health effects and sustainability of these diets are not yet understood.
Another consideration is the high variability among studies. Trials within a meta-analysis often differ in design, fasting protocol, and participant characteristics. This inconsistency makes it difficult to draw firm conclusions that apply to everyone.
Finally, more studies on diverse populations are needed, as much of the existing research has focused on adults with overweight or obesity. Understanding long-term adherence and determining which protocols are most effective for different individuals remain important areas for future investigation.