The Major IF Protocols: Evidence and Practical Guide
16:8 (Leangains/Time-Restricted Eating): The most popular IF protocol involves 16 consecutive hours of fasting daily with an 8-hour eating window. Most people achieve this by skipping breakfast and eating between noon and 8pm (or 10am-6pm). The 8-hour window provides sufficient time to meet complete nutritional needs. Evidence: multiple RCTs show significant weight loss (3-8% body weight over 8-24 weeks), improvements in insulin sensitivity, reductions in blood pressure and inflammatory markers, and equivalent muscle preservation to 3-meal diets when protein is adequate. Particularly effective for metabolic syndrome and visceral fat reduction. The eating window should ideally align with circadian biology — earlier windows (8am-4pm or 10am-6pm) show superior metabolic benefits to late windows (noon-8pm or 2pm-10pm) in several studies.
5:2 Protocol (Michael Mosley): Eat normally 5 days per week and restrict calories to 500-600 on 2 non-consecutive fasting days. The 5:2 diet has been validated in multiple RCTs, most comprehensively by Michelle Harvie at the Genesis Breast Cancer Prevention Centre. Research found 5:2 produced equivalent weight loss to continuous calorie restriction at 3 months, with some studies showing superior fat loss (particularly visceral fat) and better preservation of lean mass. Adherence may be higher than continuous restriction for some people because of the psychological freedom on non-fasting days. Potential benefit for cancer prevention: the two fasting days produce the autophagy and metabolic benefits while the normal eating days maintain muscle mass and micronutrient status.
Alternate Day Fasting (ADF): Alternating between complete or near-complete fasting days (0-500 calories) and ad libitum eating days. ADF produces the largest weight loss in shortest time of any IF protocol in clinical trials — up to 11% body weight in 24 weeks. It also produces significant improvements in LDL cholesterol, triglycerides, blood pressure, and insulin sensitivity. The primary limitation is adherence: many people find every-other-day fasting too restrictive long-term. A modified version with 25% of calorie needs on fasting days (rather than complete fasting) shows similar metabolic outcomes with meaningfully better adherence.
Prolonged fasting (24-72+ hours): While beyond the scope of most people’s regular practice, periodic prolonged fasting cycles (one 24-72 hour fast monthly or quarterly) may confer benefits beyond what daily IF achieves, primarily through more sustained autophagy and more complete glycogen depletion. Valter Longo at USC has developed the “Fasting Mimicking Diet” (ProLon protocol) — 5 days of very-low-calorie, plant-based eating (700-1100 kcal) that produces the metabolic state of fasting while maintaining energy and compliance. Clinical trials show FMD produces significant reductions in IGF-1, glucose, blood pressure, BMI, and inflammatory markers with 3-4 consecutive monthly cycles.