Intermittent Fasting

What Is Intermittent Fasting?

Intermittent fasting (IF) is an umbrella term for eating patterns that cycle between defined periods of fasting and eating, rather than specifying which foods to consume. It is primarily a temporal framework rather than a traditional diet. The most studied protocols include time-restricted eating (TRE), alternate-day fasting (ADF), and the 5:2 method. Unlike continuous calorie restriction, intermittent fasting alters the distribution of energy intake across the day or week.

During a fasting period, the absence of incoming glucose and amino acids shifts the body toward a catabolic state: glycogen stores decline, insulin falls, glucagon rises, and the body increasingly mobilizes fatty acids for fuel. After 12–16 hours of fasting, measurable ketone production begins in most individuals.

Common Protocols

• 16:8: A 16-hour fasting window followed by an 8-hour eating window. This is the most widely practiced form. A common implementation is eating between 12:00 and 20:00 and fasting overnight and through the morning.
• 5:2 diet: Five days of unrestricted eating and two non-consecutive days of severe restriction (approximately 500–600 kcal). This protocol suits individuals who prefer limiting restriction to specific days.
• Alternate-day fasting (ADF): Alternating between fasting days (zero or very low calorie intake) and unrestricted feeding days. ADF produces stronger metabolic changes than daily TRE but is harder to sustain.
• OMAD (one meal a day): A single daily meal within a 1-hour window. This approach carries higher risk of nutrient insufficiency and muscle protein breakdown.

Evidence and Practical Considerations

Randomized controlled trials comparing intermittent fasting to continuous calorie restriction generally find similar weight loss outcomes when total calorie intake is matched. The primary advantage of IF for many adherents is simplicity: fewer decisions about meals and a natural reduction in eating opportunities can make calorie control less effortful.

Consuming adequate protein (0.4 g/kg per meal, at minimum) within a compressed eating window helps preserve lean mass. Exercise timing relative to the eating window should be individualized based on performance and recovery needs. IF is generally not appropriate for individuals with a history of eating disorders, those who are pregnant or breastfeeding, or those with insulin-dependent diabetes without medical supervision.