Intermittent fasting (IF) is an umbrella term for various diets that cycle between a period of fasting and non-fasting during a defined period. Intermittent fasting can also be used with calorie restriction for weight loss.
Variations
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Intermittent fasting protocols can be grouped into 2 categories: whole-day fasting and time-restricted feeding (TRF).
- Whole-day fasting involves regular one-day fasts. The strictest form would be Alternate day fasting (ADF). This involves a 24-hour fast followed by a 24-hour non-fasting period. The 5:2 diet allows the consumption of 500รข€"600 calories on fasting days.
- Time-restricted feeding (TRF) involves eating only during a certain number of hours each day. A common form of TRF involves fasting for 16 hours each day and only eating during the remaining 8 hours, typically on the same schedule each day. A more liberal practice would be twelve hours of fasting and a twelve-hour eating window, or a stricter form would be to eat one meal per day, which would involve around 23 hours of fasting per day.
Recommendations vary on what can be consumed during the fasting periods. Some would say only water, others would allow tea or coffee (without milk or sugar) or zero-calories drinks with artificial sweeteners. Yet others would allow "modified fasting" with limited caloric intake (e.g., 20% of normal) during fasted periods rather than none at all.
The 5:2 diet became popular in the UK in 2012 after the BBC2 television Horizon documentary Eat, Fast and Live Longer. Via sales of best-selling books, it became widely practiced.
According to NHS Choices as of 2012, people considering the 5:2 diet should first consult a physician, as fasting can sometimes be unsafe. In the UK, the tabloid press reported on research claiming the 5:2 diet could reduce the risk of breast cancer, improve brain and immune functions, or extend lifespan, but there is inadequate evidence for such statements. A news item in the Canadian Medical Association Journal expressed concern that promotional material for the diet showed people eating high-calorie food such as hamburgers and chips, and that this could encourage binge eating since the implication was that "if you fast two days a week, you can devour as much junk as your gullet can swallow during the remaining five days".
Research
A 2014 review described that intermittent fasting has not been studied in children, the elderly, or the underweight, and could be harmful in these populations. It also suggested that people choosing to fast for periods of time greater than 24 hours should be monitored by a physician, as changes to the gastrointestinal system or circadian rhythm can occur. The review concluded that fasting is unlikely to have much effect on conditions other than obesity, such as aging or other chronic condition, unless combined with moderate calorie restriction and a plant-based diet such as the Mediterranean diet.
According to another 2014 review, intermittent fasting can lead to weight loss, though long-term calorie restriction can lead to slightly more weight loss compared to intermittent fasting. Intermittent fasting has been found in healthy and obese adults to reduce basal insulin, triglycerides, and blood glucose in fasting periods shorter than 24 hours.
Reductions in weight, improvements in cardiovascular and metabolic variables, such as fat mass, LDL cholesterol, triglycerides, and C-reactive protein in non-obese individuals have been recorded. Intermittent fasting has been linked directly to facilitating how the body metabolizes potential fuel sources. About 12 hours after one's last meal, the body begins to metabolize fatty acids and ketones as the primary food source. This change in energy source results in a decrease in the levels of insulin and leptin, reduced levels of inflammatory proteins and other molecules associated with oxidative stress.
See also
- Calorie restriction
- Henry S. Tanner (doctor)
References
