Intermittent fasting (IF) is an umbrella term for various diets that cycle between a period of fasting and non-fasting. Intermittent fasting can be used along with calorie restriction for weight loss.
One form of intermittent fasting, alternate day fasting (ADF), involves a 24-hour fast followed by a 24-hour non-fasting period. This is sometimes referred to as every other day fasting or every other day feeding. Alternate-day calorie restriction may prolong life span.
Another form involves eating for limited times every day. One might fast for 14, 16 to 23 hours and then eat healthily. Choosing 14 or 16 hours actually allows eating twice per day, such as eating at 12:00 noon and again about 7:00 PM of an evening finishing at 8:00 PM. The time until the next noonday meal can be mitigated by sleeping 8 or 9 hours during that 16 hours fast. One can choose fasting 23 hours and one meal per day.
In some contexts, fasting allows the consumption of a limited amount of low-calorie beverages such as coffee or tea.
Modified fasting involves limiting caloric intake (e.g., 20% of normal) on fasting days rather than none at all. A study suggests that this regimen may retain most of the benefits of intermittent fasting.
More generally, forms may choose to specify various ratios of fasting to non-fasting periods. The BBC2 Horizon documentary Eat, Fast and Live Longer covered people who committed to fasting two non-consecutive days per week. Known as the 5:2 diet, people consumed 400–500 calories (women) or 500–600 calories (men) during the days of fasting. During feed days, the diet was regular.
A 2014 review described that studies done in animal models have shown fasting improves indicators of health—blood pressure, insulin sensitivity, and inflammation—likely through adaptive cellular responses to better handle stress. These findings suggest intermittent fasting has the potential to improve health and reduce the risk of chronic diseases, however this has not been reproduced in long-term human studies. The review also concluded that intermittent fasting has not been studied in children, the elderly, or the underweight, and could be harmful in this population. They also suggest that those 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 also 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 plant-based diet such as the Mediterranean diet.
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