Intermittent fasting involves random or intermittent periods of going without food.
Clinical studies in both mice and humans have shown that intermittent fasting may have some benefits such as helping to manage body weight, controlling blood lipids, and regulating blood glucose levels.
What is Intermittent Fasting?
For medical purposes, intermittent fasting is defined as going without food for 8-12 hours. Unlike a long-term fast covering a specific time period of multiple days, intermittent fasting is characterized by the absence of food for a shorter time. Common types of intermittent fasting include:
- Full-day fasting. This type of fast recommends fasting from one dinner to the next, or approximately 24 hours. For example, the person eats an evening meal at 7 PM and then fasts the following day until another evening meal at 7:30 PM.
- Partial-day fasting. People eat during a specific “eating window” of time, in which eaters consume their daily allotment of calories over a 6-hour period and fast for the other 18 hours. People who have higher activity levels or are training for specific athletic events often plan their training around a longer window of time for eating (from 8-10 hours).
- Alternate-day fasting. Eaters eat normally one day and then fast the next day. There are usually no restrictions on calorie intake on the eating day.
The Effects of Fasting on the Body
The body requires energy to absorb or digest food. The process of metabolizing nutrients can create potentially harmful free-radicals and may affect the aging process.
A lowered food intake may slow the aging process. Some of the metabolic or physiological outcomes of a brief or intermittent fast include:
- Decreased metabolic rate
- Increased liver glycogen breakdown
- Ketone production from fat breakdown
- Decreased growth hormone
- Decreased fasting insulin levels
- Decreased protein synthesis
- Reduced glucose muscle intake
Many studies have been done concerning fasting for religious or spiritual purposes, including the Islamic ritual of Ramadan, a 4-week period of fluid and food abstinence from sunrise to sunset.
A study that examined Mormons in the American Journal of Cardiology (Horne et al 2008) suggested that groups that fast for religious purposes might also lower the risk of diabetes and coronary artery disease.
Chronic Calorie Deprivation vs. Intermittent Fasting
Intermittent fasting should not be confused with calorie restriction, which is a diet in which total calorie intake is restricted to approximately 60-80% of normal caloric intake.
During the 1940s, researcher Ancel Keys collected data on healthy men at the University of Minnesota for a period of 168 days with the men consuming 40% of their usual intake. This resulted in the development of disordered eating patterns such as an obsession with food and bingeing.
For most people, intermittent fasting might be an easier regimen to follow, as it allows for eating a surplus of calories on non-fasting days. It is easier to restrict calories on alternating days or for specific time periods rather than all the time.
Studies in mice that are restricted to an intermittent fasting or alternate–day fasting schedule of dietary restriction have shown an increase in disease resistance and longer lifespan (1).
The body weight and food intake of the mice was maintained while intermittent fasting, and resulted in metabolic benefits such as reduced insulin and blood glucose levels. It is unclear whether similar increase in lifespan would be obtained in humans without additional calorie restriction on the non-fasting days.
The concept of calorie restriction to increase lifespan and improve health is not a new one. Humans have fasted for religious purposes or during extended periods of food scarcity.
Intermittent fasting might be an easier way than extreme calorie restriction to regulate blood glucose, manage body weight, and extend life.
1. Anson RM, Guo Z, de Cabo R, et al. Intermittent fasting dissociates beneficial effects of dietary restriction on glucose metabolism and neuronal resistance to injury from calorie intake. Proc Natl Acad Sci U S A 2003;100:6216–20.