Fasting is a rising trend right now, not only for its powerful weight loss results, but for a host of other health benefits that range from mental clarity to improved metabolic risk factors. Which fasting approach is right for you? Here’s a brief overview of six types (though you should always consult with your doctor before fasting).
Intermittent Fasting (IF)
How and when to fast varies by individual preference, health concerns, and medical clearance. However, intermittent fasting (IF) is gaining popularity for its ease and benefits. When we eat regularly, our bodies burn a simple sugar known as glucose for fuel. Fasting for around 9 to 16 hours (everyone’s different) depletes those glucose stores and begins to burn fatty acids known as ketones instead. This process is known as “ketosis” and it burns fat while preserving muscle mass. IF has been shown to improve cardiometabolic risk factors that lead to diabetes and metabolic disorders, improve cardiovascular health, increase longevity, and more.
There are many different variations on IF, including time-restricted fasting, where people eat only within certain windows of time that accord with their circadian rhythms. Many people prefer to stop eating after 7 or 8 p.m., fast through the night, and skip breakfast, sometimes known as the 16:8 plan. Women may not need to fast quite as long as men (approximately 14 hours for women to men’s 16 hours) to enter ketosis. Most plans recommend fasting a couple of times per week. But there are other variations, including the next one in this list, alternate day fasting (ADF).
Alternate Day Fasting (ADF)
Alternate day fasting(ADF) is a subset of intermittent fasting in which participants restrict all calories for an entire day, the “fast day” followed by a “feed day.” One recent study, comparing ADF to basic caloric restriction found that ADF is safe and comparable to caloric restriction (CR) in weight-loss — participants lost between 3% and 8% of their body fat — particularly stubborn belly fat. Additionally, studies point to ADF as a positive support for metabolic disorders and its risk factors. And the effects may not be temporary; for up to 24 weeks after undertaking ADF, the form of fasting was not linked to an increased risk of weight regain even without professional supervision. Add in endurance exercise and ADF may be almost doubly effective. Participants have reported that an ADF diet is easier to stick to than other forms of fasting because there’s no eating to tempt you on fast days.
If you’re not sure that fasting is right for you, there’s always good old-fashioned caloric restriction (CR), reducing the number of calories you take in. In 1935, two researchers discovered that the simple act of reducing calorie intake in lab mice (without causing malnutrition) prolonged their lives by nearly double. This effect was also shown in other organisms, including worms, fruit flies, other rodents and primates. Studies have gone on to explore this effect in humans with promising results as well. To see benefits, humans must reduce their calorie intake by 40% to 60%. One study suggested that CR is “the only intervention known to date that consistently decreases the biological rate of aging and increases both average and maximal lifespan” in animal models. Researchers sought to understand why there are 4–5 times more centenarians (those 100 years old and older) in Okinawa, Japan than any other industrialized country. It turned out that there was record of a low caloric intake reported in school children on the island more than 40 years prior to the study.
Currently, a popular form of CR is the 5:2 Diet, popularized by British author Michael Mosley. Five days of the week you eat as normal, and for two days you restrict calories to 500–600 for the day.
For the ambitious fasters who are medically cleared to do so, multi-day fasts may be the right route to go. Fasting expert Valter Longo, mentioned above, published a study in the journal Cell that studied the affects of multi-day fasting on mice and humans. His research found that a multi-day fast of three days with zero to 200 calories per day reduced white blood cell counts, and encouraged the immune system to produce new white blood cells.
Longo theorized that fasting forces the body to recycle unneeded immune cells, with the help of an enzyme called PKA and a hormone called IGF-1, both of which increase with fasting. When participants begin to eat again, stem cells replenish the cells that were recycled. Of course this sort of fasting can be more mentally challenging, but the results may be worth the struggle.
Fasting-Mimicking Diet (FMD)
For those who just can’t face total fasting without some kind of food, the fasting-mimicking diet might be right for you. Fast mimicking diets essentially “trick” the body into believing it’s fasting while you still eat food, albeit a reduced amount of calories. A typical FMD lasts about five days (though it can go for much longer) and aims for a balance of carbs, protein, and calories, leaning toward foods that are high in fat. Participants take in about 40% of normal calorie intake. A popular form of FMD is the “Keto diet.”
A growing body of evidence also indicates that the FMD brings numerous health benefits. A study published by the Longevity Institute at the University of Southern California’s Leonard Davis School of Gerontology, led by Professor Valter Longo in Science Translational Medicine, found that FMD is associated with reduced risks for diseases ranging from cancer, diabetes, heart disease, and others in human participants who adopted the FMD for only five days each month for three months.
Whatever fasting method you choose, studies suggest the health benefits outweigh the challenges of going hungry.