From ancient Greece to some of the world's main religions, fasting has long been promoted as a path to spiritual enlightenment. In the early 1900s, fasting took on an important role in medicine; it was used by physicians to treat everything from diabetes to epilepsy.
The practice has experienced a recent resurgence in the weight-loss world, thanks to the popularity of diets such as Lean Gains, the 5:2 Diet and Eat Stop Eat. These diets each recommend different types of fasting that can range from not eating anything for a full 24 hours to dramatically limiting calories a couple of days a week.
The promise? Proponents of intermittent fasting say it will help you shed fat, build muscle, lower your risk of cancer and heart disease and even make you live longer. But do these diets really work better than typical weight-loss plans?
We can all agree: Dieting isn't fun. Sticking to a plan day after day makes dieters feel like there's no end in sight, and it's only a matter of time until they cave and go off the rails.
With intermittent fasting, it's almost as though you get to be on a part-time diet. Sure, there will be days of restriction, but they're always interspersed among higher-calorie days where you can eat what you crave (within reason). This seems to be the appeal for many dieters.
Here's what the science reveals so far for the three major types of intermittent fasting diets.
Don't eat for 24 hours and then eat "normally" the next 24 hours (example: Eat Stop Eat).
There isn't any evidence to show that alternate-day fasting has a benefit over other types of diets. Only three small studies have been done on this type of diet, and none of them had a comparison or control group for benchmarking.
Although one study demonstrated an average weight loss of 2.5 percent over 22 days, the participants reported being very hungry on fasting days, and this didn't get better over time.
Eat a quarter of the calories you need in a day (about 500) on two nonconsecutive days of the week and eat a normal diet the rest of the week (example: 5:2 Diet).
There are three studies on modified fasting in overweight and obese individuals that have a comparison group (either 1,200-1,500 calories per day or three-quarters of calorie needs). Only one of the studies found that the fasting group lost 4.1 percent more weight than the comparison group.
Blood sugar levels weren't significantly different between the fasting and comparison groups in any of the studies, but insulin levels were lower in the fasting group in two of the studies. (High insulin levels encourage your body to store more fat, especially around your waistline.)
This diet pattern extends your nighttime fast from 12 to 20 hours so you have less time to eat and eat fewer meals during the day (example: Lean Gains).
There have been two small studies on time-restricted feeding in people. In one study, 29 men with normal body mass index followed a nighttime fast of 11 hours or more for two weeks and also spent two weeks following a regular eating schedule. During the fasting period, the men lost 1.3 percent more than in the control period.
Another study looked at the impact of having only one meal a day in the afternoon for eight weeks in 15 adults with a healthy body mass index and found they lost 2.1 kg of fat compared to losing no weight or fat when they ate the same amount of calories divided over three meals a day. Not surprisingly, when the participants were eating one meal a day, they reported feeling much hungrier in the morning.
Overall, intermittent fasting diets don't seem to be any better than daily calorie reduction for promoting weight loss or health. A review of all of the research studies that have compared intermittent fasting to daily calorie reduction found that in the intermittent fasting groups, people lost 3 to 8 percent of their weight over three to 24 weeks while daily calorie reduction led to weight loss of 4 to 14 percent over six to 24 weeks.
Fat loss, insulin and blood sugar levels also weren't any better in the intermittent fasting groups compared to daily calorie reduction.
No harmful effects of intermittent fasting have been reported in studies in healthy adults. Still, these studies are of small groups of people and are short-term. The research has also ignored the impact of intermittent fasting on exercise habits, sleep quality and diet quality. I can guarantee that meeting all of your nutrient needs in one meal a day is impossible, making the longevity of such a diet questionable and even risky.
If you have a medical condition, are taking medications, have diabetes or problems with blood sugar control, are underweight or have a history of disordered eating, speak to your physician and dietitian before trying a fasting diet. If you're pregnant or breast-feeding, intermittent fasting isn't a good diet for you.
As promised, intermittent fasting appears to promote weight loss and improve metabolic parameters, but it doesn't seem to be as effective as the tried and true method of moderate calorie reduction on a day-to-day basis.
Limiting the amount of time you spend eating via intermittent fasting may help you eat fewer calories and result in weight loss over time. It all depends on what type of routine works best for you. Some people find they get hungry frequently or struggle with drops in blood sugar levels during the day which makes smaller, frequent meals a better option for them.
The common concern with fasting is that people may be prone to overeating on non-fasting days. Surprisingly, the research doesn't suggest that this happens. Whether this type of diet works for you probably depends on your personality, lifestyle and eating preferences.