What is Fasting?
Fasting is the practice of abstaining from food for a period of time, while caloric restriction is a dietary regimen that simply reduces caloric intake.
Rather than cutting calories day in and day out, what if, instead, you just ate as much as you wanted every other day? Or for only a few hours a day? Or what if you fasted two days a week or five days a month? These are all examples of intermittent fasting regimens, which I cover in my book How Not to Diet, and they may even be the way we were built to eat.
How Does Fasting Work for Weight Loss?
Researchers designed a study to compare the sustainability of weight loss at three different speeds: six days of fasting versus three weeks of a very-low-calorie diet of 600 calories a day or six weeks of a low-calorie diet of 1,200 daily calories. One year later, at least some of the fast-induced weight losses were maintained by the great majority, about 90 percent. But, over time, that number dropped to fewer than 1 in 10. By then, almost everyone had regained the weight they had initially fasted away.
The small minority for whom fasting led to sustainable weight loss “all admit to a radical change in previous eating habits.” It appears fasting may only work long-term if it can act as a jumpstart to a more healthful diet.
For more information on fasting and weight loss, check out my video Is Fasting Beneficial for Weight Loss?
What Are the Benefits of Fasting?
Hunger may start to dissipate within 36 hours.
Clear and alert state of mind
A few days into a fast, many people report feeling clear, elated, and alert—even euphoric. This may be in part due to the significant rise in endorphins that accompanies fasting.
Reduction in body fat
Although fasting for a week or two can actually interfere with the loss of body fat, eventually, after the third week of fasting, fat loss starts to overtake the loss of lean body mass in obese individuals.
What Are the Fasting Methods?
Not eating at all or severely limiting intake during certain times of the day, week, or month.
A back-and-forth sequence of a fasting day (up to 100% calorie restriction) and a feeding day (ad libitum food consumption), resulting in roughly 36-hour fasting periods every other day.
Eating five days of the week and fasting for the other two.
Five days a month eating a specifically-designed low-calorie plant-based diet.
Time Restricted Feeding
Fasting for periods of at least 12 hours but less than 24 hours.
Our fasting for weight loss infographic compares the advantages and disadvantages of these different types of fasting based on the latest evidence.
How Long Does it Take to Lose Weight With Intermittent Fasting?
1 pound lost per week in the first 2 weeks when eating is stopped between 7pm and 6am.
7 pounds lost in obese people after 12 weeks of eating from 10am-6pm.
What Are the Side Effects of Intermittent Fasting?
If the feeding window is late, as opposed to early, one can suffer adverse chronobiological effects of breakfast skipping and/or night eating, which includes elevations in blood pressure and cholesterol levels despite the weight loss.
The bottom line is that fasting only works long-term if it can act as a jumpstart to a healthier diet. In addition, it’s important to be aware of the risks and whether you fall into a category of people who should never fast. Check out our fasting for weight loss infographic for more details on these cautions.
To learn even more about weight loss, visit our topic page, which covers a broad range of the latest evidence-based research.