Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating.It doesn’t specify which foods you should eat but rather when you should eat them.In this respect, it’s not a diet in the conventional sense but more accurately described as an eating pattern.
Common intermittent fasting methods involve daily 16-hour fasts or fasting for 24 hours, twice per week. Fasting has been a practice throughout human evolution. Ancient hunter-gatherers didn’t have supermarkets, refrigerators or food available year-round. Sometimes they couldn’t find anything to eat.
As a result, humans evolved to be able to function without food for extended periods of time. In fact, fasting from time to time is more natural than always eating 3–4 (or more) meals per day.
FAQ:
IF is a way of eating that calls for alternating between fasting (or significant reduction of calorie intake) and eating at specific times, according to Johns Hopkins Medicine. It’s different from other diets in that it’s not about eating specific foods. IF is not about depriving yourself either. Rather, it’s about eating your meals during a certain time frame and fasting for the rest of the day and night.
Fasting has been around since ancient times and has predominantly been practiced within religions, according to a study published in the Journal of the Academy of Nutrition and Dietetics in August 2015. But the version of IF that’s talked about today arose in the past eight years or so. According to Harvard Health, IF became more popular around 2012 when the documentary Eat, Fast and Live Longer aired. The Journal of the Academy of Nutrition and Dietetics study says many books on the topic were published around that time as well, including 2013’s The Fast Diet, which added to the buzz. Research followed. “Over the past five years, rigorous research has shown the remarkable benefits of intermittent fasting, which is behind this sudden interest,” says Sara Gottfried, MD, the Berkeley, California–based author of Brain Body Diet.
There are a few different versions of IF (outlined below), but each version follows the basic premise of designating a certain period of time during the week meant for eating and certain periods when food and drink should be restricted (or severely limited), according to Harvard Health.
The most popular are:
- 16:8 This method calls for 16 hours of fasting and 8 hours of eating during the day. Followers of this method most commonly will skip breakfast and eat between the hours of 11 a.m. and 7 p.m. or noon and 8 p.m. The rest of the night and morning is spent fasting. Dr. Lowden says this approach is commonly referred to as “time-restricted eating.”
- Alternate-day fasting This involves limiting your calories to 25 percent of your usual calorie intake followed by a normal eating day, according to a study published in October 2014 in Translational Research. For instance, you might severely restrict calories on Monday, Wednesday, and Friday, and then eat normally on the other days.
- 5:2 fasting A very low number of calories (around 400 to 500 calories) is allowed on the two nonconsecutive “fasting” days of the week. The other five days have no eating restrictions, per Harvard T.H. Chan School of Public Health.
The short answer: probably. IF gets a lot of press as a weight loss tool, and I recommend it in my practice for weight loss and weight management. It’s linked to weight loss because not eating between meals forces the body to turn to the fat stored in cells for energy, according to Harvard Health. Insulin levels decline through this process as the body burns fat.
Versions of IF that restrict eating after a certain time, say 7 p.m., also help eliminate nighttime eating, which has been shown to contribute to metabolic syndrome and obesity, according to a study published in December 2018 in BMC Public Health.
Here are some of the proposed benefits of IF:
- Boost weight loss A review published in Current Obesity Reports in June 2018 found most research on IF has supported its link to weight loss, and the data suggest that it can result in between a 5 and 9.9 percent loss of body weight. The 2018 study published in Nutrition and Healthy Aging suggests alternate-day fasting may produce greater weight loss than time-restricted eating, but that alternate-day fasting may be harder to stick with than time-restricted eating. Ultimately, more research is needed on whether IF can result in real, sustainable weight loss.
- Lengthen life A study published in Cell in December 2014 suggests calorie restriction may slow aging. The study involved animals, however, and the same findings haven’t been shown among humans yet.
- Reduce insulin resistance This condition is the hallmark of type 2 diabetes, and being overweight increases the likelihood of insulin resistance, according to a study published in February 2014 in Frontiers of Medicine. IF can help with insulin resistance by reducing the number of calories consumed overall, according to a study published in Nutrients in April 2019.
- Better heart health A study published in Nutrition Journal reported IF helped study participants lose weight, lose fat, and lower cholesterol levels, leading researchers to conclude eating this way may help people lower their risk of coronary artery disease.