Does intermittent fasting work? Research doesn't have a definite answer for its long-term effects
Intermittent fasting can work as a weight-loss tool, and short-term research has found that it can improve overall health for those with obesity.
However, the long-term results are less clear, and intermittent fasting may be less sustainable than other calorie restriction diets.
It also may not be safe for everyone. Teenagers, women who are pregnant or breastfeeding, people who take medication that can cause hypoglycemia, and those with a history of eating disorders should not try intermittent fasting.
This article was medically reviewed by Melissa Rifkin, MS, RD, CDN, owner of Melissa Rifkin Nutrition LLC.
This article is part of Insider's guide on How To Lose Weight.
Intermittent fasting may be a popular weight-loss strategy, but it's not a particularly new phenomenon.
In addition to religious and spiritual reasons, many cultures throughout history have used fasting as a tool to improve health. For example, ancient Greek athletes would fast to prepare their bodies for the Olympic games.
But what is intermittent fasting, exactly? And can restricting food intake to certain hours of the day or specific days of the week really help you shed some pounds? Here's what you should know about intermittent fasting and weight loss.
What is intermittent fasting?
Intermittent fasting involves following a schedule that switches between periods of eating and periods of fasting. It's an eating strategy that doesn't put limits on what you eat, but rather when you eat.
As a weight-loss method, intermittent fasting is known to improve metabolic health and lower insulin levels, which helps the body burn more daily calories.
There are many approaches to intermittent fasting, all of which revolve alternating periods of eating and fasting. You may choose to eat during certain hours of the day or certain days of the week, as long as the schedule is consistent.
Some common types of intermittent fasting include:
16/8 fasting: Also known as time-restricted fasting, this popular strategy involves eating only during an eight-hour period, such as 10 a.m. to 6 p.m. and fasting for the rest of the day.
Alternate-day fasting: As the name implies, this style of fasting is when you fast or severely restrict your caloric intake every other day.
5:2 diet: With this method, you eat normally five days a week and choose two, nonconsecutive days where you eat only 500 to 600 calories.
What the research says
Research is limited on how well intermittent fasting works for weight loss — especially in all its different forms and methods. Some human studies indicate that it can help people lose weight in just a matter of weeks, while others found no weight-loss benefit at all:
A 2020 study of 116 adults found no significant difference in weight loss between a group that limited all calorie intake to between 12 p.m. and 8 p.m. each day and a control group that was allowed three meals per day, plus snacks. Notably, the 12-week study found fasting could lead to loss of muscle mass.
A 2017 study that compared an alternate-day fasting plan with a typical calorie restriction diet found no significant difference in weight loss between participants. The study involved 100 obese participants, and there was a significant dropout rate — only 69% of the subjects stayed until the end.
A 2013 study found that, from a sample of 115 overweight women, those who tried a 5:2 intermittent fasting diet for 3 months lost more fat and experienced a greater reduction of insulin resistance than women who followed a standard calorie-restricted diet for the same time period.
A small 2009 study published in The American Journal of Clinical Nutrition found that 16 obese people who ate a very restricted diet three days per week lost an average of 12 pounds over 8 weeks. The dieters also saw improvements in their cholesterol and blood pressure.
Another small 2005 study published in The American Journal of Clinical Nutrition showed that eight non-obese participants lost weight and burned body fat after fasting every other day for 3 weeks. However, they reported feeling incessant hunger the entire time, suggesting that this form of intermittent fasting may not be sustainable.
Research around intermittent fasting has its limitations, from participant size to length of studies. What's more, different studies used different methods of intermittent fasting, signaling that more restrictive fasting plans aren't necessarily better.
Another thing to keep in mind? Effectiveness is also largely dependent on whether you can actually stick to the plan.
For example, according to registered dietitian Natalie Allen, a clinical assistant professor at Missouri State University, most people choose some version of the 16/8 method, because it's easier to follow than some other versions.
The amount of weight you lose while fasting intermittently may also depend on which version of the diet you choose:
For example, when fasting two days per week, you can expect to lose between 9 and 13 pounds in the first 2 to 3 months, says Leonie Heilbronn, an associate professor at Adelaide Medical School.
If you fast three days per week, Heilbronn says you could lose 13 to 18 pounds in the first 2 to 3 months.
So when deciding about whether intermittent fasting is right for you, it's crucial to keep your lifestyle and eating habits in mind. "Some people can struggle with not eating for prolonged periods," says Heilbronn. Ultimately, more research is needed to determine if this diet can work over the long-term.
Intermittent fasting isn't safe for everyone
According to Heilbronn, intermittent fasting is not for everyone. "People who take medication that can cause hypoglycemia (i.e. low insulin levels) shouldn't do this diet without talking with their doctors," she says. You should also avoid intermittent fasting if:
You're pregnant or breastfeeding
You're not fully grown (teenagers)
You have a history of eating disorders
Insider's takeaway
Intermittent fasting can be a safe and effective weight loss strategy, with the caveat being that the long-term benefits are less clear. To start, you may want to try a simple form of the 16:8 plan, which limits the hours of the day when you eat.
If you believe intermittent fasting may be unrealistic for your lifestyle, you may want to check out less restrictive eating plans like the Mediterreanan diet.
It's important to note that beyond weight loss, intermittent fasting has other potential health benefits. Researchers behind a 2019 review surveyed 70 published studies and found intermittent fasting of all types can help slow aging and reduce the risk of some diseases, including cancer, diabetes, and heart disease.
But when it comes to weight loss, your own tolerance for hunger pangs may guide you in deciding if the eating plan is right for you. While intermittent fasting is generally safe for most healthy adults, it's not meant for everyone.
Related stories from Health Reference:
What to eat when intermittent fasting for health and hunger pains
Can you drink coffee while intermittent fasting? Yes, and other drinks you can have while fasting
The different types of intermittent fasting and which may be more beneficial to your health
What is the keto diet and how it helps your body burn fat through ketosis
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