What Is Intermittent Fasting?

Intermittent fasting

Verywell / Debbie Burkhoff

At Verywell, we believe there is no one-size-fits-all approach to a healthy lifestyle. Successful eating plans need to be individualized and take the whole person into consideration. Prior to starting a new diet plan, consult with your healthcare provider or a registered dietitian, especially if you have an underlying health condition.

What Is Intermittent Fasting?

Intermittent fasting (IF) is a method of calorie restriction that requires you to go for extended periods of time without eating. There are different types of IF programs—some recommend fasting for a few hours or for a certain part of each day, some require fasting for a few days each week, and some involve fasting for several days each month.

Intermittent fasting has gained momentum because of popular beliefs that the regimes can help you improve your level of fitness, boost longevity, and get faster and longer-lasting weight loss results. However, research shows that it is more complicated than that.

What Experts Say

"Intermittent fasting, restricting food intake for certain periods of time, has been studied for potential effects on longevity and other health outcomes but is often used for weight loss. Many experts agree food restriction is not sustainable and frequent fasting could lead to social isolation or binge eating."

Willow Jarosh, MS, RD

What You Can Eat

You can eat basically anything you'd like or would normally eat on an intermittent fasting diet. The restriction is entirely based on when you eat, not what you eat.

What You Cannot Eat

There is nothing specifically off-limits on an intermittent fasting diet. There are several variations in books and popular programs that encourage avoiding processed foods or some other types of foods. Intermittent fasting is sometimes combined with a Paleo diet.

How to Prepare an Intermittent Fasting Diet & Tips

Almost every other eating plan requires you to increase your intake of certain foods (such as high protein diets) or limit your intake of others (low-carb diets). But on an IF program, you can eat as much or as little of any food or food group as you like, as long as it is consumed in the "feasting" window. There are no macronutrient recommendations and there is no list of foods to limit or avoid. Instead, intermittent fasting simply regulates the timing of your food intake.

Different methods of intermittent fasting exist, but all of them include a "feast" phase and a "fast" phase. During the feast phase, most programs recommend that you eat an "ad libitum" diet, meaning that you don't limit or restrict your food intake. But you do avoid eating past the point of fullness and overeating unhealthy foods. A balanced diet is still recommended during this time. In the fasting phase, you either severely restrict or avoid food altogether.

The 5:2 Plan

One popular approach to IF is called the 5:2 diet. On this plan, you eat a less-restricted, healthy diet five days of the week and then spend two days per week fasting. However, in this program, fasting does not mean total abstinence from food. It means severely restricting your food intake. For women, that represents about 500 daily calories, for men, about 600 calories. On the other days, you consume a typical healthy diet, although a maximum calorie intake is suggested.

Other variations of IF include alternate-day fasting (ADF) plans that require you to completely abstain from food or severely restrict food every other day, or time-restricted plans where food is eliminated during certain hours of the day. Severely restricting your food intake may mean that you consume roughly 25% of your daily calorie needs per day.

Pros of Intermittent Fasting

Although this eating style may seem extreme, at least in certain versions, it does have some potential benefits.

  • Does not restrict foods: Intermittent fasting is a popular approach to dietary change because it allows followers to continue to eat the foods that they love. Other programs may require avoiding beloved foods. While that is sometimes tolerable in the short-term, many people have a hard time giving up familiar foods over the long term.
  • May work for weight loss: While there is still little evidence on the long-term effectiveness or safety of the diet, several studies have suggested that intermittent fasting works as well as continuous calorie restriction for weight loss. Some researchers have suggested that these diets may be a better approach to the treatment of obesity and obesity-related conditions.
  • May create a natural calorie deficit: In one study, subjects who ate 20% to 30% of their normal calorie requirements on fasting days generally ate just 10% more than usual on their non-fasting days. In addition, many people reported that their feelings of hunger on low-calorie days dramatically diminished over time.
  • May reduce muscle loss: A 2011 review study found that 90% of the weight lost through intermittent fasting was fat (rather than muscle), compared with only 75% in daily calorie restriction. This suggests that conventional dieting causes greater muscle loss than IF programs.
  • May boost longevity: The research is very new and ongoing, but IF could offer some benefits to longevity. One study found that "IF lasting from 12 to 48 hours and repeated every 1 to 7 days and PF [periodic fasting] lasting 2 to 7 days and repeated once per month or less have the potential to prevent and treat disease." More research is necessary.

Cons of Intermittent Fasting

As with any diet, there are also drawbacks and risks to intermittent fasting. It's important to be aware of these before attempting this diet.

  • Lack of guidance: Experts worry that there is no guidance provided about making healthier food choices. Someone following an intermittent fasting plan might meet their nutritional needs, but these plans do not provide encouragement to meet healthy eating guidelines.
  • Lacks scientific validity: Intermittent fasting has been studied extensively for its potential weight loss benefits and its effect on longevity. Many of the earlier studies were performed on rodents, but more studies are now being conducted on humans. Long-term studies are still needed to see if IF is safe or effective in the treatment of obesity and other conditions.
  • Could lead to unhealthy food habits: A primary concern among researchers and nutrition experts is the feast-or-famine approach to eating. Starving yourself for limited periods may lead to overeating or binge-eating at other times.
  • Not superior to other diets: Current evidence suggests that those who follow traditional calorie-restriction diets lose about the same amount of weight as those who follow intermittent fasting programs. Several studies have found that while there is a slight advantage for those doing IF, the advantage isn't significant. Additionally, experts still don't know if IF programs are sustainable.
  • May cause social isolation: If you cannot eat during regular meal times, you may feel socially isolated. It can be challenging to avoid eating lunch with co-workers, sharing dinner with your family, or participating in social gatherings centered around food.

Is Intermittent Fasting a Healthy Choice for You?

The U.S. Department of Agriculture provides guidelines for the daily intake of certain foods (such as fruits and vegetables) and important nutrients (such as fiber, protein, and fat). Certain types of intermittent fasting require that you avoid food except for water and clear liquids on certain days of the week or month. It would, therefore, be impossible to meet your nutritional guidelines on those days. Other variations of IF would allow you to meet your nutritional needs, but only if you were very careful in your food choices.

Most IF programs make it impossible (or nearly impossible) to meet USDA nutritional guidelines on days when you are fasting. However, with careful planning, you may be able to make up for the differences on your non-fasting days.

A Word From Verywell

There has been substantial research conducted on different variations of intermittent fasting, but much of it has been conducted on animals. Long-term studies are needed to determine if there is enough scientific evidence to recommend this eating style. Intermittent fasting is not superior to other calorie-controlled diets and is therefore unnecessarily restrictive. Instead, consuming a healthy, well-balanced diet filled with whole foods that you eat at normal meal times is more sustainable.

Remember, following a long-term or short-term diet may not be necessary for you and many diets out there simply don’t work, especially long-term. While we do not endorse fad diet trends or unsustainable weight loss methods, we present the facts so you can make an informed decision that works best for your nutritional needs, genetic blueprint, budget, and goals.

If your goal is weight loss, remember that losing weight isn’t necessarily the same as being your healthiest self, and there are many other ways to pursue health. Exercise, sleep, and other lifestyle factors also play a major role in your overall health. The best diet is always the one that is balanced and fits your lifestyle.

10 Sources
Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Sharon Basaraba
Sharon Basaraba is an award-winning reporter and senior scientific communications advisor for Alberta Health Services in Alberta, Canada.