Intermittent Fasting 101: What It Is and How to Do It

By Elizabeth Ward, MS, RDN
Updated on 02/4/22
Fajita-Style New York Strip Steaks

The Spruce / Ali Redmond

A Note From the Editors

The Spruce Eats does not endorse this diet; rather, we are providing some information that can contribute to your decision. Please talk with your doctor or a registered dietitian before making any significant changes to your diet.

Intermittent Fasting Basics

While Intermittent Fasting may sound like a new trend, humans have fasted for both health and spiritual reasons for centuries. And we've been studying the health effects for a long time. For example, in the early twentieth century, doctors studied long-term fasting as a treatment for diabetes. More recently, research has uncovered the benefits of shorter fasting intervals—what we now call Intermittent Fasting, or IF.

Intermittent fasting isn’t about what you eat, it’s about when. With no calorie cap or special foods to make or buy, IF is more of a lifestyle than a prescriptive diet. There are several types of IF, including time-restricted eating—consuming all of your food for the day within a designated window—and going with no, or very little, food for entire days. Here are some of the most common IF plans.

16:8 On this plan, you consume only calorie-free beverages for 16 hours, and then you eat during an eight-hour period of your choosing. (The same goes for the 20:4 regimen, which is an extreme eating schedule that we don’t recommend.)  

5:2 This plan consists of eating as you usually do on five days of the week. On the other two (preferably nonconsecutive) days you reduce your intake to 25 percent of your typical daily calories (about 500 for women and 600 for men). On a similar program called Alternate Day Fasting (ADF) you consume only calorie-free beverages every other day of the week; on the alternate days you eat your as usual. 

The goal of IF is to lower your insulin levels. Eating raises insulin levels in the blood. This helps move the glucose created by digestion into your cells to satisfy energy needs.  Fasting for more than 12 hours suppresses insulin and triggers the body to burn stored fat, creating ketones that cells can also use for energy.

Weight loss and reduced body fat typically attract people to IF, but this way of eating may also improve energy, reduce blood glucose levels, tamp down inflammation, lower blood pressure, improve cardiovascular health, and slow the aging process.

Pros and Cons of Intermittent Fasting

Pros

  • You get to choose the IF style that works best for you.
  • It’s possible to shed pounds without counting calories. A study of people with obesity who ate only between the hours 10 AM to 6 PM, without tracking what they ate, consumed an average of 350 fewer calories daily, lost about 3 percent of their body weight, and lowered their blood pressure during a 12-week period.
  • Achieving ketosis on a regular basis may reduce body fat stores, resulting in a leaner body.
  • People who shorten their eating window may sleep better and feel more alert during the day.
  • When you eat less overall, IF saves time spent shopping and on food prep, and could cut food costs.
  • The 5:2 plan may work for people who find it easier to be strict about their eating on just two days of the week, or who like to eat whatever they want on the weekends, or both. 
  • Because of its effect on cholesterol, insulin, and glucose levels in the blood, IF may help protect against diabetes and cardiovascular disease.

Cons

  • Fasting can leave you feeling jittery and cranky and may cause headaches. 
  • Going entire days without eating can make it difficult to satisfy nutrient needs. A multivitamin and other single vitamin and mineral supplements help to fill dietary gaps for micronutrients on every type of IF, but you may come up short on protein, healthy fats, and fiber.   
  • Preoccupation with the timing of eating can exacerbate existing anxieties about food, or create new ones, such as obsessing about eating. 
  • Exercise reduces glucose and insulin levels, and you may need to change the intensity, and timing, of your exercise routine to prevent fatigue. 
  • Adults who live with and care for children—a group that requires a steady supply of nutrients—may send conflicting messages to kids about food. 
  • Depending on the length and timing of the eating window, IF could put a crimp in family and social activities that involve food. 
  • IF is not always more beneficial than daily calorie restriction. A one-year study found that ADF produced the same rate of weight loss as reducing calorie intake by 25 percent.

Common Intermittent Fasting Mistakes

Lack of consistency. You need a regular fasting schedule to sufficiently suppress insulin levels and promote fat burning. Pick a pattern and stick to it, including on weekends. 

Eating too much at night. Our bodies are designed to process food better during the day. Eating until 8 or 9 PM may reduce the effectiveness of time-restricted eating plans.  In a study of women, those who ate most of their calories before 3 PM lost 30% more weight  than those who ate most of their food after 3 PM, despite eating a similar number of calories. 

Not easing into IF. If you want to do a plan like 16:8, start by fasting for 12 hours. Then, increase that time gradually over a series of days, working up to your target. 

Who Should Not Do Intermittent Fasting

Certain groups need a steady flow of nutrients for good health. While going without food for is safe for many people, forgo intermittent fasting if:

  • You’re under 18 and over 75
  • Pregnant or breastfeeding 
  • You take blood pressure medication or need to take medication with food at certain times of the day
  • You’re an endurance athlete
  • You have a chronic medical condition, including kidney disease, and type 1 or type 2 diabetes 
  • You have a history of disordered eating
How to Start to Intermittent Fasting

The Spruce / Ellen Lindner

What to Eat

No food is off-limits on any IF regimen. However, a balanced eating plan is important for getting the nutrients your body needs, and even more so if you are eating less overall. Include adequate protein, fiber, and healthy fats at every meal, and consume three servings of low-fat dairy foods and at least five servings of fruits and vegetables daily to help keep you nourished and full, and to limit cravings for salty or sweet low-nutrient foods. If you want treats, enjoy small portions within your eating time-frame. 

What Not to Eat

While you can include any food on IF, avoid an eating free-for-all mindset during your eating window, which can result in weight gain and negate other potential IF benefits. Choose satisfying, nutritious foods that control hunger, and eat them on a regular basis. 

Bottom Line

During intermittent fasting, you focus more on the clock than on calories, and every type of food is allowed. It’s possible that designated times makes it easier for people to control their food intake, especially in the evening.  Certain groups should not try IF, but as long as you consume adequate calories and satisfy your nutrient needs, and IF doesn’t interfere with your mental health, or your work, family, or social life, it’s probably safe and sustainable. If going long stretches without eating isn’t appealing, a shorter daily eating window may provide benefits similar to more drastic IF plans.

Article Sources
The Spruce Eats 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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