Intermittent Fasting: Answers to 5 Frequently Asked Questions.

Every January, our Nutrition team answers an influx of questions from patients resolving to improve their health and lose weight in the new year. Intermittent fasting—a dietary approach that cycles between periods of fasting and eating—has become one of the most popular diet trends.

While celebrities including Jimmy Kimmel and Jennifer Aniston have created a buzz about the weight loss benefits of intermittent fasting, people have fasted for religious reasons for centuries. 

Clinical research on intermittent fasting is limited—and mixed. Some studies have shown that it can lead to improved health and mild or moderate weight loss. Others suggest that fasting has no significant long-term health benefits.

If you’re considering intermittent fasting to improve your overall health or lose weight, it’s important to understand:

• The plan’s basic principles  
• How to find accurate information about intermittent fasting
• That everyone responds differently to eating patterns

Intermittent fasting is not a quick fix for weight loss (there’s no such thing) or a plan that “allows” you to eat large amounts of processed or fast foods within limited time frames. Knowing how to start fasting and what guidance to follow can be confusing; there’s no shortage of advice available from both medical professionals and non-experts.

To help you learn how to incorporate it into your life safely, I’ve answered the five most common questions I get about intermittent fasting as a registered dietitian.

Related reading: Mindful Eating for Healthy Weight Loss

1. Is there a ‘right way’ to fast?

Intermittent fasting is popular because it's less about what you eat and more about when. Most people in the U.S. eat during a 12-hour window each day: 8 a.m. to 8 p.m., for example. Intermittent fasting changes this pattern by limiting the eating window or restricting calories during certain days. 

But there’s no “right” way to fast. People choose different patterns based on their lifestyle and preferences. Here are four methods to consider.

• 5:2 diet: With this approach, you eat as you normally would for five days each week. For the two remaining days—which are typically non-consecutive—you consume between 500 and 600 calories. On these days, choose low-carb, high-fiber foods, such as vegetables; grilled or steamed fish; boiled eggs; natural yogurt; low-calorie soups; and black coffee or tea. This approach tends to help those who respond best to only having to follow the “rules” during these two days. 
• Alternate-day fasting: A subset of the 5:2 diet, this plan alternates a “feast” day with a “fast” day every other day. Like the 5:2 plan, you consume 600 calories or less on fast days. 
• Circadian fasting: This pattern aligns your eating schedule with your natural hormone cycles. Our circadian rhythm—the body’s 24-hour clock—controls our sleep, digestion, hormones, and stress levels. Time your meals for early in the day when your energy levels are likely to be higher. Fast when the sun goes down and your energy decreases and digestion slows.
• 16:8 diet: This time-restricted approach is best for people who like routine. You choose an eight-hour eating window each day and consume most of your calories in the middle of the day. Some people skip breakfast, eat from noon to 8 p.m., and then fast until noon the next day. Others may eat from 10 a.m. until 6 p.m. Combined with regular exercise, this method has been linked to reduced fat mass and body weight and can be easier to follow consistently than more restrictive patterns.
  

I recommend trying different patterns to see which one—if any—works best for your lifestyle.

When combined with endurance exercise, most of these approaches can result in significant weight loss and help lower the risk of heart disease. They have also been linked to improved insulin sensitivity and blood pressure.

2. Can I eat whatever I want on non-fasting days?

Like any successful diet plan, no foods are off limits for intermittent fasters. But we always recommend that you eat more fruits and vegetables and fewer processed foods, whether or not you are trying to lose weight. 

Eating more plant-based foods rich in nutrients, vitamins, and minerals helps keep your immune system healthy. It also reduces your risk of chronic inflammation, which is associated with diseases such as diabetes, heart disease, liver disease, and several types of cancer.

While many people try fasting to improve their health, some try fasting to lose weight. We rarely recommend that weight loss be the primary goal of a diet plan, as studies show this approach typically does not result in long-term weight loss or improvements in health. In fact, it often negatively impacts mental health. But if you’re fasting to lose or maintain your weight, even if it’s not the primary goal, you should also be mindful of the amount of food you eat. 

Some people who choose more restrictive patterns of intermittent fasting eat more than they normally would on non-fasting days in anticipation of—or to make up for—eating less on fasting days. Because they’re consuming the same number of calories overall, their weight stays the same. 

Related reading: Fight Harmful Inflammation with These 10 Healthy Eating Tips

3. What are the health benefits of intermittent fasting?

When people fast, the body swaps its source of energy from glucose to ketones. This process, combined with exercise, is known as metabolic switching. Theoretically, it can change your body by flushing out damaged cells and replacing them with newer, healthier cells, making it possible to lose weight and reap long-lasting health benefits such as:

• Improved sleep
• Reduced inflammation
• Strengthened immune system
• Improved insulin sensitivity
• Increased energy
• Decreased blood pressure
• Improved digestion

Keep in mind that some people only experience one or two of these health benefits—or none at all. Everybody’s metabolism functions differently, and you might need to eat more often than someone else. 

I know people who say fasting helps them feel more energetic and other people who tried fasting and said it made them feel anxious and jittery. If any diet plan makes you feel worse mentally or physically, it is not safe to continue.

It usually takes two to four weeks for your body to become accustomed to new eating habits. Any side effects you may experience when you start fasting, such as headaches or irritability, usually disappear after a week or two. But if you’re miserable a week in, don’t force yourself to keep going. Accept that it’s simply not the right eating plan for you.

4. Is intermittent fasting sustainable?

The flexibility of intermittent fasting patterns makes it easier than other diet plans to stick to long term. If you’re eating a balanced diet, sleeping well, exercising, and in good health overall, it’s perfectly safe to fast long term, as long as you remain in good health, both physically and mentally. 

There’s not a lot of long-term research or peer-reviewed studies about fasting. However, studies have shown that the 5:2 diet often results in short-term weight loss because people have a hard time sticking to it over time.

The 16:8 method is often easier to maintain than the 5:2 plan because you’re eating every day. Yet even this approach can be difficult when you factor in weekends; some people fast only during the work week. The key is choosing a pattern that’s easily sustainable for your particular lifestyle.

5. Who should not try intermittent fasting? 

While intermittent fasting can be tailored to individual lifestyles, it is not safe for certain populations, including:

• Anyone under age 18. Their metabolism and hunger cues are not fully developed.
• People who are prone to restrictive eating patterns or have a history of eating disorders. Fasting can lead to or exacerbate binging and starving, over-restricting food intake, or eating/food anxiety.
• Those who need to take medication with food at specific times.
• People who are pregnant or breastfeeding. Fasting can cause low blood sugar. Plus, growing and feeding a baby requires sufficient—not restricted—calorie intake.
• Anyone who becomes irritable, shaky, or anxious when they don't eat for a certain amount of time. 
• People prone to constipation. While some report improved digestion after intermittent fasting, others have experienced constipation.

If you fall into one of these categories and want to change your diet, talk with your primary doctor or a registered dietitian about sustainable ways to do so safely. People with any type of health condition should not try intermittent fasting or any other diet plan without close monitoring by their doctor.

Many “wellness experts” provide well-meaning advice about intermittent fasting that can be harmful. In addition to speaking with a professional, when seeking more information:

• Ensure research you read is peer-reviewed from a trusted source (such as the National Institutes of Health or a peer-reviewed medical journal).
• View online searches with skepticism. If an “expert” is only presenting the benefits of a diet plan and also selling that diet plan, the information should be taken with a grain of salt. Many websites want to sell you a diet they claim is easy and magical for weight loss. However, in most cases the only loss you’ll see is in your wallet.

Bottom line: Intermittent fasting isn’t for everyone.

It can be easy to get swept into someone else’s excitement about a diet that works for them. But don’t get discouraged if you don’t experience the same results or if you find it difficult to stick to full-time intermittent fasting.

I’ve tried—and enjoyed—intermittent fasting in the past, but it wasn’t conducive to my lifestyle long term. When changing your eating habits for better health or weight loss, do what works best for you with the guidance of a credible medical professional who knows your medical history.