Intermittent fasting: The complete guide for beginners

• Intermittent fasting is an approach to eating when you switch between periods of fasting and eating.
• The research on intermittent fasting is limited, but it may have behavioral, cognitive, and overall health benefits.
• Intermittent fasting can be a safe and effective weight loss strategy for many healthy adults—but it’s not for everyone.

Maybe you’re on a mission to push past a challenging weight loss plateau, or trying to lower cholesterol or boost overall health. Whatever the case, intermittent fasting has piqued your curiosity. 

It’s a unique approach to health and weight loss—so if you’re a beginner, you probably have questions. What is intermittent fasting? Are there any benefits? Is it actually a healthy way to lose weight? 

We spoke with our very own Noom Weight Coach, Kendra Gutschow, Registered Dietitian Nutritionist (RDN), to get answers. 

Note: Consult with your medical provider before making dietary changes.

What is intermittent fasting?

Intermittent fasting is an approach to food where you switch between scheduled periods of fasting and eating. It focuses on when you eat, instead of what you eat.

The thinking behind this approach is that if you don’t eat for specific periods of time, it can boost your health and help you lose weight. The simplicity is appealing to many.

In recent years, intermittent fasting has also become a popular trend among fitness and health influencers.

An example of an intermittent fasting schedule might be to only eat in a six-hour window each day—say from 9 a.m. to 3 p.m. The rest of the day is a fasting period with limited or no food.

However, there are many different ways to practice intermittent fasting, as you’ll learn below.

Potential benefits of intermittent fasting

The research on the impact of intermittent fasting is limited, and we still don’t have concrete evidence that it has lasting benefits for your health.

However, there are some promising benefits suggested by studies.

Weight loss benefits

A meta-analysis examined several studies on the effects of alternate-day fasting (eating every other day) on adults. 

After six consecutive months following an alternate-day fasting plan, study participants saw weight loss, which led to improved body mass index (or BMI) scores.

Participants also saw positive improvements in their blood pressure and low-density lipoprotein (LDL) cholesterol levels, both of which can potentially boost heart health. 

Longevity benefits

If you think your life span boils down to genetics alone, we have good news—research suggests that just under 25% of what leads to a long life is genetic. 

That means how long you live largely depends on factors you can control, like healthy lifestyle choices and your environment. 

A small study suggests intermittent fasting may help you live a long, healthy life by cutting risk factors for age-related diseases like heart disease, cancer, and diabetes. 

Other research shows promise as well. For example, one study suggests intermittent fasting may have anti-aging effects on the brain. 

Behavioral benefits

Aside from physical benefits, some people find that fasting helps them be more mindful of their eating habits and patterns. 

Gutschow says that intermittent fasting may help control patterns like snacking past regular meal times (which can lead to overeating). 

Anti-cancer benefits

One study suggests that intermittent fasting may offer hope for preventing and treating cancers related to metabolic syndrome. Metabolic syndrome is a cluster of common health conditions (including high blood sugar, high blood pressure, and high cholesterol) that can lead to major health issues. 

A study in mice suggests that intermittent fasting may also slow tumor progression, which could pave the way for exciting studies in the future. 

Ultimately, more research is needed to understand how intermittent fasting may affect cancer.

How does intermittent fasting work?

Intermittent fasting may help with weight loss simply because if you’re not eating for a lot of the day, chances are you are taking in less calories overall. This can lead to a calorie deficit, which results in weight loss.

Aside from calories, intermittent fasting is also thought to trigger a process called a lipolysis response.

When this happens, it means your body has run out of sugar from food and is breaking down stored fat for energy. This breakdown of stored fat may contribute to weight loss.

Experts think many benefits of intermittent fasting may come from this switch in energy sources—from burning sugar to burning fats. However, more research is needed to understand the exact impact on the body.

Common intermittent fasting plans

On an intermittent fasting plan, you set aside specific time periods for fasting and eating. 

During your eating windows, eat as you usually would. In the fasting periods, you drink water and eat limited or no food.

There are multiple options for fasting and eating windows, and each of them works in different ways. 

One strategy isn’t necessarily better than the other. Gutschow says choosing your fasting plan ultimately depends on your health needs and personal preferences.

The most common methods for intermittent fasting include:

• The 16/8 method is a popular approach with one rule for every day of the week—eat for eight hours, then fast for 16.
  
• The 5:2 method allows you five non-fasting days per week. On the remaining two days, limit yourself to a single 500 to 600 calorie meal per day. 
  
• The eat-stop-eat method requires normal eating five to six days per week, followed by a 24-hour fast. If you opt to fast for two 24-hour periods, be sure to space them out on non-consecutive days.
  
• Skipping breakfast is a less formal approach that involves only eating lunch and dinner.

Although there are other approaches, like alternate-day fasting, Gutschow does not recommend plans with fasting windows of 24 hours or longer.

How safe is intermittent fasting?

Intermittent fasting is likely safe when done under the supervision of a medical professional. 

Research also shows that intermittent fasting is likely a healthy weight loss strategy for most people, but there are important exceptions.

According to Gutschow, intermittent fasting isn’t appropriate if you: 

• Have any history of eating disorders.
• Have blood sugar instability.
• Are pregnant or breastfeeding.

Gutschow also stressed the importance of talking with your doctor before starting an intermittent fasting plan, especially if you take medications that are affected by foods.

How to get started with intermittent fasting

Let’s say you want to give intermittent fasting a shot. How should you get started? 

It comes down to shortening the number of hours you eat in a day.

Gutschow recommends starting with small fasts to see how your body reacts. On your first day, try skipping breakfast in the morning and see how you feel.

From there, you can build up to longer fasts. Maybe the following day, you also stop eating after the last meal of the day—for example, at 7 p.m. That means no post-dinner or late night snacks.

After taking small steps into intermittent fasting, you can try working your way up to a plan like 5:2 or 16/8 if it feels right for you. 

What should you eat while intermittent fasting?

Because intermittent fasting targets when you eat, you might wonder if you can eat whatever you want on an intermittent fasting plan.

It’s true that some people eat lots of sweet and processed foods while on intermittent fasting. However, experts say feeding your body nutritious foods is essential. 

Gutschow recommends trying to strike a balance between nutrient-dense options (think whole grains, vegetables, and lean proteins) and the treats you crave. 

Opting for healthy foods during your non-fasting window can help you keep your calorie intake in check while giving your body what it needs to function at its best. 

If you need more guidance, our intermittent fasting guide can help.

FAQs: Side effects of intermittent fasting 

Some question the safety of intermittent fasting while others swear by it, so it’s natural to wonder if it’s a safe and effective approach. 

To help us learn more, Gutschow shares answers to common questions about the side effects of intermittent fasting.

Does intermittent fasting cause muscle loss? 

Intermittent fasting research is limited—current studies tend to focus on weight loss, so we don’t have a clear picture of how intermittent fasting may affect muscle mass (yet). 

Here’s what we do know: When you reduce your calorie intake, it’s possible for you to lose muscle mass in addition to fat. But by introducing muscle-building exercises (like strength training) and consuming lean protein, you can lower your chances of losing muscle. 

That may explain the results of one study, which suggests that intermittent fasting doesn’t have much effect on your ability to maintain muscle mass when combined with resistance training. 

If keeping or increasing muscle mass is important to your weight loss journey, it’s a good idea to chat with your healthcare provider for guidance.

Does intermittent fasting cause mood swings or irritability? 

If you’ve ever forgotten to pack a lunch, you’re familiar with the mood swings that can come from feeling extra hungry. 

Gutschow says that for some, intermittent fasting can cause similar feelings of irritability—but if this happens, these feelings are usually short-lived and resolved within a few weeks. 

Moodiness and irritability can be caused by sudden drops in blood sugar, so it’s important to make sure you’re getting enough calories during your eating window. 

On the bright side, one small study also found that participants on intermittent fasting plans experienced positive feelings, like a sense of pride and achievement. 

Will intermittent fasting give me headaches?

According to Gutschow, headaches are possible in the first few weeks of an intermittent fasting plan. 

But research shows most headaches occur after 16 hours of fasting and get better after eating. Limiting your fasting window to 16 hours or less can help reduce your chances of headaches. 

Make sure you’re also eating enough and staying hydrated. 

Does intermittent fasting slow down your metabolism?

The short answer? Maybe.

Studies seem to differ when it comes to metabolism and intermittent fasting. 

Some indicate that long-term intermittent fasting may slow your metabolism, while others conclude that short-term fasting can potentially give it a boost. 

Why is this? 

Short-term fasting may increase the stress hormone norepinephrine. When elevated, norepinephrine causes a spike in blood sugar, giving your body extra energy. These events temporarily raise your body’s resting energy needs and boost your metabolism.

On the other hand, Gutschow says restricting food for an extended period (24 hours or longer) may do the opposite. Long fasts can cause your body to adapt to having fewer calories. 

The result? Your body slows metabolism and holds onto excess calories for energy. 

Remember, this adaptation happens as a result of long-term calorie restrictions—which means it can happen with any restricted eating plan, not just intermittent fasting.

Does intermittent fasting cause diarrhea or constipation?  

While it’s not a sure thing, Gutschow explains that fasting may cause temporary digestive upsets. 

Your gut microbiome is ultra-sensitive to changes in eating patterns and food, so any big changes to your habits can mess with your bowel habits, especially early on.

Intermittent fasting can positively affect gut bacteria, but even good changes can make things feel a little out of balance at first.

The good news is, you can take steps to make the transition easier. Gutschow suggests drinking plenty of water and eating plenty of fresh fruits and vegetables to aid in digestion. 

Be patient, and any temporary discomfort should pass. Also, be sure to choose foods carefully when breaking your fast. Gutschow notes that it can also be helpful to limit foods that commonly lead to digestive upset, like overly fatty, fried, or processed foods.

How Noom can work with intermittent fasting

If you’re trying intermittent fasting, it can be helpful to get support.

Noom Weight has a personalized curriculum (along with food logging, guides, and coaching) that can support you on your journey. Noom’s color system and focus on calorie density can also help guide food choices during the times you’re not fasting.

Learn more about how Noom can work with intermittent fasting.