What is intermittent fasting for weight loss and is it safe?

If you are trying to drop pounds, fasting for weight loss has probably crossed your mind, in part because you’ve probably seen intermittent fasting all over your social media and in the news. This “diet” limits food consumption to a certain timed schedule, such as between specific times or alternate days. 

While we all know that restricting calories often leads to weight loss, it’s a little less clear whether fasting does the same. But if you’re interested in trying an intermittent fasting diet, it’s important to understand the pros and cons and how to go about it safely in order to get the best results. Keep reading for everything you need to know about fasting for weight loss.

Intermittent fasting involves having set periods of time (such as between certain hours of the day) where food is not consumed. Lauren Harris-Pincus, MS, RDN, a Ro advisor and author of The Everything Easy Pre-Diabetes Cookbook, explains that there are typically three different methods of intermittent fasting:

• Alternate-day fasting: Alternate-day fasting is when someone doesn’t eat for 24 hours at a time, for example, from noon one day until noon the next day.  

• 5:2 intermittent fasting: This form of intermittent fasting is when someone consumes 25% of their calorie needs—typically 500 for women and 600 for men—two days per week. The other days of the week, they eat the way they normally would and don’t restrict calories. 

• 16:8 intermittent fasting: Also known as time-restrictive eating, this method involves limiting eating to an eight-hour window throughout the day and fasting for the following 16 hours. “It’s common to skip breakfast and choose a window from 12 to 8 p.m., though any eight-hour window is ‘allowed,’” Harris-Pincus says.

If you’re considering which intermittent fasting schedule is best for you, Harris-Pincus recommends one with an eating window that allows you to eat breakfast, for example having an eating window between 9:30 a.m. to 5:30 p.m. or between 10 a.m. and 6 p.m.

“There are metabolic benefits to eating breakfast and foods typically chosen in the morning such as whole grain cereal, yogurt, cottage cheese, fruit, milk and eggs, which are nutrient rich and contribute to meeting our micronutrient needs,” she says. 

It’s also important to consider which method fits your lifestyle. For example, if you have nightly family dinners at 7:30 p.m., choosing an intermittent fasting method where your eating window stops at 6 p.m. isn’t realistic. “This is a lifestyle, not a diet. If you are not able to eat with your family or have to pass on social occasions, then maybe a certain schedule may not be ideal for you,” Harris-Pincus says.

While intermittent fasting can be beneficial for weight loss, there are certain factors to keep in mind in order for it to be done safely. Harris-Pincus says it’s important to make sure you’re still getting all the essential nutrients your body needs each day. (MyPlate.gov is a great resource for guidance on this.) “It's important to make an effort to fit adequate protein, fiber, fruits, veggies and vitamins and minerals into that limited eating window,” she says. 

Intermittent fasting isn’t for everyone. Harris-Pincus says intermittent fasting is not a good fit for children as well as women who are pregnant, trying to get pregnant or are breastfeeding. She adds that it’s also not a good fit for people with diabetes or insulin sensitivity. If you have a history of disordered eating, Harris-Pincus warns against intermittent fasting.    

Even if none of the above applies to you, intermittent fasting still may not be a good fit for you. “Each body is different so it’s important to learn what works for you,” Harris-Pincus says. If intermittent fasting is making you miserable or not giving you the energy you need to do daily activities that are part of your life, it’s worth it to consider a different weight loss method or eating plan instead.

Side effects of intermittent fasting can include low energy, mood swings and dizziness—all of which stem from low blood sugar. If you do start feeling any of these side effects, consider it a sign to eat something. Even if it’s outside your eating window, it’s more important to listen to your body than to stick to restrictions. 

How long is it safe to fast for weight loss?

Harris-Pincus says that when done safely, intermittent fasting can be done long-term; however, she reiterates that there is a lack of scientific research on the effects of intermittent fasting long-term.

Now that you know what intermittent fasting is, the big question is: Can intermittent fasting help with weight loss? While Harris-Pincus says it’s possible that intermittent fasting can help with weight loss, she notes that more research needs to be done to really confirm the connection. That said, based on scientific research, there are a few ways that it could be beneficial, which are all outlined below.

It can aid in calorie restriction

Perhaps the most obvious way that intermittent fasting could help with weight loss is because it serves as a way to put a cap on how many calories you consume. If your eating window ends at 6 p.m., for example, that eliminates consuming calories from late-night snacks you may have indulged in otherwise. “Theoretically, if someone limits their eating time, they may consume fewer calories per day. Especially for people who choose an earlier eating window like 10 a.m.-6 p.m. which doesn’t permit evening snacking,” Harris-Pincus says, to this point. 

A systematic review of 27 different scientific studies on intermittent fasting and weight loss shows that intermittent fasting can serve as a way to restrict calories and help with weight loss, at least in the short term. However, more research is needed to confirm a connection between intermittent fasting and long-term weight loss.  

While intermittent fasting can help minimize overconsumption, Harris-Pincus says that the weight loss outcome is similar to other forms of calorie restriction, such as sticking with a calorie deficit each day. What you eat during your eating window matters, too. Consuming lots of ultra-processed foods during the eating window likely won’t lead to weight loss as effectively as consistently maintaining a healthy diet without staying within a specific eating window.

It’s possible to maintain long-term 

When someone makes long-term lifestyle changes that are conducive to their daily life (as opposed to a crash diet, for example), it’s often easier to stick with them. Studies have shown that while crash diets can lead to short-term weight loss, they typically backfire and lead to weight gain in the long term. Clearly, deprivation doesn’t work. With intermittent fasting, no food is off limits as long as it’s consumed within your specific window, which, for some, may be a less restrictive approach.

It could speed up metabolism

Some studies show that intermittent fasting may help maintain a healthy metabolism. When the body is in a fasted state, it switches from burning glucose for energy to burning fat for energy. Regular fasting may help reduce blood sugar spikes and lower insulin levels, which can improve insulin sensitivity and could aid in weight loss over time. 

Another benefit of intermittent fasting is lowering oxidative stress in the body, which may lead to less inflammation. This effect is likely due to the reduction in glucose and insulin levels during fasting, which can lead to improved cellular function and stress resilience. If true, intermittent fasting could help lower the risk of certain diseases (cardiovascular disease and type 2 diabetes, for example), although more scientific research is needed to confirm this connection.

It could help with sleep

If you want to lose weight, it’s important to get consistent, good sleep. Research shows that not getting enough sleep may lead to metabolic changes, such as an alteration of appetite-regulating hormones, that make it harder to lose weight.

What does intermittent fasting have to do with this? “Eating close to bedtime may result in symptoms of reflux which can cause discomfort and affect sleep quantity and quality,” Harris-Pincus says. Simply put, when your body doesn’t have to be up at night digesting a late-night meal, it’s able to rest—which means so can you.

Of the different eating pattern methods, Harris-Pincus favors the 16:8 method. “I generally don’t support alternate day fasting or the 5:2 method because food intake is so limited that it may result in low blood sugar events or inability to safely go about your daily activities,” she says.

She also reiterates the importance of meeting your nutrient goals within your eating windows. This may require planning what you’re going to eat in advance, including your snacks. Planning also helps avoid reaching for nutrient-poor foods such as chips or other processed foods just because they’re readily available when you’re overly hungry. 

As Harris-Pincus pointed out earlier, the main reason why intermittent fasting can lead to weight loss is because it’s a form of calorie restriction. You can maintain a calorie deficit without fasting by being conscious of what you’re eating and prioritizing nutrient-rich foods over nutrient-void ones.

Some scientific research shows that following a low-carbohydrate diet may be beneficial for weight loss. If you have any questions about diet and weight loss, it can be helpful to meet with a registered dietitian who can offer guidance based on your individual health and goals.

If you’ve tried endless diets and still feel like you’re struggling to lose weight, a weight loss injectable medication may be an option. If you qualify, medications such as Wegovy and Zepbound can help some turn off “food noise” and therefore consume fewer calories, leading to weight loss. These medications are not suitable for everyone, so you should be sure to discuss your options with a healthcare provider to learn more.

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Current version

November 07, 2024

Written by

Emily Laurence

Fact checked by

Raagini Yedidi, MD

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About the medical reviewer

Raagini Yedidi, MD

Raagini Yedidi, MD, is an internal medicine resident and medical reviewer for Ro.