Does Intermittent Fasting for Weight Loss Work?

Craig Primack, MD, FACP, FAAP, FOMA

Reviewed by Craig Primack, MD, FACP, FAAP, FOMA

Written by Lauren Panoff

Published 04/07/2024

Intermittent fasting for weight loss has gained attention as an alternative to deliberate calorie restriction. It’s also thought to benefit heart health, help regulate blood sugar, reduce inflammation and more.

Does intermittent fasting work for weight loss? It can. But it’s only one (potential) piece of a holistic approach to healthy weight management.

And despite fasting’s recent surge in popularity, there are limited human studies examining its health benefits.

We’re exploring the ins and outs of intermittent fasting for weight loss, including different ways to do it, the pros and cons, safety and what the science says.

Intermittent fasting (IF) is a way of eating that alternates between periods of eating and fasting (not eating). Some people still eat up to 25 percent of their daily calorie needs during fasting periods, while others may only have water during that time.

How does intermittent fasting work for weight loss? The idea is that it extends the period your body has to burn food energy from your last meal, after which it switches to burning fat for fuel instead.

Many people wonder, How long should you fast for weight loss? Well, there are several ways to approach an intermittent fasting diet pattern. The most common involve splitting the day or week into eating and fasting periods.

There’s 5:2 fasting, alternate-day fasting and daily time-restricted eating. Here’s what to know.

5:2 Fasting

With 5:2 intermittent fasting, you eat normally five days of the week and restrict your calorie intake the other two days. The fasting days are typically non-consecutive — like Monday and Friday, for instance.

During fasting days, people often consume around 500 to 600 calories, usually in the form of small meals or snacks spread throughout the day.

Alternate-Day Fasting

Alternate-day fasting alternates between fasting and non-fasting days — meaning every other day. When fasting, you consume minimal to no calories, eating normally on non-fasting days.

This approach allows for intermittent fasting without necessarily restricting food on a daily basis. Some may find it harder or easier than other methods.

Daily Time-Restricted Eating

Time-restricted eating restricts your eating window to a specific block of time each day, often ranging from eight to 12 hours. During the fasting period, no calories are consumed. But water, tea and black coffee are usually allowed.

Lots of folks incorporate their sleep time into their fasting period. This helps align your eating patterns with your body’s natural circadian rhythm (your internal sleep-wake cycle) and might make a longer fast more tolerable.

What’s the Best Intermittent Fasting Schedule?

The best intermittent fasting schedule is a personal choice. Consider how long you can go without food in one sitting and how the fasting days or windows will fit into your work schedule or lifestyle.

For instance, 24-hour fast benefits may include weight loss, but for many, that’s a long time to go unfed — especially with a demanding job or family obligations. As a result, it could trigger fatigue, irritability or difficulty concentrating.

Intermittent fasting offers potential health benefits like weight loss and improved metabolic health, but results can vary.

Like any diet trend, you’re wise to look at the scientific evidence for its effectiveness, safety and long-term health outcomes.

Here are some highlights from our intermittent fasting research:

  • A 2020 systematic review of trials found IF helpful for weight loss. Many participants reduced their weight with no major negative side effects, and results were comparable to or better than calorie restriction. Intermittent fasting was also found to improve blood sugar control for people with type 2 diabetes.

  • A review of four studies from 2000 to 2018 looked at IF effectiveness for weight loss. It found that IF significantly reduced LDL (low-density lipoprotein or “bad”) cholesterol, weight and fat mass, regardless of the participants’ initial body mass index (BMI). BMI measures body weight in relation to height, roughly categorizing people as underweight, normal weight or having overweight or obesity.

  • A 2023 review of 22 studies found that intermittent fasting — especially alternate-day fasting and time-restricted eating — reduced body weight, body fat and BMI. Intermittent energy restriction, like 5:2 fasting, had inconsistent results. Most studies also showed improvement in glucose metabolism and blood pressure.

  • A 2023 study looked at the effects of doing three non-consecutive days of intermittent fasting a week for 12 weeks in people with insulin-treated type 2 diabetes. It found that IF was not only safe but also effective for improving blood sugar control.

  • A 2022 meta-analysis of 22 studies with over 700 participants with overweight or obesity found that the amount of weight lost was more significant for those who did IF versus those doing caloric restriction.

Intermittent fasting might affect women differently than men, but it depends on the person.

Women may need to approach IF with more caution, especially during pregnancy, breastfeeding or menopause. The best intermittent fasting schedule for women might be different than for men, such as with a larger daily eating window.

What about intermittent fasting for men? Guys should pay attention to signs of hormonal imbalances or effects on their mood, energy levels or sex drive.

First and foremost, both men and women should listen to their bodies. Adjust fasting windows accordingly and prioritize nutrient-dense foods.

And if you’re experiencing weird, uncomfortable or concerning side effects, intermittent fasting may not be the right weight loss approach for you.

Intermittent fasting isn’t for everyone. It’s not a quick fix for weight loss or a fast track to your health goals — but many people find it helpful for certain things.

Carefully consider the pros and cons before jumping in.

Potential Benefits of Intermittent Fasting

Intermittent fasting probably wouldn’t be so popular if it didn’t offer at least some tangible benefits.

So, is intermittent fasting healthy, and what does fasting do for your body, exactly? Here are some things you might experience:

  • Weight loss. IF might reduce your overall calorie intake and support appetite-regulating hormones in a way that helps you lose weight. Time-restricted eating and alternate-day fasting more consistently lead to weight loss compared to other approaches, like 5:2 fasting.

  • Improved metabolic health. IF could help improve blood sugar control, insulin sensitivity and inflammation.

  • Healthier blood fats. Some studies show that alternate-day fasting may help lower LDL cholesterol and triglycerides (fatty acids in the blood).

  • Better brain health. More human research is needed, but animal studies show that IF can help produce brain-derived neurotrophic factor (BDNF), a protein associated with better brain function. Losing BDNF with age is linked to a higher risk of Alzheimer’s disease and other dementias.

  • Improved cellular health. Intermittent fasting supports autophagy, your body’s process of getting rid of damaged, potentially disease-ridden cells.

  • Increased lifespan. Some evidence suggests IF may improve markers of aging, making your cells more resistant to damage and potentially helping you live longer.

  • Simplification. Many people like that following a structured IF pattern helps simplify their meal-planning and food-prep efforts.

  • More mindfulness. Mindfulness means being more present while eating. If you’re a fan of structure, IF might help encourage mindfulness and foster a healthier relationship with food.

Potential Downsides and Side Effects of Intermittent Fasting

While the potential benefits of IF can be intriguing, it’s also important to consider the possible downsides. This includes:

  • Risk of nutrient deficiencies. Without proper planning, restricting food intake during fasting periods could mean you’re not getting enough of certain vitamins, minerals and macronutrients. Not getting enough protein, for instance, may lead to loss of lean muscle mass.

  • Side effects. Some people report low blood sugar, dizziness, weakness or increased urination when fasting. These side effects usually aren’t severe enough to require medical attention, but they could make you want to return to normal eating.

  • More cravings. Some people find that fasting increases hunger and cravings, which could lead to overeating.

  • Disrupted social life. Following a strict fasting schedule could interfere with social interactions, like meeting friends for dinner or attending a work party.

  • Inconsistent energy levels. Extended fasting periods or inadequate calorie intake could result in fatigue or weakness, making it harder to be active.

  • Interrupted hormones. While intermittent fasting can improve metabolic health (meaning it might help you burn energy more efficiently), some people may experience the opposite. Think increased cortisol levels, impaired thyroid function or a disrupted menstrual cycle.

  • Disordered eating. IF may exacerbate or trigger disordered eating patterns in higher-risk individuals (for instance, if you’ve struggled with this previously).

  • Rebound weight gain. Some might experience weight regain after ending an intermittent fasting plan, particularly if they return to previous eating habits without other nutrition and lifestyle changes.

Is Intermittent Fasting Safe?

Intermittent fasting may not be appropriate for pregnant or breastfeeding women, people with a history of eating disorders, certain medical conditions, those who are underweight or people with specific dietary requirements.

It’s best to check with your healthcare provider before starting any new eating plan.

Prescribed online

Weight loss treatment that puts you first

Intermittent fasting can be effective for weight loss. But it may not work for everyone, and it’s not the only option.

We’ve highlighted four things proven to be safe and helpful for sustainable weight loss.

1. Better Sleep

More rest makes you less cranky. It’s also a crucial aspect of weight loss.

Getting better sleep helps regulate appetite-regulating hormones like ghrelin and leptin.

Additionally, getting enough shut-eye reduces the likelihood of craving unhealthy foods that may counteract your weight loss efforts. On average, adults should get seven to nine hours of sleep a night.

2. More Movement

Exercise is essential for healthy weight loss.

Experts recommend at least 150 minutes of moderate-intensity activity or 75 minutes of vigorous aerobic exercise a week for general health. If weight loss is the goal, you might want to bump it up even more.

We realize this is quite a lot, especially if you’re just starting out. Incorporate daily movement into your routine, even just a few minutes a day, and work your way up.

Besides heart-pumping activities, you might want to add strength training (like lifting weights or using resistance bands) a couple times a week.

3. More Nutrient-Dense Foods

One meal or snack isn’t going to make or break you, but the types of foods you eat can support or hinder weight loss efforts.

Nutrient-dense foods — like fruits, vegetables, whole grains, nuts, seeds, legumes and lean proteins — provide vitamins, minerals, fiber and antioxidants that support healthy weight management and wellness. Good nutrition also promotes a full feeling that can help prevent cravings and overeating.

4. Weight Loss Medication

Weight loss medications can be highly effective when combined with healthy eating and lifestyle habits. They work by suppressing appetite, reducing fat or carbohydrate absorption of fat or boosting metabolism.

Some of the most effective weight loss medications include a combination of bupropion and naltrexone (sold as Contrave®), metformin and topiramate.

Another option is injectable glucagon-like peptide 1 (GLP-1) agonists — like liraglutide (Victoza®, Saxenda®) or semaglutide (Ozempic®). Weight loss injections work by mimicking the effects of the naturally occurring hormone GLP-1 to boost insulin secretion (aka lowering blood sugar), suppress appetite and promote weight loss in people living with type 2 diabetes or obesity.

IF isn’t a perfect science and has some gray areas. Here are some answers to things many people are still wondering about.

Can You Drink Water While Intermittent Fasting?

Yes! You can and should drink water during your no-food window. It’s important to stay hydrated while fasting. Plus, water is calorie-free, so it’s not going to break your fast.

What Can You Have While Fasting?

During a fast, you’re usually not supposed to eat or drink anything that could break your fast. In addition to water, some intermittent fasting plans allow non-caloric drinks like herbal tea or black coffee. These keep you hydrated and help curb hunger until your next eating window.

How Long Does It Take for 16/8 Intermittent Fasting to Work?

The time it takes for a 16/8 fast (when you eat during an eight-hour daily window and fast for the other 16 hours) to work can vary from person to person. It’ll depend on things like your starting weight, dietary habits and activity level. You might notice changes within a few weeks of sticking to a fasting schedule — like some weight loss or more energy. Significant changes in your health and weight loss will likely take longer.

Can Intermittent Fasting Cause Weight Gain?

It’s possible for IF to lead to weight gain. For instance, if your food intake is overcompensating for hunger during fasting periods, this could result in higher-than-intended daily calories.

Furthermore, even though IF focuses on timing, it’s crucial to pay attention to the nutritional quality of what you’re eating. Fasting is only one aspect of holistic weight loss.

Intermittent fasting is effective for weight loss, particularly time-restricted eating and alternate-day fasting. However, longer-term human studies are needed. If you’re considering IF for weight loss, keep these points in mind:

  • It’s not a quick fix for weight loss. Like any lifestyle change, IF requires consistency and patience to see noticeable changes. The best intermittent fasting schedule for weight loss is a personal decision.

  • It’s only one piece of the puzzle. If you decide to try IF, it should be used with other healthy habits, like better sleep, nutrition and movement.

  • There are other options. Many people find IF helpful for weight loss, but it’s not for everyone. Weight loss medications, when used with nutrition and exercise, can also be effective. Speak with your healthcare provider to figure out the best approach.

Ready to start your personalized weight loss plan? Take our free assessment to kick things off.

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