Intermittent Fasting - familydoctor.org (2024)

You may have heard of the term intermittent fasting. It’s a popular topic nowadays. Some people talk about it when looking for a way to lose weight. Others talk about it as a way to improve their overall health. But what is intermittent fasting? Is it good for your body? What are the benefits? Are there side effects? Is it right for everyone?

Path to improved health

Intermittent fasting is not a diet plan. While it may have some of the same health benefits as a diet, it’s really an eating pattern. It means you fast (don’t eat) during a certain period of time each day (usually an extended period of time). You then eat during another period of time each day (usually a smaller period of time). While you’re fasting, you can drink beverages that don’t contain calories, including water, black coffee, and unsweetened tea.

How fasting and eating is divided each day is called an eating schedule. One of the most common, easy-to-follow schedules is 16:8. This means you fast for a 16-hour period of time and eat your daily meals during an 8-hour period of time. For example, you may want to fast from 7 p.m. until 11 a.m. the next day. You would then eat a healthy lunch and dinner between 11 a.m. and 7 p.m. You wouldn’t eat anything after 7 p.m. until 11 a.m. the next day. This is just an example of times. You can select any 16-hour and 8-hour block of time that works best for your schedule. But it’s important to keep your eating window at the same every day.

Other intermittent fasting schedules include 18:6 (when you fast for 18 hours and eat for 6 hours) or alternating days. With alternating days, you fast for 24 hours, then eat a healthy diet for the next 24 hours, then fast again for the following 24 hours. This schedule continues using the every-other-day format. Another schedule option is 5:2. This is when you fast for two days a week, and eat a normal, healthy diet the other five days. This is a little different, though, as this schedule allows you eat one small meal of 500 to 600 calories on your fasting days.

The time you’re allowed to eat is called your eating window. During your eating window, focus on eating a healthy diet and maintaining portion control. Don’t eat too many calories and avoid junk food and fast food. While you don’t need to eat anything in particular, you do need to make sure you’re getting the nutrition you need. Some people choose to use the Mediterranean diet as a guideline for what to eat. This plan focuses on fruits, vegetables, beans, whole grains, and fish. You can also choose lean proteins and healthy fats.

How intermittent fasting works

Fasting for at least 12 hours changes how your metabolic system works. Your metabolic system is how your body changes the foods and beverages you consume into energy. Most of the time, your body gets its energy from a sugar called glucose. Glucose is found in the foods you eat and beverages you drink. When you eat three meals during a day, your body maintains a steady glucose level because you’re eating and drinking frequently.

However, when you fast for more than 12 hours, your body’s glucose levels start to dip because you’re not eating as frequently. When your body doesn’t have the glucose it needs for energy, it taps into your body’s fat for energy. When this happens, the fatty acids in your body are absorbed into your blood. They produce a chemical called ketones. Your body then uses the ketones as its energy source. This is called a metabolic switch. Your body is switching from glucose to ketones.

When your body uses ketones instead of fat, you may lose weight. But, behind the scenes, the ketones also may be having a positive effect on your body’s organs and cells.

To get the benefits of intermittent fasting, you need to fast for at least 12 hours. That’s how long it takes your body to switch from using glucose for energy to using fat for energy. Additionally, it will take your body a while to get used to this new eating schedule. So don’t expect results right away. You may need to wait between 2 and 4 weeks to see or feel any results.

Health benefits of intermittent fasting

While researchers are still studying intermittent fasting, some research has shown it offers some health benefits. For starters, it’s common to lose weight when following intermittent fasting. That’s because your body is using fat—not glucose—as its energy source. Additionally, if you make wise food choices when you do eat, you’re likely eating fewer calories than before you started intermittent fasting. If you add exercise to the mix, it’s a great combination for not only weight loss, but also improved health benefits. Intermittent fasting may help people who have cardiovascular disease, neurological disorders, and some cancers. Intermittent fasting may also help lower your bad cholesterol and improve symptoms of arthritis.

Be sure to talk with your doctor before you begin intermittent fasting. They will help you decide if it’s a good fit for you. They will consider your current health, medicines, and health history when making their recommendation. If you have certain chronic health problems such as diabetes or heart disease, you may need to adjust or monitor your eating patterns.

Intermittent Fasting - familydoctor.org (2024)

FAQs

Why don t doctors recommend intermittent fasting? ›

Intermittent fasting is safe for many people, but it's not for everyone. Skipping meals may not be the best way to manage your weight if you're pregnant or breast-feeding. If you have kidney stones, gastroesophageal reflux, diabetes or other medical problems, talk with your doctor before starting intermittent fasting.

How much weight can you lose in a month with intermittent fasting? ›

If you consistently stay active while following intermittent fasting, you can lose as much as 6-8 pounds per month. This estimation is shockingly high when you consider that all you have to do is not eat for select hours throughout the day.

Is it better to fast 12 or 16 hours? ›

A good rule of thumb for those wanting to fast to improve their metabolic and overall health is to leave at least 12 hours between meals – ideally more – on a regular basis, according to Dr Adam Collins, Principal Teaching Fellow in Nutrition at the University of Surrey.

How long does it take for 16:8 intermittent fasting to work? ›

Additionally, it will take your body a while to get used to this new eating schedule. So don't expect results right away. You may need to wait between 2 and 4 weeks to see or feel any results.

Why intermittent fasting is not healthy? ›

Longer periods without food, such as 24-, 36-, 48- and 72-hour fasting periods, are not necessarily better for you, and may be dangerous. Going too long without eating might actually encourage your body to start storing more fat in response to starvation.

Why is 16 hours the magic number for fasting? ›

While both 12-hour and 16-hour fasting can promote fat burning, it's possible that a 16-hour fast may be more effective. This is because the longer fasting period may lead to a greater depletion of glycogen stores, which can trigger higher levels of fat burning.

How long does it take to lose 10 lbs intermittent fasting? ›

Weinandy says that results are usually apparent around the 10-week mark, adding that she sees an average weight loss of seven to 10 pounds over that period through any of the three approaches to intermittent fasting mentioned previously. It might take longer than that since everyone's metabolism works differently.

What is the best intermittent fasting window to lose belly fat? ›

A 2023 review of research found that the 16/8 method and 16/8 combined with restricting calories were effective strategies for weight control in adults with overweight or obesity. An eating window starting before noon led to greater weight loss than one that began after noon.

How long does it take to lose 30 pounds with intermittent fasting? ›

People who do some form of full-day fasts usually lose about 10 to 30 pounds over a three-to-six-month period, says Varady. Those who do time-restricted fasting, or fast for a window of time daily, tend to drop about 10 to 15 pounds. Fasting turns out to be surprisingly simple in practice (if a little intimidating).

What foods should you avoid during intermittent fasting? ›

It's best to limit ultra-processed foods like packaged snacks, deep-fried items, sugary drinks, and most frozen meals. These negate the positive effects of 16/8 intermittent fasting. To begin 16/8 intermittent fasting, choose an 8-hour window and limit your food intake to that time span.

What are the negatives of intermittent fasting? ›

As much as IF is filled with benefits, there are mild side effects that can happen during fasting, which do not generally require medical or surgical treatment. Possible side effects can include dizziness, nausea, insomnia, headache, weakness, etc.

Does sleep count as fasting? ›

The easiest way to do the 12-hour fast is to include the period of sleep in the fasting window. For example, a person could choose to fast between 7 p.m. and 7 a.m. They would need to finish their dinner before 7 p.m. and wait until 7 a.m. to eat breakfast but would be asleep for much of the time in between.

Why am I not losing weight on 16:8 fasting? ›

If you've tried intermittent fasting but aren't losing weight, possible reasons why include overeating during your eating window and poor food choices. To help, you can try eating fewer calories, work on balancing your meals, or create a smaller or larger eating window.

How can I speed up weight loss on intermittent fasting? ›

Also, note that the key to weight loss with intermittent fasting is not to overeat during your eating windows. Eating fewer calories than you expend remains the basis for losing weight. Shortening the eating window may make it difficult to get the vitamins and minerals you need.

What happens after 1 month of intermittent fasting? ›

The 20/4 fasting method is linked with cardiovascular improvements, such as reduced blood pressure and cholesterol levels. These changes directly contribute to a lower risk of developing heart disease, strokes, and other cardiovascular conditions, underscoring the heart health benefits of this fasting regimen.

What is the controversy with intermittent fasting? ›

A new study links intermittent fasting to a higher risk of early death instead of longevity. Some experts are skeptical, citing major limitations with the study like other lifestyle variables. It's too early to say if fasting has long-term risks, so focus on what you eat instead of when.

Is there no benefit to intermittent fasting? ›

Health claims not as significant as once thought. New research published in Aging Biology finds weight loss experienced by people on intermittent fasting diets, such as the popular 5:2 diet, does not bring the same health benefits as other weight loss programs.

Is intermittent fasting hard on your heart? ›

On March 18, the American Health Association announced study results linking intermittent fasting with an increased risk of dying from cardiovascular disease.

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