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

Written by Dr. Michael Ruscio, DC on February 7, 2025

Written by Dr. Michael Ruscio, DC on February 7, 2025 — Reviewed by Hilary Watts

• How Long Does It Take for 16/8 to Work?|
• What Is Intermittent Fasting?|
• Gut Health and IF|
• What Is the Best Way to Do 16/8?|
• Is Intermittent Fasting Safe?|

Intermittent fasting is popular among health enthusiasts for helping with weight loss, fitness, inflammation, and longevity. 

Intermittent fasting, or IF, simply means cycling between periods of eating and periods of avoiding or limiting food. It’s essentially, time-based calorie restriction. You can practice this type of fasting in a variety of ways, such as the twice-a-week method (5:2), alternate-day fasting, or time-restricted eating. 

The focus of this article, 16/8 intermittent fasting, is a popular form of time-restricted eating where you have an 8-hour eating window followed by 16 hours of fasting every day.

How long does it take for 16/8 intermittent fasting to work? Keep reading to find out. 

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

Research on how long it takes for various types of intermittent fasting to work gives us some hints about what’s possible when following the 16/8 method:

• If calming irritable bowel syndrome (IBS) symptoms is your goal, then you may feel better within 1 week. 
• If you’re hoping to lose weight, improve your metabolic health, and lower inflammation, you’ll likely need to practice 16/8 intermittent fasting for at least 3 weeks. 
• If you’re looking for sustained benefits, like preventing disease and boosting overall health, an intermittent fasting schedule may be a key part of your long-term diet strategy.

Every human body is unique. The answer to how long it takes for 16/8 intermittent fasting to work for you depends on many health and wellness factors, including your heart health, extra weight, genetics, hormonal fluctuations, and stress levels. These factors not only play a role in the diet’s impact, but they may also affect how willing and able you are to stick with it long enough to reap benefits.

When weight loss is your goal, how quickly it happens depends on your starting weight, body composition, metabolism, and how well you stick to the schedule. Therefore, most people might expect weight loss to start between a few weeks and a few months 1.

That’s a pretty broad timeline, but it’s encouraging to note that certain metabolic functions can change within the first 24 hours of short-term fasting 2. For example, an 8-hour eating window can positively shift your ability to burn sugar and fat almost immediately, so sticking with it could yield the fat loss you’re seeking.

If, on the other hand, you’re experimenting with intermittent fasting to address irritable bowel syndrome, you might see symptom improvements in just seven to 10 days 3.

Here’s a chart showing what research says about the health benefits of different kinds of intermittent fasting and the period of time it often takes to see results:

Short-Term vs Long-Term Intermittent Fasting

The studies we looked at considered short-term fasting to be three months or less and long-term fasting to be longer than three months.

The benefits of short-term intermittent fasting are promising for goals like 2:

• Weight loss
• Improving gut health
• Lowering cholesterol
• Reducing inflammation
• Improving metabolic health

Long-term intermittent fasting can provide those same benefits and also enable the body to break down and get rid of dysfunctional cells through a process called autophagy. Autophagy also helps destroy bacteria, viruses, or other microorganisms that could cause infection. 

This process is a healthy part of cellular metabolism that can reduce inflammation and help prevent cancer, Type 2 diabetes, and degenerative diseases 23.

What Is Intermittent Fasting?

Instead of completely avoiding food for days like in a traditional fast, intermittent fasting is an eating plan in which you eat during short time periods called eating windows. 

For example, with the 16/8 method, you eat within an 8-hour window and fast for the remaining 16 hours. 

Another version is to eat normally on most days but then eat very few calories (500 or less) for one or two non-consecutive days each week.

Intermittent fasting is an effective mode of calorie restriction, so the quality of foods you eat matters. Following a healthy, balanced diet during your eating windows is important for the plan to work well.

An example of a healthy dietary pattern to follow on an intermittent fasting plan is a Paleo diet. If you fill your eating window with unhealthy foods or with overeating, you’ll likely slow down your weight loss or symptom relief. 

Similarly, regular physical activity, good sleep, and stress management can greatly improve your chances of success.

The most popular approaches to intermittent fasting include the following 23:

• Time-restricted eating (TRE) involves set eating windows and fasting windows and can be done anywhere from one to seven days a week. With the 16/8 fasting method, you have an eight-hour window for eating, followed by a 16-hour fasting window.
• Twice-a-week (5:2) method means cutting your calorie intake to about 500 calories on two nonconsecutive days each week. On the other five days, you eat a whole-foods diet that includes healthy fats, high-quality carbohydrates (veggies, fruits, and whole grains), and lean proteins.
• Modified alternate-day fasting means eating about 500 calories every other day or on two nonconsecutive days of the week while eating a whole-foods diet on the other days.
• 24-hour fast (Eat: Stop: Eat method) involves staying in a fasted state (only drinking water) for 24 hours once or twice a week while eating a whole-foods diet on the other days.

Intermittent fasting appears to work better than traditional calorie restriction, where you just eat smaller portions at regular meal times. This could be because it’s easier for people to follow long-term. Plus, the body seems to react more positively to shorter eating windows than to just cutting calories.

How Does Intermittent Fasting Work?

Here are some scientific theories about how intermittent fasting works:

1. The ketosis theory, which is popular among both scientists and nonscientists, suggests that the short-term benefits of intermittent fasting may arise as a result of eating fewer calories. The shift in how your body works boosts fat burning and improves insulin sensitivity. As a result, you burn more fat, store less body fat, and lose weight 23.
2. The oxidative stress and circadian rhythm theories suggest that the longer-term benefits of intermittent fasting come from eating fewer calories over time, which reduces oxidative stress, boosts antioxidant activity, and helps improve the body’s 24-hour sleep-wake cycle (circadian rhythm). These benefits can reduce inflammation in your cells and promote autophagy (the process of breaking down and removing dysfunctional cells), which may lower the risk of chronic diseases 23. 
3. The improved gut health theory comes from studies showing that intermittent fasting can improve the balance of gut bacteria 7, the hormones that control appetite 10, and likely the health of the gut lining 8 9. These changes may help explain the benefits of intermittent fasting for body composition and metabolic health.

Gut Health and Intermittent Fasting

As I discuss in Healthy Gut, Healthy You, periodic fasting can give your gut the chance to rest and heal. I like to compare giving your gut a break to resting a sprained ankle. If you keep running on a sprained ankle every day, it probably won’t heal properly, or it will take much longer to heal than if you rested it.

The same goes for your gut—if it’s damaged and you’re eating foods that take a lot of energy to digest three times a day, it will be much harder for your gut to heal quickly.

An effective form of intermittent fasting for improving gut symptoms is the elemental diet. Some people feel much better by eating only two meals a day, replacing the third (usually breakfast) with an elemental diet. For some people, this simple change can make a big difference 24.

Patient Success Story 

I had a patient named Kacheena who struggled with severe IBS symptoms for most of her adult life. She tried different home remedies, like cutting out certain foods, but her symptoms kept getting worse, especially during her menstrual cycle. Kacheena often had to miss work during this time and even planned to get a hysterectomy. 

She decided to try Elemental Heal, a liquid diet designed to mimic fasting while providing easily absorbed nutrients. She started by replacing her breakfast with it and noticed improvements in her symptoms within a few days. After about two weeks, her symptoms and overall quality of life had improved so much that she canceled her surgery. 

This may sound like an extreme example, but many of my patients feel like they’ve run out of options and are left only with medical treatments like surgery. If fasting could be the change that makes a real difference, it’s worth trying, ideally with the support of a healthcare provider.

What Is the Best Way to Do 16/8 Intermittent Fasting?

Research suggests that eating earlier in the day gives you the best results. 

Studies comparing regular eaters, 16/8 fasters who started eating before 10 am, and 16/8 fasters who started eating at 12 pm or later have found that 16/8 fasters who ate earlier in the day had the best results 15.

Compared to the regular eaters, the early 16/8 fasters had:

• Notable weight loss (-4 lbs)

• Reduced inflammation 
• Lower fasting glucose (-5 mg/dL)
• Improved insulin resistance and blood pressure 
• No changes in cholesterol, triglycerides, HbA1c, or fasting insulin

Compared to the late 16/8 fasters, those who started eating before 10 am also had:

• More weight loss (by 1.5 lbs)
• Lower fasting glucose (by 2.77 mg/dL)
• Improved insulin resistance and fasting insulin levels

Early 16/8 intermittent fasting can also improve gut microbiota diversity better than the late type 16.

Is Intermittent Fasting Safe?

The benefits of intermittent fasting may be numerous, but little is known about the potential negative side effects in humans. Although one study found no serious side effects in people with multiple sclerosis, everyone may react differently 25. 

Possible side effects of intermittent fasting include 19:

• Low blood sugar (hypoglycemia)
• Dizziness
• Weakness
• Loss of muscle mass (unlikely if you get enough lean protein during your eating windows)

We looked further into safety after reading an unpublished study presented at an American Heart Association meeting. The study relied on self-reported data and didn’t account for stress or sleep issues, but it suggested that people eating within an 8-hour window had a 91% higher risk of death from heart disease compared to those eating within a 12- to 16-hour eating window 26.

These results are concerning, but the study’s methods made us question its findings. After comparing it to higher-quality research, we believe this alarming finding is likely based on flawed research.

Regardless, it’s always a good idea to talk with your doctor or healthcare provider before starting a new diet plan.

Who Should Avoid Intermittent Fasting?

Certain folks have a greater risk of side effects from 16/8 intermittent fasting. Everyone should talk to a knowledgeable healthcare provider before starting an intermittent fasting diet, but take extra caution if you 19 27:

• Have hormonal imbalances
• Are under the age of 25
• Are pregnant or breastfeeding
• Use insulin or other diabetes medications to control your blood sugar levels
• Have a seizure disorder
• Work the night shift
• Operate heavy machinery
• Are elderly
• Have an immune deficiency or use immunosuppressive drugs
• Have disordered eating
• Have dementia

Additionally, people with diabetes may be more susceptible to blood sugar swings while intermittent fasting, especially if you’re taking insulin.

Regardless of your health status, it’s important to listen to your body. If you begin to feel negative side effects, tell your doctor. They may advise you to slow down, take a break, or consider a less intense intermittent fasting protocol. 

Start Your 16/8 IF Journey Safely

How long does 16/8 intermittent fasting take to work? The time it takes to see intermittent fasting results will be unique to you and what health benefits you’re seeking. IBS can be calmed within a week and weight loss can happen around three weeks along with lower inflammation and metabolic health improvements. Some people stick to an intermittent fasting schedule long-term to boost overall health and prevent disease.

While intermittent fasting is generally safe for people with chronic illness, the effects of intermittent fasting are still not well understood. Discuss intermittent fasting with your healthcare provider and listen to your body. If you need more personalized guidance, please contact us at our virtual clinic.

The Ruscio Institute has developed a range of high-quality formulations to help our clients and audience. If you’re interested in learning more about these products, please click here. Note that there are many other options available, and we encourage you to research which products may be right for you. The information on DrRuscio.com is for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment.