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

Intermittent fasting, which involves cycling between periods of eating and periods of avoiding or limiting food, is popular among health enthusiasts for supporting weight loss, fitness, longevity, and reducing inflammation.

There are several intermittent fasting approaches, including the twice-a-week method (5:2), alternate-day fasting, and time-restricted eating. One of the most popular is the 16/8 method, a form of time-restricted eating where you eat within an 8-hour window and fast for the remaining 16 hours each day.

But the most common question we hear is simple: How long does 16/8 intermittent fasting actually take to work?

The short answer is that it depends on both the goal and the person. Some people notice digestive changes within a week, while metabolic and weight-related benefits typically take a few weeks to appear. Longer-term benefits tend to build over months with consistency.

Fortunately, there are studies available that offer helpful insight into when different benefits tend to appear. Understanding these can help you set realistic expectations before getting started.

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

Research on intermittent fasting gives us a useful framework for what most people can expect.

Within the first week, some individuals report reduced bloating or irritable bowel syndrome (IBS) discomfort within the first week of adopting a time-restricted eating pattern, though research is still developing on this 1 2. 

Given a few weeks, many people experience modest changes in body weight along with early improvements in insulin sensitivity and markers of inflammation, as seen in clinical trials of fasting interventions 3 4.

Over several months of intermittent fasting, systematic reviews suggest that improvements in metabolic health and cellular repair can build and persist with consistent adherence, though long-term disease prevention outcomes require more research 4.

Individual results vary. Factors like starting weight, metabolic health, hormones, stress, sleep, and schedule consistency all influence how quickly benefits can appear 5.

When weight loss is the goal, many people can begin to see changes between a few weeks to a few months. Certain metabolic shifts, like improved fat burning, can occur within the first 24 hours of fasting 6.  

For gut-focused goals, particularly related to abdominal discomfort or IBS, symptom improvement has been observed within a week to 10 days in some studies 1.

Research is helping develop a useful framework for what most people can expect from 16/8 intermittent fasting results:

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”. The concept of intermittent fasting may have been inspired by specific religious ceremonies, such as Ramadan.

One example of this dietary habit is the 16/8 method, where you eat within an 8-hour window and fast for the remaining 16 hours. 

Another version, called the 5:2 method, is to eat normally on most days but then eat very few calories (500 kcal 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 and avoid potential unwanted health symptoms.

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, results may not be as effective regarding weight loss or symptom relief. Keeping balanced with healthy fats, proteins, and essential micronutrients can help get you to where you want to be.

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

The most popular approaches to intermittent fasting include the following 26:

• Time-restricted eating (TRE) involves setting eating windows and fasting windows. This method can be followed anywhere from one to seven days a week. Specifically with the 16/8 fasting method, you have an eight-hour window for eating, followed by a 16-hour fasting window.
• The 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 involves alternating between low-calorie days (often around 500 calories) and regular eating days throughout the week, creating a more frequent fasting pattern than the 5:2 approach.
• 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.

Before beginning any form of fasting or periods of caloric deficiency, especially long-term, it is best to discuss your plans with a clinician. 

Intermittent fasting may be more effective than traditional calorie restriction–eating smaller portions at regular meal times. One reason may be that it’s easier for people to adhere to long-term. Shorter eating windows may also lead to more favorable metabolic responses than calorie reduction alone.

Short-Term vs Long-Term Intermittent Fasting

Most studies define short-term intermittent fasting as three months or less, and long-term fasting as longer than three months 3.

Short-term fasting may be promising for weight loss, gut health, cholesterol balance, inflammation reduction, and metabolic improvements 6.

With longer-term consistency, intermittent fasting may also activate autophagy, a normal cellular cleanup process that helps remove damaged cells and can support long-term disease prevention.This process is a healthy part of cellular metabolism that can reduce inflammation and help prevent cancer, Type 2 diabetes, and degenerative diseases 26.

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 during periods of fasting, calorie intake drops and the body begins to shift from relying primarily on glucose to using stored fat for energy. The shift in how your body works boosts fat burning and improves insulin sensitivity. When the body is in ketosis, excess fat is used for energy, and this can lead to weight loss 26.
2. The oxidative stress and circadian rhythm theories suggest that the longer-term benefits of intermittent fasting come from eating fewer calories over time. This can lead to reduced oxidative stress, boosted antioxidant activity, and improvement of 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 26. 
3. The improved gut health theory stems from studies suggesting that intermittent fasting can improve the balance of gut bacteria 10, the hormones that control appetite 13, and potentially the health of the gut lining 11 12. 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 the digestive system time to rest and recover. When you are constantly digesting food—especially foods that are difficult to break down—the gut has fewer opportunities to repair the lining, rebalance microbes, and calm inflammation.

I often compare this to a sprained ankle. If you keep putting stress on it every day, healing tends to stall. Giving it time to rest allows recovery to happen more efficiently. The same principle applies to the gut.

For some people, traditional intermittent fasting accomplishes this well. For others, especially those with sensitive digestion, irritable bowel syndrome (IBS), or ongoing gut inflammation, fasting completely can feel too stressful on the GI system or lead to symptom flares.

This is where an elemental diet can be helpful.

An elemental diet is designed to supply many of the benefits of fasting while still providing essential nutrients in a form that requires very little digestive effort. Because the nutrients are already broken down, the gut receives nourishment without the usual workload of digestion.

In practice, some people use an elemental formula to support a modified fasting approach. This can look like replacing one meal per day, often breakfast, while eating whole foods at other meals. This approach may allow the gut to rest, similar to fasting, without the strain that complete food restriction can place on the body 27.

For certain individuals, this combination can feel more sustainable and gentler, while still supporting improvements in digestive symptoms.

Personal Success Story 

In clinical practice, we sometimes see these effects play out quickly.

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

While you may not have gotten to the point of pursuing a surgical approach just yet, you may relate to many of my clients who feel as though they’ve run out of ideas and are left only with options such as medical treatments. If fasting could be the change that makes a real difference, it’s worth trying. Consider this ideally with the support of a healthcare provider.

Frequently Asked Questions About 16/8 Intermittent Fasting

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

When it comes to timing, earlier eating windows appear to work better than later ones.

A meta-analysis comparing several randomized controlled trials suggests that earlier 16/8 eating windows, typically starting before 10 a.m., may be associated with better results than those who pushed their eating schedule to later in the day 18. 

Additional evidence shows that compared to people who did not fast, early 16/8 fasters experienced modest weight loss, lower fasting blood sugar, reduced inflammation, and improvements in insulin sensitivity and blood pressure. When early and late 16/8 fasters were compared directly, the early group still came out ahead, with slightly more weight loss and better blood sugar control 18.

There is also evidence that earlier eating windows may support healthier gut microbiota diversity compared to later ones 19.

Is 16/8 Intermittent Fasting Safe?

For many people, intermittent fasting is well tolerated. That being said, outcomes and tolerability can vary by person.

Some people notice side effects, especially early on. These can include low blood sugar, dizziness, or feeling weak. Loss of muscle mass is less likely when protein intake is adequate during eating windows 22.

You may have seen headlines about an unpublished study suggesting that eating within an 8-hour window was linked to a higher risk of heart-related death 28. That finding raised concerns, but the study relied on self-reported data and did not account for factors like stress, sleep, or overall diet quality.

When we compare that study to higher-quality research, we believe the result is more likely due to methodological limitations than intermittent fasting itself.

Additionally, research from a large epidemiological study suggests IF may be associated with increased disordered eating symptoms in some individuals, particularly adolescents and young adults. Psychological factors such as impulse control difficulties may further influence how appropriate or safe this approach is on an individual level, but it is best to approach this with some caution 29 30.

It is always wise to talk with a healthcare provider before starting any fasting plan.

Who Should Avoid 16/8 Intermittent Fasting?

Some people need to approach intermittent fasting more carefully, or avoid it altogether.

Take extra caution if you 22 31:

• 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 a history of disordered eating
• Have been diagnosed with dementia

People with diabetes, especially those using insulin, may be more sensitive to blood sugar swings while fasting.

No matter your health history, paying attention to how your body responds is key. If symptoms appear or worsen, your healthcare provider may recommend adjusting the fasting window or choosing a more suitable approach.

Why Am I Not Losing Weight on 16/8 Intermittent Fasting?

Not losing weight on 16/8 intermittent fasting is more common than people expect, especially early on.

While 16/8 can support fat loss, it does not guarantee weight loss on its own. Several factors can slow or stall progress. This may include eating more calories than expected during the eating window, frequently choosing ultra-processed foods (UPFs), chronic stress, poor sleep, or hormonal and metabolic differences 32 33.

It is also worth noting that some people experience metabolic improvements, such as better blood sugar control or reduced inflammation, before seeing any weight changes. In these cases, weight loss may arrive later compared to other health benefits.

If weight loss is the primary goal and progress has not occurred after several weeks, adjusting food quality, portion sizes, protein intake, or the timing of the eating window may help. In some cases, a less restrictive or more supportive approach may be a better fit.

What Are the Signs That 16/8 Intermittent Fasting Is Working?

Signs that intermittent fasting may be working include 6:

• Reduced bloating or digestive discomfort
• More stable energy levels between meals
• Improved hunger awareness or fewer cravings
• Weight loss
• Better blood sugar control
• Improvements in lab markers such as insulin, triglycerides, or inflammation
• Better tolerance of longer periods between meals

For gut-focused goals, some people notice symptom relief, such as less bloating or abdominal pain, within the first one to two weeks. For metabolic goals, changes often emerge over several weeks.

If symptoms worsen or new issues develop, that can be a sign that the fasting approach needs to be adjusted or paused.

Start Your 16/8 IF Journey Safely

So, how long does 16/8 intermittent fasting take to work? For some, digestive relief may begin within a week. Weight loss and metabolic improvements often emerge over the course of a few weeks. Longer-term benefits build with consistency.

Intermittent fasting may not be appropriate for everyone, and its effects can vary based on health history, certain medication use, and lifestyle factors. We recommend discussing any fasting approach with a knowledgeable healthcare provider and paying close attention to how your body responds.

If you would like individualized guidance, our virtual clinic team can help determine whether intermittent fasting–or a modified approach–fits your health goals.

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