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How Long Does Intermittent Fasting Take to Work?

Written by Dr. Ruscio on November 2, 2022

Written by Dr. Ruscio on November 2, 2022 — Reviewed by Hilary Watts

Dr. Michael Ruscio is a DC, Naturopathic Practitioner, researcher, and clinician. He serves as an Adjunct Professor at the University of Bridgeport and has published numerous papers in scientific journals as well as the book Healthy Gut, Healthy You. He also founded the Ruscio Institute of Functional Health, where he helps patients with a wide range of GI conditions and serves as the Head of Research.

When You Can Expect To See Results from Intermittent Fasting

• How Long Does Intermittent Fasting Take to Work?|
• What is Intermittent Fasting?|
• Gut Health and Intermittent Fasting|
• Is Intermittent Fasting Safe?|
• Bottom Line|

Intermittent fasting has become popular among health enthusiasts for everything from weight loss and fitness to longevity and inflammation. Intermittent fasting simply means you’re cycling between periods of eating and periods of avoiding or limiting food (essentially caloric restriction). This type of fasting can be practiced in a variety of ways such as time-restricted eating (TRE), the twice-a-week method (5:2), and alternate-day fasting. 

But how long does intermittent fasting take to work? It’s different for everyone and really depends on your overall goal. If you’re using intermittent fasting to improve your gut health and symptoms, you may notice benefits after just a few days. For weight loss and improved metabolic health, it could take a month. If your goal is to increase longevity and overall health, you’ll likely need to incorporate intermittent fasting consistently for the long term. 

It’s important to note that intermittent fasting results also depend on your overall diet. If you’re eating processed and inflammatory foods during your feeding windows, you’re not going to experience the maximum result, and you may not meet your health goals at all. 

In this article, I’ll introduce you to the various types of intermittent fasting diets and their potential health benefits. I’ll also discuss what the research says about how long it takes intermittent fasting to work, touch on safety concerns, and share how my patient Kacheena used a form of intermittent fasting to heal her lifelong IBS symptoms in a matter of days. 

How Long Does Intermittent Fasting Take To Work?

While the effects of intermittent fasting will be unique to each individual and highly depend on your individual health goals, the research suggests you may realize benefits after a few days. A 2022 literature review found just one to two fasts could alter sugar (glucose) and fat metabolism, hormone levels, and your sleep-wake-cycle [1]. 

And in one randomized control trial of patients with irritable bowel syndrome (IBS), just seven to ten days of intermittent fasting led to significant improvements in abdominal pain, bloating, diarrhea, nausea, and anxiety [2].  

Whether you’re hoping to decrease your body weight or improve your health for the long-run, here’s a chart summarizing what the research says about fasting benefits and the period of time it takes to see results:

As you can see, the benefits of short-term intermittent fasting are promising when it comes to things like weight loss and gut and metabolic health. Long-term intermittent fasting will provide those same benefits and also enable autophagy, which may contribute to lifelong health benefits like decreased inflammation and reduced risk of chronic disease [23]. 

It’s unclear if you’ll still reap the benefits of short-term intermittent fasting after going back to your usual eating pattern. I suspect you’ll only see sustained benefits by creating an intermittent fasting routine that you can continue consistently. If you sabotage your efforts by eating unhealthy foods or over-eating during your feeding window, it’s highly likely that you will hinder or even negate your efforts.

You’ll probably get more benefits from intermittent fasting by incorporating some additional lifestyle changes like an anti-inflammatory diet (such as Paleo), healthy sleep, adequate sun exposure, and stress management.

Now that you’ve had a sneak peek into how long it takes to experience the potential benefits of intermittent fasting, let’s define it and discuss the different types of intermittent fasting.

What is Intermittent Fasting?

Intermittent fasting is an eating pattern where you cycle between periods of eating and periods of avoiding or limiting food [23]. Rather than a traditional fast where you avoid all food intake for days at a time, intermittent fasting is a dietary pattern that allows for the consumption of food during an eating window. 

Intermittent fasting is pretty flexible and seemingly easier to incorporate into your routine when compared to strict calorie restriction and traditional fasting.

The most popular types of intermittent fasting include [23]:

• Time-restricted eating (TRE): Includes set fasting and eating windows and is practiced between one and seven days per week. For example, 16/8 fasting means your eating period is an eight-hour window followed by a 16-hour fasting window.
• Twice-a-week (5:2) method: Includes restricting your calorie intake (often to 500 calories) for two nonconsecutive days per week and then eating a whole-foods diet (healthy fats, high-quality carbohydrates, whole grains, and lean proteins) the other five days per week.
• Modified alternate-day fasting: Includes a modified fast (about 500 calories) every other day or two nonconsecutive days of the week and then eating a whole-foods diet on the other days.
• 24-hour fast (eat: stop: eat method): Includes a fully fasted state (consuming only water) for 24 hours once or twice a week and then eating a whole-foods diet on non-fasting days.

There are no strict rules when it comes to an intermittent fasting schedule. You may initially want to experiment with each type to see what works best for you, but there’s no reason you can’t alternate between different forms if you find you prefer more than one intermittent fasting method. It’s probably best to go slowly to determine your individual response [24, 25].  

Keep in mind, taking the time to figure out which method is best for you will vary and can affect how quickly you see results. Now let’s review some of the theories as to how intermittent fasting may provide its benefits.

How Does Intermittent Fasting Work?

As I discuss in Healthy Gut, Healthy You, periodic fasting can give your gut the chance to rest and heal. The analogy for the effects of fasting I like to use is healing a sprained ankle. 

If you run three miles a day with a sprained ankle, how effectively would it heal? Similarly, if your gut is ‘injured’ and you’re eating three meals a day with limited time to rest, how well will your gut heal?  While my analogy can drive the point home, here’s what the science says about how intermittent fasting works.

1. The ketosis theory (the most popular theory among scientists and laypeople) describes the short-term benefits of intermittent fasting by suggesting that intermittent fasting leads to fewer calories being consumed, which shifts the metabolism toward more fat-burning and greater insulin sensitivity. This metabolic shift allows you to burn more fat, store less body fat, and ultimately leads to weight loss [23].
2. The oxidative stress and circadian rhythm hypotheses describe the longer-term benefits of intermittent fasting by suggesting that intermittent fasting leads to lower caloric intake, which reduces oxidative stress and increases antioxidant activity while also improving circadian rhythm (the 24-hour sleep-wake cycle). These benefits lead to less cellular inflammation and more autophagy (breakdown and disposal of dysfunctional cells), which may ultimately lower the risk of chronic diseases [23]. 
3. The improved gut health theory is based on human clinical trials where intermittent fasting appears to improve gut microbiota composition [26], [15, 17, 27, 28], the gut hormones that regulate appetite [9], and gut wall health [2, 4, 10], which may partly explain the benefits of intermittent fasting on body composition and metabolic health.

It’s one thing to hear about intermittent fasting in promising research, but you may be wondering if it works in the real world. While everyone will respond differently to intermittent fasting, it can be helpful to hear from a real person’s experience. 

Let’s take a closer look at a patient of mine, Kacheena, who used intermittent fasting to heal her IBS symptoms.

Gut Health and Intermittent Fasting

When it comes to improving gut symptoms, there’s another very effective form of intermittent fasting, the elemental diet. Some people feel much better when they don’t eat frequent meals but rather only eat two meals per day and/or substitute a liquid elemental diet for the other meal. In fact, for some people, this one change can make a world of difference [29].

Let’s take a look at this in action with Kacheena who suffered pretty severe IBS symptoms for most of her adult life. She tried various home remedies like limiting certain foods, but her symptoms continued to get worse, especially during her menstrual cycle. Kacheena routinely had to miss work during these times and she contacted her gynecologist for a hysterectomy. 

Kacheena decided to try Elemental Heal, a liquid diet with partially digested nutrients that make absorption so easy, it’s almost like fasting (sometimes thought of as a “fast-mimicking” diet). She initially used it to replace her breakfast meal and noticed symptom improvement within days. After about two weeks, her symptoms and quality of life had improved so much that she made the decision to cancel her surgery. 

This may sound like a pretty extreme example, but many of my patients come in feeling they have no avenues left to pursue beyond medical interventions like surgery. If there’s a chance that fasting is the change that ultimately makes the difference, it seems worth exploring.

Kacheena continues to use Elemental Heal as a meal replacement two to three times per week for maintenance and prevention, which is a really great example of how intermittent fasting can be modified to fit your needs and specific situation. 

Many nutritionists recommend eating three meals a day, so you may be wondering if it’s safe to practice intermittent fasting. Let’s take a look at what the research says.

Is Intermittent Fasting Safe?

While the benefits of intermittent fasting seem great, little is known about the potential negative side effects in humans. One randomized controlled trial found no serious side effects of intermittent fasting in chronically ill people, but everyone will respond differently [30]. Here are some possible side effects of intermittent fasting to watch for, especially if you’re a beginner [20]:

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

If you want to consider an intermittent fasting plan, speak with your healthcare provider first and use caution if you [20, 31]:

• Have hormonal imbalances
• Are under age 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

As mentioned, it’s important not to push yourself or rush the process of integrating intermittent fasting into your life. Listen to your body — if you begin to feel side effects slow down, take a break before trying again, and/or consider a less intense intermittent fasting protocol, like Elemental Heal. 

You may be more susceptible to blood sugar swings while intermittent fasting if you are diagnosed with diabetes (especially if taking insulin), so use your discretion. And always feel free to discuss your options with a nutrition-savvy healthcare provider or dietician.

Intermittent Fasting Can Provide Benefits in a Matter of Days

How long does intermittent fasting take to work? The time it takes to see intermittent fasting results will be unique to you, but the research suggests you can experience some health benefits (like improved gut symptoms) after just a few days. 

If you’re hoping to lose some weight, improve your metabolic health, and lower inflammation, you’ll likely need to practice intermittent fasting for at least a month.  When it comes to sustained benefits like disease prevention and overall health, it’s probably best to practice intermittent fasting in some form consistently for the long term. 

It’s worth mentioning that you’ll gain even more benefits from intermittent fasting if you’re following a whole-food diet, avoiding processed and junk foods, and incorporating other healthy lifestyle foundations like physical activity, time in nature, and quality sleep. 

While intermittent fasting has been found to be safe for chronically ill people, the effects of intermittent fasting are still not well understood. It’s best to first discuss intermittent fasting with your healthcare provider, then take it slow and practice caution.  If you’re in need of more personalized guidance, please contact us at the Ruscio Institute for Functional Healthcare.

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