18/6 Intermittent Fasting: How Effective Is It?

Intermittent fasting, or time-restricted eating, has become popular as a diet strategy for achieving a myriad of health benefits. Among the various fasting protocols, 18/6 intermittent fasting seems to be one of the most popular methods. 

While I could never fast for such a long period of time, it’s my responsibility as a dietitian to understand the ins and outs of it. Many people find it helpful in achieving their desire body weight, while research studies have shown it can improve metabolism, lower inflammation, decrease blood sugar levels, and improve asthma.  

A clock next to a plate of nutrient dense food with the title text box 18/6 intermittent fasting: how effective is it?

This post explores all the latest research regarding 18/6 intermittent fasting – how effective is it? What benefits can it provide? Does intermittent fasting promote a faster weight loss? What are the reasons you should try it? 

Let’s dive into the the intricacies of 18/6 intermittent fasting and explore if it can significantly impact your health and well-being.

What Is 18/6 Intermittent Fasting?

There are quite a few ways intermittent fasting can work – with 18/6 intermittent fasting being a moderately strict approach. This method requires you to fast for 18 hours, with a 6-hour eating window. 

During this fasting period, individuals abstain from consuming calories with the proposed notion that the body will experience a number of physiological changes. The whole idea of intermittent fasting is allowing our insulin levels to decrease for a long enough period of time where we actually start to burn fat. 

This 2019 study from the New England Journal of Medicine states that there is emerging evidence that eating during a 6-hour period and fasting for 18 hours leads to a metabolic switch from glucose-based to ketone-based energy – which may increase longevity and decrease risk of certain diseases. 

Choosing Your Fasting Window

While 18/6 intermittent fasting is quite strict, there are no rules when choosing your fasting window. It’s best to select  a 6-hour window that fits well into your daily routine and lifestyle. It could be from noon-6 pm, or 2-8 pm. However, some experts claim that selecting the eating period earlier in the day may promote better results when it comes to improving blood sugar levels. 

Prioritizing Nutrient-Dense Foods

Because the eating window is so small, every food should pack a punch to ensure you’re consuming vital nutrients. Think a variety of fruits and vegetables, whole grains, and 3-4 serving sizes of lean protein. 

Protein can assist with positive changes to our body composition, so you might need to get creative with your meals. Incorporating Greek yogurt or cottage cheese to increase the protein content of meals, or adding chia or hemp seeds for additional fiber and omega-3 fatty acids. 

Eating adequately with the 18/6 intermittent fasting method may require a bit more planning and preparation so that nutritionally dense foods are prioritized.

A nutrient dense meal for 18/6 intermittent fasting: avocado toast with a poached egg on a plate.

Staying Well-Hydrated

It’s extremely important to stay hydrated during those fasting hours, as you’re not consuming foods which often play a role in our hydration. A recent study showed that some of the initial weight loss effects of intermittent fasting may be from the loss of body water from fluid deprivation. 

Therefore, make sure you’re drinking plenty of water or non-caffeinated tea during those fasting hours.

Sample 18/6 Intermittent Fasting Meal Plan

10 am: Cottage cheese scrambled eggs with whole wheat toast and one cup of mixed berries. 
12 pm: Banana with two tablespoons of peanut butter and a glass of skim milk.
2 pm: Hearty salad topped with 4 ounces of chicken breast and olive-oil based dressing. 
3:30 pm: Cucumber, bell-pepper and chickpea quinoa salad topped with two tablespoons of hemp hearts.

4 pm-10am: Black coffee, water, herbal teas (no cream or sugar). 

A chart explaining 18/6 intermittent fasting and the proposed pros and cons.

Benefits of Intermittent Fasting

Here’s the thing: there are many proposed health benefits of intermittent fasting, however, most of the research conducted to date are smaller studies. It has been suggested that not all intermittent fasting approaches are the same, but some might provide substantial health benefits if they are sustainable. However, like most hot topics in nutrition, further studies are needed.

Weight Management

The most popular reason people turn to intermittent fasting is usually to promote changes in body composition. Many people find that that time-restricted eating can kickstart weight loss again after reaching a weight-loss plateau. 

However, this year-long study published in 2022 found no difference between time-restricted calorie restriction (6-hour eating window) and normal calorie restriction. All participants were instructed to follow a specified calorie-restricted diet with no significant differences between the two when measuring waist circumference, BMI, lean body mass, blood pressure, and metabolic risk factors. 

So is 18/6 intermittent fasting better than traditional calorie restricted diets? I think the jury is still out on that one.

Metabolic Health

There are quite a few research studies that have concluded that intermittent fasting can improve our cardiovascular health, however, it is still not clear if this is related to the actual fasting protocol itself or the weight loss that has resulted from it. 

Both animal and human studies have shown it can reduce blood pressure, resting heart rate, cholesterol, triglycerides, markers of systemic inflammation and oxidative stress. Improvements typically become evident within 2-4 weeks of starting IF.  

Gut Health

Intermittent fasting might have a positive impact on our gut microbiome – altering the composition of it and increasing bacterial diversity. 

Improved Cognitive Function

There is some evidence that alternate-day fasting may delay the onset or progression of Alzheimer’s disease, stroke, and Parkinson’s disease. Animal studies have consistently shown the benefits of fasting on brain and cognitive function, but longitudinal and randomized-clinical studies are needed for further clarification.

Prevention of Diabetes

One of the most interesting findings of intermittent fasting is its’ potential to prevent type II diabetes. One study found that individuals who were fasting had significantly lower insulin levels and were less hungry than those on a non-fasting and calorically restricted diet with the 6 hour eating window.  

This study also found that just changing the timing of the meals to earlier in the day, and extending the overnight fast, significantly improved metabolism even if the person didn’t lose a single pound.

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Do You Have to Intermittent Fast Forever?

A downside to intermittent fasting is that it may be unsustainable to maintain for years and years. The long-term effects of intermittent fasting have not been studied in humans, although the animal studies do seem promising if sustained for a long period of time.

What Are Other Potential Side Effects?

Some of the reported side effects of intermittent fasting are:

  • hunger
  • fatigue
  • decrease concentration
  • nausea
  • constipation
  • headaches

It’s important to note that most of these side effects go away within the first month. However, working with a health profressional – like a doctor or dietitian – will ensure that you are doing it safely. It is not recommended for those under the age of 18, or with a history of disorered eating.


Most of the research that has been conducted on 18/6 intermittent fasting is on overweight or obese young and middle-aged adults. However, clinical studies have shown a lot of potential health benefits –  specifically in the prevention of diabetes and the treatment of prediabetes.

Does intermittent fasting have an edge over other popular diets, like the widely-researched Mediterranean diet, when it comes to weight loss? In my opinion, no – the best diet is the one you can stick to – and intermittent fasting is definitely not for everyone!

Several of the suggested health benefits associated with intermittent fasting have been demonstrated to be accurate when adopting a healthier lifestyle (such as, again, following a Mediterranean diet).

However, if you’re someone who is overweight, obese, or struggling with their blood sugars – 18/6 intermittent fasting could prove to be very beneficial. 

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