Registered Dietitian (RD) since 2010
You can do plenty in 4 hours: binge-watch your favorite TV series, finish a good book, or even run a marathon.
But if you had to compress all your eating for a day into this brief window, how would you manage it?
A 20-hour fast sounds challenging, and for some, it may seem downright impossible. Yet research has shown some pretty impressive benefits of adopting this intermittent fasting pattern.
From improved cognitive function to enhanced weight loss and better heart health, the rewards can be substantial (2) (9).
If you’re thinking of working your way up to a 20-hour intermittent fasting schedule, or you’re simply curious about the myriad of health benefits it could potentially offer, then you’ve come to the right place.
Here’s everything you need to know about whether this popular form of time-restricted eating is worth the challenge.
After 21 hours of fasting, there’s a lot going on in your body (15).
During the initial 12 hours of fasting, you enter a metabolic state that is known as the post-absorptive state. This is when your body has absorbed all the nutrients from your last meal and starts to rely on its internal fuel sources for energy (16).
Once you reach the 18-20 hour mark, you’re in the fasted state and it’s at this point your body starts to experience the benefits of fasting.
During a 20-hour fast, your insulin levels drop significantly, allowing your body to tap into its fat stores for energy. This process is known as ketosis and is responsible for the weight loss benefits that are typically associated with fasting.
With only a 4-hour eating window, 20/4 intermittent fasting weight loss results after 1 month may be fast or slow, depending on individual factors, in addition to the types of food you consume during the eating window.
Studies have shown that individuals who follow this type of intermittent fasting pattern can lose anywhere from 0.18 – 13% of their body weight without any serious health effects (11). This is significant as obesity has been linked to an increased risk of heart disease and other health issues (13).
That being said, the benefits of a 20-hour fast extend beyond 20/4 intermittent fasting weight loss.
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Studies have shown that this type of fasting may also help support:
Fasting has been found to increase the production of a protein called brain-derived neurotrophic factor (BDNF), which is responsible for the growth and development of new neurons in the brain. This may help support improved cognitive function, memory, and overall brain health (19).
Chronic inflammation is linked to a higher risk of developing various diseases, including cancer, diabetes, and Alzheimer’s (3). Fasting has been found to reduce markers of inflammation in the body, which potentially reduces the risk of developing these conditions (20).
Longer fasting periods have been linked to autophagy (18), a process where your body breaks down damaged cells and recycles them for energy. This has been proven to have numerous health benefits, including reducing inflammation and improving overall cellular function (1).
Research has found that intermittent fasting can lower blood pressure, reduce cholesterol levels, and improve other risk factors for heart disease (7). This is believed to be due to the reduction in inflammation and improvement in overall health that comes with fasting.
During fasting, your body’s natural detoxification processes may be enhanced (6). This is because during fasting the synthesis of detoxification enzymes is triggered (17), which has a positive impact on overall health and well-being.
There are also some side effects of a 20-hour fast that are worth considering (8).
During the fasting period, it’s normal to experience hunger pangs. However, these typically decrease over time as your body adapts to the new eating pattern.
Some people may experience headaches during a 20-hour fast. This can be a result of dehydration or caffeine withdrawal. Drinking plenty of water and gradually reducing your caffeine intake can help alleviate these symptoms.
Fasting can lead to a decrease in energy levels, particularly during the first few days. This is because your body is adjusting to using fat as its primary source of fuel instead of glucose. However, once your body becomes more efficient at burning fat, this fatigue will typically subside.
During ketosis, your body produces a ketone body called acetone that can result in a distinctive “fruity” breath odor (12). This is temporary and can be managed by drinking more water and practicing good oral hygiene.
Over time (usually several weeks or longer), 20/4 fasting can lead to nutrient deficiencies if you don’t consume a balanced and calorically adequate diet during the eating window (g).
Read more: Green Tea Intermittent Fasting: Benefits and Side Effects
The best intermittent fasting plan depends on the individual. For some people, a 20-hour fast may be better than 16 hours. For one, the speed of weight loss while intermittent fasting 20/4 every day may be faster than when fasting for just 16 hours.
Think of it this way: creating a calorie deficit is the key to weight loss (14), and the longer you fast, the more time your body has to tap into its fat stores for energy.
The same theory applies to most intermittent fasting benefits: the longer you fast, the greater the potential benefits.
In our piece on Intermittent Fasting for Women, we discuss how extended fasts may not be ideal for women.
So, we advise you to approach the question of which fast length is better with caution. While a 20-hour fast may offer more potential benefits overall, it’s not sustainable or suitable for everyone.
A personalized, individualized approach to intermittent fasting is recommended, and you should experiment with different fast lengths and find what works best for your body and lifestyle.
In another article, we discuss Intermittent Fasting Exercise For Safe Weight Loss.
Studies on the long-term effects of daily 20-hour fasts are limited, and all we can do is draw conclusions from the available research and anecdotal evidence.
In terms of weight loss, you may want to consider the fact that over time, your body may adapt to the 20-hour fast, which makes it less effective for weight loss.
What happens is your body may start to conserve energy, slowing down your metabolism and burning fewer calories (4).
Fasting for 20 hours every day can also be challenging for some people to sustain in the long term. It requires significant discipline and adjustment to your lifestyle and eating habits.
However, if you choose to follow a daily 20-hour fast, you’re bound to experience the various previously mentioned benefits. Most people who follow this daily fast find that they feel less hungry and find a 4-hour eating window quite manageable after a while.
With only 4 hours to fit in your daily nutrient intake, a meal plan is essential. We’re not just talking about knowing what breaks a fast and what doesn’t, we’re also talking about ensuring you get a balanced, healthy meal in the short window.
Although there are no specific guidelines for what you should eat during your eating window, it’s essential to prioritize nutrient-dense whole foods and limit ultra-processed foods and added sugars.
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Yes, a 20-hour fast is a form of calorie restriction and will lead to weight loss over time (10). However, fasting daily for months or years may lead to a slower metabolism and reduced weight loss results.
The exact weight loss results are dependent on several factors such as your overall calorie intake, exercise level, and individual body composition.
Yes, you can drink black coffee or unsweetened tea during a 20-hour fast. These beverages don’t contain any calories, so they won’t break your fast. However, it’s essential to limit your intake of caffeine as too much of it can lead to headaches, jitters, and disrupted sleep patterns (21).
Fasting for 20 hours will probably affect your energy levels at first, as your body is adjusting to using fat as its primary source of fuel. You may also experience some side effects such as headaches, fatigue, and irritability (10), but they will typically subside over time.
On a positive note, fasting for 20 hours a day for weight loss can also have several benefits such as improved blood sugar control, weight loss, and potential longevity advantages. However, as individual results vary, not everyone will necessarily experience the same benefits.
Research has shown that the effects of autophagy are more enhanced during longer fasts, so a 20-hour fast could potentially have higher levels of this cellular repair process (18). However, more research is required to fully understand the optimal timing for autophagy during a fast.
The main cons of 20/4 fasting include potential nutrient deficiencies, difficulty in sustaining it in the long term, and negative side effects such as headaches and fatigue.
It may also not be suitable for certain groups, such as women with hormone imbalances or individuals with a history of disordered eating.
You should break a 20-hour fast gently. Starting with broth (bone or vegetable) is a good idea, as it’s easy on the stomach and helps prepare your digestive system for solid food.
From there, you can gradually add nutrient-dense foods such as vegetables, protein, and healthy fats to create a balanced meal. The idea is to eat slowly, listen to your body’s hunger and fullness cues, and avoid overeating after a fast.
So, is a 20-hour fast worth the challenge?
For some individuals, yes, it is. The potential benefits of improved weight loss, heart health, and brain function may make it a worthwhile endeavor.
However, for others, a shorter fasting window may be more suitable and sustainable. Ultimately, the decision to adopt a 20-hour fast should be made based on individual needs and preferences and careful consideration should be given to the potential risks and side effects.
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