18 Hour Fast Weight Loss: How Much Will You Lose and How Quickly?

Weight loss is one of the main reasons many people fast. Surely, it works for some people. Unlike counting calories obsessively or adhering to strict dietary protocols, Intermittent Fasting (IF) simplifies the approach to eating by focusing on when you eat rather than what you eat.

This tends to be simpler to integrate into a busy lifestyle, and for some, more effective in promoting significant weight loss and improving certain health markers.

But what should you expect when you adopt an 18-hour fasting regimen? How quickly will you see results, and more importantly, how much weight can you actually lose? What should you do if the results are not immediately noticeable, or if you find the process challenging to maintain?

Here’s what you need to know about the potential of 18-hour fasts for weight loss, including the science behind fasting, realistic expectations, and strategies for success.

How Many Calories Do You Burn In an 18 Hour Fast?

Fasting itself doesn’t directly “burn” calories in the way that exercise does. Instead, calorie burning during a fast happens primarily through your body’s basal metabolic rate (BMR), which is the amount of energy your body uses to perform basic functions such as breathing, circulation, cell production, and nutrient processing while at rest (5).

Your BMR accounts to about 60% to 75% of your total daily energy expenditure and varies from person to person (1).

To estimate how many calories you burn in an 18-hour fast, you would need to calculate your BMR and then adjust for any additional activities, if you are not completely at rest. Various online calculators can help estimate your BMR, requiring you to input details like your age, gender, weight, and height.

For example, a man weighing 70 kg (about 154 lbs) with a height of 175 cm (about 5’9″) and aged 30 years might have a BMR of roughly 1,700 calories per day.

During an 18-hour fast, assuming minimal physical activity (resting), this individual could theoretically burn approximately 1,062 calories from their BMR alone (1,700 calories/day * 18 hours / 24 hours).

Remember, these are rough estimates, and actual calorie expenditure can vary significantly.

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Can You Lose Weight Fasting 18 Hours a Day?

Fasting 18 hours a day for a month or more will certainly result in weight loss for many people. When we go hours without food, several processes occur in the body that promote fat burning and weight loss.

Calorie Intake Reduces

Fasting usually leads to reduced calorie intake, as you are restricting the hours in which you can consume food. This ultimately results in a calorie deficit, where your body burns more calories than it consumes, which promotes weight loss (13).

The reduction in calorie intake is the primary mechanism through which intermittent fasting leads to weight loss. 

Note, calorie reduction isn’t always guaranteed, and some individuals may end up consuming more calories during non-fasting periods, leading to little or no weight loss.

We’ll discuss this in more detail in the next section.

Fat Burning Lipolysis Increases

During fasting, levels of the hormone insulin drop significantly. Insulin is responsible for regulating blood sugar and encourages fat storage in the body. When insulin levels decline and you aren’t eating anything, your body eventually enters a state of ketosis or “fat-burning mode,” where it turns to stored fat for energy instead of glucose from food (15).

As this process continues over time, the body breaks down fat cells to release fatty acids into the bloodstream for energy, which can contribute to weight loss (15).

Metabolic Rate May Increase

Fasting may influence the body’s neuroendocrine responses, which include possible increases in norepinephrine (noradrenaline) levels (10). 

Norepinephrine is a key hormone and neurotransmitter involved in the body’s fight or flight response. It has profound effects on energy expenditure and fat loss (12).

Increased norepinephrine enhances the body’s ability to break down stored fat for energy, a process known as lipolysis (14). Essentially, with higher levels of norepinephrine, the body becomes more efficient at utilizing its fat stores, possibly leading to fat loss while preserving lean mass.

Metabolic Flexibility Improves

With regular fasting, your body is thought to become more metabolically flexible, meaning it can switch between using different sources of fuel for energy. This flexibility may be beneficial as your body can better adapt to changes in food availability and use stored fat efficiently for energy (7).

Digestion May Improve

When you space out meals, like in an 18:6 fasting schedule, you might support the function of the migrating motor complex (MMC), which is responsible for clearing out your digestive tract between meals (22). 

A healthy MMC promotes better digestion and gut motility (9)

Read more: Green Tea Intermittent Fasting: Benefits and Side Effects

How Fast Can You Lose Weight With 18:6 Fasting?

You can aim to lose about 1 to 2 pounds per week with an 18:6 fasting schedule, depending on your starting weight and body composition. However, this is not a guaranteed outcome for everyone.

If you have underlying health conditions that affect metabolism and weight management, you may experience slower weight loss results. Even if you are perfectly healthy, everyone is different and experiences weight loss differently.

Additionally, the success of fasting for weight loss also depends on your adherence to the diet and lifestyle changes. If you find it challenging to maintain an 18-hour fast, you may not see significant weight loss results. It may be better to find an alternative schedule or diet that you can stick to consistently. 

Extreme or rapid weight loss is also not recommended as it can have adverse effects on your health (16). Sustainable and gradual weight loss is the key to achieving long-term success (11).

Check out our previous blog: 2 Day Fast Benefits where we discourage the use of extreme fasting solely for weight loss purposes and promote a balanced approach to health and wellness.

Why am I Not Losing Weight on 18 Hour Fast?

If you’ve been practicing 18:6 fasting for one month or more, and are not seeing the weight loss results you were hoping for, there could be several potential explanations.

You’re Eating Too Many Calories During Non-Fasting Periods

As mentioned earlier, calorie intake reduction is the most crucial factor in weight loss during fasting. If you are eating more high-calorie foods or too much food during your non-fasting periods, your body may not enter a caloric deficit, leading to no weight loss or even weight gain.

Fasting for 18 hours does not give you free rein to eat whatever you want during the remaining six hours. To see results, eat a balanced and healthy diet during your non-fasting periods. Don’t overcompensate for the hours of fasting by consuming excessive calories.

A good rule of thumb is to stick to mostly nutrient-dense foods and listen to your body’s hunger and fullness cues.

You’re Not Fasting Consistently or Effectively

The rules of fasting can be flexible, but consistency is key. If you are only fasting for 18 hours a few days a week and not consistently following the schedule, your body may not have enough time to enter a fat-burning state and promote significant weight loss. Or your results may be slower compared to following the schedule every day.

Additionally, how you fast matters too. Consuming calorie-containing beverages or snacks during your fasting period (even in small amounts) can break your fast and might prevent you from seeing desired results.

To ensure effective fasting, follow these rules:

1. Hydrate with water and unsweetened beverages during the fasting period.
2. Avoid any food or calorie-containing drinks during the fasting period.
3. Consume enough protein, healthy fats, and whole grains, and vegetables during your eating window to support weight loss and overall health (4).
4. Don’t overeat or binge during the non-fasting hours.
5. Limit added sugars and ultra-processed foods during the eating window.
6. Stay consistent with your fasting schedule every day.
7. Listen to your body’s hunger and fullness cues, and adjust your eating habits accordingly.

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You’re Not Getting Enough Sleep

Sleep is crucial for weight loss, as it impacts hormones that regulate appetite, metabolism, and energy balance (17). When we don’t get enough sleep, our hunger hormone ghrelin increases, leading to increased cravings and overeating. At the same time, our fullness hormone leptin decreases, making it harder to feel satisfied after meals (18).

If you’re not getting enough sleep while fasting, these hormonal changes may hinder weight loss efforts. Aim for at least 7-9 hours of quality sleep each night to support your weight loss goals.

You Have an Underlying Health Condition

Sometimes, underlying health conditions can make it challenging to lose weight, even with fasting. For example, conditions like hypothyroidism or polycystic ovary syndrome (PCOS) can slow down metabolism and make it harder to lose weight (3).

If you have been following an 18:6 fasting schedule consistently and not seeing results, it may be worth discussing your concerns with a healthcare provider and getting any necessary tests done to rule out any underlying health issues.

You’ve Hit a Weight Loss Plateau

Weight loss is not always linear, and it’s common to hit a plateau where your weight stays the same, despite following an 18:6 fasting schedule. This can be frustrating, but don’t let it discourage you.

What happens when you’ve been fasting for a while is that your body adapts to the new eating pattern, leading to slower weight loss. To break through this plateau, you can try:

• Intermittent calorie restriction: Instead of consuming the same number of calories every day during your eating window, alternate between higher calorie days and lower-calorie days.
• High-intensity interval training (HIIT): Incorporate short bursts of high-intensity exercises into your workout routine to increase metabolism and burn more calories (8).
• Add strength training: Building muscle through strength training can boost metabolism and help with weight loss (19).
• Increase fasting time: If you have been consistently following an 18:6 schedule, you could try increasing your fasting window by an hour or two to see if it helps kick start weight loss again.

Check out our, Fasting Workout blog for more tips on how to break through a weight loss plateau while fasting.

You’re Focusing Only on the Scale

While weight loss is a common goal for many who practice 18:6 fasting, it’s essential to remember that weight is not the only indicator of health. Changes in body composition, such as losing fat and gaining muscle, can influence the number on the scale.

Instead of solely focusing on weight, pay attention to other factors like energy levels, the way your clothes fit, and overall well-being. These off scale victories can be just as rewarding and motivating.

Is 18 Hour Fast Better Than 16?

An 18:6 fasting schedule might lead to more significant results than a 16:8 schedule, as it allows for a longer period of fat-burning. However, the duration of fasting may not be the only determining factor in weight loss success.

Both 16:8 and 18:6 fasting schedules have led to weight loss and improved metabolic health for some people. The best approach would be to choose the schedule that fits your lifestyle and preferences best, as consistency is crucial for results.

Read more: Intermittent Fasting and Running: A Winning Combination or a Terrible Mistake?

FAQs

• What Happens When You Go 18 hours Without Eating?

When you go 18 hours without eating, your body enters a fasted state. During this time, insulin levels drop, and the body starts burning stored fat for energy (15).

Other changes in the body during an 18-hour fast may include increased release of growth hormone, upregulated cellular repair processes, and a decrease in inflammation. These changes might have numerous health benefits, including weight loss and improved metabolic health (20).

• How Often Should You Do an 18 Hour Fast?

Ideally, an 18 hour fast should be done consistently, with a goal of following the schedule for at least five days a week. This consistency can help your body adapt to the fasting pattern and promote better results.

However, one may also choose to do intermittent fasting with shorter or longer fasting periods, depending on individual goals and preferences. It’s essential to listen to your body and adjust as needed for optimal results. It’s also important to speak to your healthcare provider before starting a fasting regimen or making any major dietary changes.

• What Happens To The Body After 18 Hours of Fasting?

After 18 hours of fasting, your body goes through several changes to adapt to the lack of food. These may include increased fat burning, reduced insulin levels, and possibly a decrease in inflammation (2) (6).

Additionally, the body may also upregulate cellular repair processes during this time, which might have various health benefits.

The Bottom Line

18:6 fasting is a popular and potentially effective method for weight loss, but it may not work for everyone. It’s essential to remember that weight loss is a complex process influenced by various factors, including genetics, lifestyle habits, and underlying health conditions. 

If you’re not seeing results with an 18-hour fast, don’t get discouraged. Try adjusting your fasting routine, diet, or exercise habits and consult with a healthcare provider if needed. Remember to focus on overall health and not just weight loss, as that will lead to sustainable results in the long run.