Fasting is one of the most ancient and widely adopted practices for improving health and longevity (12). Research links Intermittent Fasting (IF) to a variety of health benefits, from weight loss and improved metabolic health to increased longevity and cognitive function (7).
Other benefits that often go unmentioned include the cultivation of mental discipline and the breaking of unhealthy eating patterns.
There’s no rule as to how long or how frequently one should fast, but some fasting regimens are more popular and seemingly effective than others. 15/9 intermittent fasting is one such popular method, where one eats for 9 hours and fasts for the remaining 15 hours of the day.
Here’s everything you need to know to determine if the 15/9 intermittent fasting approach is right for you.
Fasting for 15 hours is okay; it may even be beneficial if you do it consistently over a while.
During a fast, your body goes through several changes that can promote weight loss and improve overall health. For example, during the elongated periods of fasting, your body depletes its glycogen stores and begins to burn stored fat for energy (9).
This process is known as ketosis and has been linked to numerous health benefits, including weight loss, improved insulin sensitivity, and reduced inflammation (9).
Additionally, 15/9 intermittent fasting may help individuals overcome unhealthy eating patterns and achieve better control over their food intake.
Experts encourage beginners to start slowly with intermittent fasting, and the 15/9 approach may be a good way to ease into it.
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15 hours of fasting is effective for most purposes and can be a stepping stone to longer fasts that unlock more benefits.
Some 15/9 intermittent fasting benefits include:
When you limit your eating window to 9 hours, you naturally consume fewer calories. Say, for example, you only eat from 11am to 8pm. During this time, you might consume two meals and a snack.
By cutting out those late-night snacks or early-morning breakfasts, you reduce your overall calorie intake without having to count calories or restrict specific foods.
If you’ve been struggling with weight loss, try 15/9 fasting for weight loss to help you sustainably achieve your goals.
Find out more about fasting and calories in our previous blog post – Intermittent Fasting Macros.
Ketosis is a metabolic state where the body starts burning fat for energy instead of glucose. It’s a desirable goal for many as it can lead to weight loss and improved health markers.
According to several health resources, your body might go into ketosis after just 12 hours of not eating, such as during an overnight fast before a morning meal (2) (16).
In general, if you eat 20-50 grams of carbs per day, it could take 2-4 days to enter ketosis. However, some people may find it takes a week or longer to reach this state.
The 15/9 intermittent fasting method may help you enter ketosis faster, especially if you’re already following a low-carb or ketogenic diet.
Our Beginner Intermittent Fasting Meal Plan is a useful resource for those needing help finding easy meals and snacks that fit into the 15/9 approach.
Research has shown that intermittent fasting can have a positive impact on an individual’s metabolic health, particularly when it comes to insulin sensitivity and blood sugar control (4).
Insulin sensitivity refers to how responsive your cells are to insulin, the hormone responsible for regulating blood sugar levels (11).
This matters because insulin resistance is a risk factor for type 2 diabetes, obesity, and other chronic diseases (10).
By improving insulin sensitivity, 15/9 intermittent fasting may help you reduce your risk of these conditions and improve overall metabolic health.
Research has also linked intermittent fasting to increased longevity in animal studies (5).
One theory suggests that by promoting cellular repair processes such as autophagy (the breakdown and recycling of old cellular components), intermittent fasting may help protect against age-related diseases and promote healthy aging (8).
Unfortunately, autophagy doesn’t occur until after 24-48 hours of fasting, so those who follow the 15/9 approach may not experience this benefit.
Using the 15/9 intermittent fasting method as a starting point to build up to longer fasting periods may be a more effective approach for those looking to increase longevity through IF.
Read more: Green Tea Intermittent Fasting: Benefits and Side Effects
Some research suggests that intermittent fasting may also benefit brain health and cognitive function (14).
Animal studies have shown that intermittent fasting can increase levels of a protein called brain-derived neurotrophic factor (BDNF) (15), which is crucial for promoting the growth and survival of neurons (3).
Longer fasts may also stimulate the production of new nerve cells in the brain, potentially improving memory and learning abilities (6).
While more research is needed to confirm these findings in humans, it’s possible that incorporating 15/9 intermittent fasting into your routine may have positive effects on your cognitive function.
Intermittent fasting can also help individuals develop mental discipline and control over their eating habits.
By learning how to delay gratification and overcome the urge to eat during the fasting period, individuals can apply this same discipline in other areas of their life. This includes resisting temptations and improving self-control.
Many individuals struggle with unhealthy eating patterns, such as snacking late at night or constantly grazing throughout the day.
By implementing a set eating window with intermittent fasting, individuals can break these patterns and develop more structured and healthier eating habits.
This can also be beneficial for those who tend to emotionally eat or use food as a coping mechanism.
With the 15/9 approach, individuals have a set time to consume their meals and snacks, which can help prevent mindless eating throughout the day.
To practice 15/9 intermittent fasting, you can choose any 15-hour period during the day to fast and a nine-hour window for eating.
For example, you could stop eating at 8 pm and resume eating at 11 am the next day.
During the fasting period, you can only consume the following zero calorie liquids:
Once your eating window begins, you can consume your meals and snacks as normal. You should eat until you feel satisfied, not overly full.
There’s no calorie counting or restriction on specific foods, but it’s still important to aim for balanced and nutritious meals during your eating window.
Some examples of meals that fit into the 15/9 intermittent fasting approach include:
Not everyone can fit 3 meals and a snack in their 9-hour window, and that’s okay. Such individuals can opt to have 2 larger meals and no snacks instead.
You may also consider adjusting your fasting and eating windows based on your individual needs and lifestyle.
For example, if you prefer an early dinner and breakfast, you could eat from 7 am to 2 pm and fast from 3 pm to 6 am the next day.
Other tips for successful 15/9 intermittent fasting include:
In our guide – How to Prepare for a Fast, we discuss in detail how to successfully prepare for your 15/9 intermittent fast, including tips on gradually building up to longer fasting periods and choosing the right foods to break your fast.
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16/8 is one of the most popular methods of intermittent fasting, where individuals fast for 16 hours and eat within an eight-hour window.
So what’s the difference between 15/9 and 16/8? Is one method better than the other?
The main difference is that with 15/9, you have a longer eating window of nine hours compared to eight hours with the 16/8 intermittent fasting method.
While this may not seem like a significant difference, it can be more challenging for some individuals to fit all their meals and snacks into a shorter eating window. So the extra hour in 15/9 may be more manageable for those with busy schedules or specific dietary needs.
Additionally, some individuals may find it easier to gradually work their way up to a 16-hour fast by starting with 15 hours first.
Ultimately, longer fasts have shown more significant benefits in terms of weight loss and other health markers, but the most important factor is finding a fasting method that works for you and your lifestyle.
Read more: Intermittent Fasting and Running: A Winning Combination or a Terrible Mistake?
The benefits of 15/9 intermittent fasting include potential improvements in weight loss, blood sugar regulation, brain function, and mental discipline. It can also help individuals break unhealthy eating patterns and develop more structured and healthier eating habits.
During the first 12 hours of a fast, your body uses up glucose stores for energy. From 12-16 hours, your body starts burning stored fat for fuel, leading to potential weight loss and other health benefits.
Beyond 16 hours, autophagy, a process where your body breaks down and recycles old or damaged cells, begins (13). This can have significant anti-aging and disease-preventing effects (1).
Furthermore, 16 hours is a good starting point for intermittent fasting as it’s a manageable length of time for most individuals, and they can still fit in three meals during the eight-hour eating window.
Black, unsweetened coffee is allowed during the 15-hour fasting period, as it contains zero calories and does not cause an insulin response. However, adding sugar, cream, or other additives can break your fast and disrupt the potential benefits of intermittent fasting.
The hours spent sleeping do count towards your fasting period, making it easier to reach the 15-hour mark. For example, if you stop eating at 8 pm and sleep for eight hours, you only need to fast for another seven hours before breaking your fast at 11 am.
15/9 intermittent fasting is a popular and flexible approach to intermittent fasting that can offer potential health benefits and fit into various lifestyles. It involves fasting for 15 hours and eating within a nine-hour window, with no restrictions on specific foods or calorie counting. With some preparation and planning, this method can be easily incorporated into your daily life.
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