What Is Periodic Fasting?
Periodic fasting, also known as whole-day fasting, is a diet that involves fasting food for an entire 24-hour period once a week. For the rest of the week, you eat without restriction.
Periodic fasting has become an increasingly popular diet that many people claim is excellent for weight loss benefits, chronic disease prevention, and overall health improvement. As fasting becomes more popular, more research is being conducted to confirm how periodic fasting affects and benefits the body.
How to do Periodic Fasting
Periodic fasting consisted of fasting for 24 hours once a week . It is highly flexible because you can pick any day of the week and time frame that works best for your lifestyle or schedule.
For example, you may choose to eat breakfast before fasting. You could begin your fast at 9 AM and continue to fast until 9 AM the following morning. Or, you could choose to make lunch your last meal before fasting and fast from 1 PM to 1 PM the next day. It is entirely up to you. Some people may find it easier to begin their fast after lunch, as they may have more energy throughout the day.
Research has not concluded how many consecutive weeks people should participate in periodic fasting to see benefits. It is up to you to listen to your body and assess how you feel to determine how many weeks you should continue.
On the remaining days of the week that you are not fasting, you are allowed to eat an ad libitum diet, meaning unrestricted . However, this does not mean that you should consume high-calorie, unhealthy junk food during that time [1, 3]. You should eat an unrestrictive healthy, balanced diet without a caloric deficit [1, 3].
When periodic fasting, you should consume your daily recommended caloric intake on non-fasting days of the week. The USDA Dietary Guidelines for Americans suggests that adults between the ages of 19 and 30 should consume about 1,800 to 3,000 calories per day, depending on their sex, height, and weight . You can use a calorie calculator to estimate your ideal daily caloric intake for weight maintenance. Be sure to calculate your caloric intake for weight maintenance, even if you want to fast for weight loss. The 24-hour fasting period creates enough of a caloric deficit in your week.
Note that it is not required to count your calories when periodically fasting, but if you want to track your calories, there is no harm in doing so.
If you want to experience potential benefits from periodic fasting, eating a healthy and nutritious diet is essential. The USDA recommends that people eat a diet rich in whole grains, vegetables, fruits, nuts, and protein . Added sugars, excess sodium, and saturated fats should be avoided .
When fasting, it is very important to stay hydrated. You are permitted to drink water within the fasting period . You may also drink black coffee or tea . Just be careful when consuming caffeine because caffeine can increase dehydration.
Benefits of Periodic Fasting
The main benefit that attracts people to periodic fasting is weight loss. The weight loss that people experience when periodic fasting could be a result of two factors. Either being in a state of ketosis or having a caloric deficit.
Research shows that fasting for an extended duration causes your body to enter into a state of ketosis. The duration depends on the individual, but generally, a person can enter ketosis anywhere after 12 to 36 hours of fasting [4, 9]. When your body enters ketosis, your body switches from using the calories consumed from your last meal for energy to using fat stores for energy [3, 4, 9].
This metabolic shift leads to fat loss, as fat is used for energy by the body . This is one benefit that a prolonged periodic 24-hour fast has an advantage over other shorter-duration intermittent fasting diets .
Multiple studies have shown that people who did intermittent fasting with calorie restriction experienced weight loss . However, those who intermittently fasted and restricted calories did not lose any more weight than those who engaged in a daily calorie-restrictive diet . Those who participated in whole-day fasting and maintained their regular eating pattern lost a significant amount of body weight compared to those on a daily calorie-restrictive diet . This may be because those who participated in whole-day fasting entered into a state of ketosis.
There is a chance that even when fasting for 24 hours, your body may not enter a state of ketosis, as everyone’s body will respond differently. But, you may lose weight if you are eating in a caloric deficit throughout the week . Any time you consume fewer calories than your body needs throughout the day or week, you will likely lose weight due to the caloric deficit .
Overall, there is limited evidence that whole-day fasting is more beneficial than daily calorie restriction for weight loss . There is still no confirmation on what time frame of fasting is best for weight loss either .
The good news is that even if weight loss does not occur, many other benefits could be gained from periodic fasting independent of weight loss .
Chronic Disease Prevention
Periodic fasting has been found to improve the biomarkers of chronic disease.
Studies have found that 24-hour fasts can result in biological changes that improve health and prevent chronic disease when fasts are practiced over many years [5, 9, 12]. Biological changes include reduced cardiac risk factors, activation of inflammatory pathways, decreased insulin resistance, increased cell regeneration, increased cell promotion, decreased resting heart rate and blood pressure, and increased red blood cell parameters [5, 12].
Improved Blood Sugar
Many studies have found fasting can decrease insulin sensitivity in insulin resistance.
One study stated that fasting decreases insulin resistance and improves insulin sensitivity . Another suggested that periodic fasting may even be a future treatment for people with type 2 diabetes if further research is conducted .
Another trial found that the participants who fasted for 24 hours had decreased insulin sensitivity the following morning by 54% .
While research shows promise, more research is needed to confirm if the benefits of fasting are greater than the risk for those who have been diagnosed with type 2 diabetes, as fasting can greatly increase an individual’s risk of hypoglycemia .
Those who have not received a diagnosis of diabetes may reduce their risk of developing diabetes if they fast regularly .
Improved Memory and Learning
A few studies found that fasting can help protect and improve memory, improve learning functionality, and increase cognitive performance [4, 12].
Potential Risks of Periodic Fasting
In general, fasting diets are safe for healthy individuals, but there are risks involved for specific individuals .
Those who have been diagnosed with type 2 diabetes should enter into a fast cautiously under medical supervision, as fasting can greatly increase the risk of hypoglycemia (low blood sugar) [5, 10, 13]
Women who are pregnant or breastfeeding should avoid fasting diets as their bodies require constant nutrition to support pregnancy and lactation . Adolescents should also avoid fasting diets as they need consistent energy for growth .
Individuals who have been diagnosed with eating disorders should also be cautious of fasting diets as food restriction can increase the risk of unhealthy self-restriction and obsession with food intake .
For healthy individuals, periodic fasting may cause headaches, feelings of hunger, and irritability due to hunger.
Learn more on Fasting:
- 16:8 Intermittent Fasting Guide and Tips
- 20:4 Intermittent Fasting Guide and Tips
- Which Sweeteners Can Break Your Fast? Find Out Here.
- The OMAD Approach: Is One Meal A Day an Effective Weight Loss Strategy?
Is fasting for 24 hours good?
There are very few risks and many benefits associated with fasting for 24 hours once a week. It is safe to assume that fasting for 24 hours is good for your body and that you may experience benefits.
How much weight can you lose fasting for 24 hours?
If you fast for one 24-hour period, you may lose a fraction of weight because you have not consumed any food, only excreted. If this is the first 24-hour fast you have ever completed, the initial weight loss you experience may be due to losing water weight.
How often should you do a 24-hour fast?
You can do a 24-hour fast once or twice per week safely.
How do I know if I’m in ketosis?
The best way to determine if your body is in a state of ketosis is to test the ketone levels in your blood or urine.
What should I eat after fasting?
When breaking your 24-hour fast, you will want to start with a small, nutrient-dense meal. This will activate your digestive system again and replenish the nutrients your body may lack from fasting.
Periodic fasting offers many benefits. Weight loss is the main reason that people gravitate towards periodic fasting. However, there are many other benefits that you can experience from fasting that are independent of weight loss. Overall, evidence shows that whole-day fasting once a week is a safe diet for most people .
Some studies even support that periodic fasting may benefit people as a lifestyle instead of simply a short-term diet . If you have any pre-existing health problems or concerns, or if undesirable reactions come up from trying this diet, speak with your health provider before beginning or continuing fasting.
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- U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020-2025. 9th Edition.
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- Anton, S. D., Moehl, K., Donahoo, W. T., Marosi, K., Lee, S. A., Mainous, A. G., Leeuwenburgh, C., & Mattson, M. P. (2017). Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity, 26(2), 254–268. https://doi.org/10.1002/oby.22065
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