12 Hour Fast vs 16 Hour Fast: How Do The Benefits Compare?

Both 16/8 fasting and 12/12 fasting are popular methods of intermittent fasting with documented weight-loss and health benefits, but what are the key differences between the two methods, and which one is right for you?

Choosing an intermittent fasting diet mainly involves deciding how many hours you will fast per day and how many hours you will permit eating to occur.

There can be benefits to both the 12/12 intermittent fasting diet and the 16/8 intermittent fasting diet, which involve 12 hour fasts vs 16 hour fasts, respectively.

But, what are the primary differences between a 12 hour fast vs 16 hour fast? Are there more significant health benefits of either a 16 hour fast or 12 hour fast? Is one fasting method better than the other?

A person holding a plate over another platter with pieces of food at each hour marking, making it represent a clock.

What Are The Health Risks Of Fasting?

Whilst there are many health and wellness benefits to fasting, there are also some health risks, so it is recommended to speak to your doctor before trying it.

As fasting can cause significant weight loss, it is not recommended if you are already underweight, and it can be dangerous for those with eating disorders.

Furthermore, intermittent fasting can lead to significant overeating during non-fasting periods.

Skipping meals or limiting calories can also be dangerous for people with certain conditions such as diabetes.

Additionally, if you are taking medications that should be taken with food, fasting may not be recommended. Furthermore, some blood pressure and heart medications can make you more prone to mineral imbalances during fasting.1Harvard Health Publishing. (2020, December 4). 4 Intermittent Fasting Side Effects to Watch Out for. Harvard Health. https://www.health.harvard.edu/staying-healthy/4-intermittent-fasting-side-effects-to-watch-out-for

The longer the fasting period, the greater the risk factors.

What Are The Defining Differences Between a 12 vs 16-hour Fast?

Before we look at the differences in the logistics, fasting physiology, and health benefits between the two types of fasting, let’s briefly cover the differences in definition between them.

Intermittent fasting 16/8 is stricter than 12/12 intermittent fasting because you only get eight hours to eat per day vs 16 hours of fasting time.

However, compared to many common intermittent fasting schedules, both the 12 and 16-hour schedules are much less extreme and more along the lines of how many people naturally schedule their food intake relative to intermittent fasting diet schedules like 18/6 or 20/4.2Gill, S., & Panda, S. (2015). A Smartphone App Reveals Erratic Diurnal Eating Patterns in Humans that Can Be Modulated for Health Benefits. Cell Metabolism22(5), 789–798. https://doi.org/10.1016/j.cmet.2015.09.005

A clock, plate and itensils.

What Are The Health Benefits Of a 12-Hour Fast vs 16-Hour Fast?

Although there are benefits to fasting for 12 hours, you will experience more physiological effects and health benefits when fasting for 16 hours. 

This is because many of the health benefits of fasting, such as enhanced autophagy, take closer to 18-24 hours or more to significantly take effect.3de Cabo, R., & Mattson, M. P. (2019). Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine381(26), 2541–2551. https://doi.org/10.1056/nejmra1905136 Furthermore, anything much less than a 12-hour fast will not result in any appreciable physiological benefits of fasting.

One important thing to keep in mind when discussing the differences between fasting 12 hours vs fasting 16 hours is that most biological and biochemical processes that occur do not operate on an “on/off“ switch that is flipped a certain amount of time after your last meal, but rather a “dimmer“ switch.

What this means is that the various effects of fasting on the body don’t suddenly occur or cease altogether at a given fasting duration.

Rather, it’s more of a continuum, and the longer you fast, the more pronounced certain fasting benefits will be.4Kerndt, P. R., Naughton, J. L., Driscoll, C. E., & Loxterkamp, D. A. (1982). Fasting: the history, pathophysiology and complications. The Western Journal of Medicine137(5), 379–399. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1274154/

A clock and utensils on the plate.

Additionally, the specific 12-hour fast vs 16-hour fast benefits you personally experience will depend on your unique biochemistry, genetics, body size and composition, dietary intake when you aren’t fasting, exercise routine, and overall health status.

In other words, any two individuals may experience somewhat different metabolic and hormonal effects of fasting, whether or not they are doing a 12-hour fast, a 16-hour fast, or a more extended fast.

Therefore, while we can make general comparisons about the overarching differences between a 12 vs 16-hour fasting diet, not every individual body will necessarily respond in the exact same way.

5 Key Differences Between a 12-Hour Fast and a 16-Hour Fast

While 12 and 16-hour fasts have many important similarities, research suggests that there are differences between the two to consider, especially if you are new to fasting.

A clock, measuring tape and salad.

#1: Difficulty and Logistics

The most obvious answer to the question: “What Is the difference between a 12-hour fast and a 16-hour fast?”, is the fasting duration.

Of course, a 16 hour fast is four hours longer than a 12 hour fast. From a practical perspective, it is more difficult to do a 16 hour fast than a 12 hour fast because there will be more hours in the day when you are awake and unable to eat.

Furthermore, when fasting for 16 hours vs fasting for 12 hours, when you are eating, you will only have eight hours to consume all of your calories for the day instead of 12 hours.

Together, this means that you will likely be hungrier and will have to put a more deliberate focus on abstaining from eating with 16:8 intermittent fasting vs 12:12 intermittent fasting.

To this end, women who are pregnant or breastfeeding, people with chronic health conditions, and those with issues regulating blood sugar should speak with their healthcare provider before fasting for 12 hours or more.

A clock, measuring tape, salads, juices and a fork.

#2: Autophagy Benefits

One of the key benefits of intermittent fasting or prolonged water fasting is that it activates the biological process known as autophagy.

This is a cell cleanup process that helps rid the body of damaged and dead cells, which can help protect cells, proteins, and DNA/RNA against premature aging.

Most evidence suggests that fasting for 12 or 16 hours is not long enough to induce an appreciable degree of autophagy.

In fact, predictive modeling suggests that the fasting autophagy peak isn’t reached until closer to 36-72 hours of fasting.5https://www.researchgate.net/profile/Basavarajaiah-Dm/publication/348034205_Citation_Basavarajaiah_DM_Narasimhamurhy_B_2020_Predictive_Modeling_of_Autophagy_Interrelation_with_Fasting/links/5fed714d45851553a009cc01/Citation-Basavarajaiah-DM-Narasimhamurhy-B-2020-Predictive-Modeling-of-Autophagy-Interrelation-with-Fasting.pdf

However, a human study found that an 18-hour fast activates autophagy, and another recent study about fasting during Ramadan similarly found that fasting anywhere between 17 and 19 hours increased autophagy activity. 6Jamshed, H., Beyl, R. A., Della Manna, D. L., Yang, E. S., Ravussin, E., & Peterson, C. M. (2019). Early Time-Restricted Feeding Improves 24-Hour Glucose Levels and Affects Markers of the Circadian Clock, Aging, and Autophagy in Humans. Nutrients11(6), 1234. https://doi.org/10.3390/nu11061234 7Erlangga, Z., Ghashang, S. K., Hamdan, I., Melk, A., Gutenbrunner, C., & Nugraha, B. (2023). The effect of prolonged intermittent fasting on autophagy, inflammasome and senescence genes expressions: An exploratory study in healthy young males. Human Nutrition & Metabolism32, 200189. https://doi.org/10.1016/j.hnm.2023.200189

A clock on a plate with a sign that says fasting.

#3: Weight Loss

Any intermittent fasting diet has the potential to help you lose weight, but you will likely see greater weight loss benefits with a 16 hour fast per day vs 12 hour fast per day.

This is because the period of time in which you can consume calories is truncated with longer fasting, potentially helping reduce caloric intake.

Indeed, a crossover study in healthy individuals found that fat loss was better with 16:8 vs 12:12 intermittent fasting.8Tovar, A. P., Richardson, C. E., Keim, N. L., Van Loan, M. D., Davis, B. A., & Casazza, G. A. (2021). Four Weeks of 16/8 Time Restrictive Feeding in Endurance Trained Male Runners Decreases Fat Mass, without Affecting Exercise Performance. Nutrients13(9), 2941. https://doi.org/10.3390/nu13092941

Of course, how much you eat and what you eat during your eating window (relative to your body size and activity level) will ultimately dictate your 12-hour fast weight loss vs 16-hour fast weight loss results.

#4: Fat Burning

You will generally experience more fat-burning with a 16 vs 12-hour fast because the longer fasting window allows you to burn through more of your glycogen stores. 

This is a crucial first step before significant body fat-burning and ketosis can set in.9Anton, S. D., Moehl, K., Donahoo, W. T., Marosi, K., Lee, S. A., Mainous, A. G., Leeuwenburgh, C., & Mattson, M. P. (2018). Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring, Md.)26(2), 254–268. https://doi.org/10.1002/oby.22065

A plate of food next to a clock.

#5: Hormonal Impact

Fasting can cause several positive changes to your hormones

For example, prolonged fasting will decrease insulin levels and increase levels of human growth hormone.10Rothschild, J., Hoddy, K. K., Jambazian, P., & Varady, K. A. (2014). Time-restricted feeding and risk of metabolic disease: a review of human and animal studies. Nutrition Reviews72(5), 308–318. https://doi.org/10.1111/nure.12104 11Møller, N., & Jørgensen, J. O. L. (2009). Effects of Growth Hormone on Glucose, Lipid, and Protein Metabolism in Human Subjects. Endocrine Reviews30(2), 152–177. https://doi.org/10.1210/er.2008-0027

It can also improve insulin sensitivity over time, resulting in better monitoring of blood sugar levels and decreasing the risk of insulin resistance and type 2 diabetes.

Growth hormone helps repair cells and tissues, expedite cell turnover, and facilitate muscle growth and repair after exercise, among other benefits.

However, some of the hormonal benefits of fasting take longer to kick in and may not be significant with 12-hour fasting or even 16-hour fasting. That said, you are more likely to see a greater beneficial hormonal impact with a 16-hour fast than a 12-hour fast.

Conclusions

Overall, the main benefit of a 12-hour fast is that it is often a more sustainable and beginner-friendly approach to an intermittent fasting diet. You likely won’t need to adjust your dietary patterns all that much.

A clock next to a plate and utensils.

The benefits of 16/8 intermittent fasting vs 12/12 intermittent fasting are that you will see greater fat burning and lower levels of insulin because your body will have more time with no energy intake, meaning that you will burn through more of your glycogen and fat stores.12Weltan, S. M., Bosch, A. N., Dennis, S. C., & Noakes, T. D. (1998). Influence of muscle glycogen content on metabolic regulation. American Journal of Physiology-Endocrinology and Metabolism274(1), E72–E82. https://doi.org/10.1152/ajpendo.1998.274.1.e72

You may also lose more body weight with a 16-hour fast vs 12-hour fast because you will have less time permitted for eating, which can help prevent overeating and binging on carbs, further improving your metabolic health.

If you want to try an even more aggressive intermittent fasting diet than a 12-hour fast or 16-hour fast per day, check out our guide to the 18/6 intermittent fasting diet here.

FAQs

Can I fast every day?

Most medical and nutrition experts say that it is safe to follow an intermittent fasting diet such as the 12/12 or 18/6 diet every day. However, once you get up to 24 hours of fasting or more, you will need to fast much less often to give your body a break.

With prolonged water fasting, you are not taking in calories, so there is only so often you can be fasting 48 hours or 72 hours while still meeting you are nutrient needs for health.

You can still reap many of the benefits of fasting without doing it every day, for example through alternate-day fasting or fasting for just a few days of the week.

References

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Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a NASM-Certified Nutrition Coach and UESCA-certified running, endurance nutrition, and triathlon coach. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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