12 Hour Fast vs 16 Hour Fast: Comparing Fasting Length Benefits

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reviewed by Katelyn Tocci

Choosing an intermittent fasting diet mainly involves picking the intermittent fasting hours in terms of 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 a 12 hour fast vs 16 hour fast, respectively.

But, what are the primary differences between a 12 hour fast vs 16 hour fast? Are there significant benefits of a 16 hour fast vs 12 hour fast? Is it better to do 12/12 intermittent fasting or a 16/8 intermittent fasting diet?

In this article, we will compare the benefits of intermittent fasting for 12 vs 16 hours, and discuss how intermittent fasting hours affect the benefits of a 12 hour fast vs 16 hour fast.

We will look at: 

  • How Does a 12 Hour Fast vs 16 Hour Fast Compare?
  • What Are the Differences Between a 12 Hour Fast vs 16 Hour Fast?
  • Key Differences Between a 12-Hour Fast and a 16-Hour Fast

Let’s get started!

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How Does a 12 Hour Fast vs 16 Hour Fast Compare?

Before we look at the differences in the logistics, fasting physiology, and health benefits that occur between a 16 hour fast vs 12 hour fast, let’s briefly cover the basics of what 12 hour fasting vs 16 hour fasting refers to.

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

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.

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What Are the Differences Between a 12 Hour Fast vs 16 Hour Fast?

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

This is because many of the health benefits of fasting, such as enhanced autophagy, take closer to 18-24 hours or more to really take effect. Furthermore, anything much less than a 12 hour fast will really 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 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.

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Additionally, the specific 12 hour fast vs 16 hour fast benefits you personally experience will depend on your own 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, 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.

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

While the 12 and 16-hour fast have many important similarities, the research suggests that there are differences between the two to consider, especially when first starting out.

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#1: 12 Hour Fast vs 16 Hour Fast: Difficulty and Logistics

The most obvious answer to the question: “What Is the difference between a 12 hour fast and 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 vs 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 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.

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#2: 12 Hour Fast vs 16 Hour Fast: 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.

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.

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#3: 12 Hour Fast vs 16 Hour Fast: 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 window 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.

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: 12 Hour Fast vs 16 Hour Fast: Fat Burning

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

This is a crucial first step before significant fat burning and ketosis can set in.

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#5: 12 vs 16 Hour Fasting: 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.

It can also improve insulin sensitivity over time, which in turn can decrease 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 16 hour fast vs 12 hour fasts.

Overall, the benefits of a 12 hour fast vs 16 hour fast are that it is often more sustainable and a beginner-friendly approach to an intermittent fasting diet. You likely won’t need to adjust your dietary patterns all that much.

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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 stores.

You may also lose more weight with a 16 hour fast vs 12 hour fast because you will have less time permitted for eating, which can help prevent overeating.

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.

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Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a 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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