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Why Does Intermittent Fasting Work?

Most people easily lose weight with intermittent fasting, even after struggling with their weight for their whole life. What’s the secret?
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Intermittent fasting is very popular when it comes to weight loss. How is intermittent fasting different from other “diets”? Constant snacking blocks our energy storage, which makes it difficult to lose weight. By increasing the time between meals, intermittent fasting allows us to use stored body fat. The insulin hormone plays a key role in this process.

Written by
Sarah Neidler, PhD
Freelance Science and Medical Writer

Many people who start with intermittent fasting have tried countless diets. They may have helped them lose some weight temporarily, but it always came back. For them, intermittent fasting is the first “diet” that works. Intermittent fasting helps people with weight issues to control their appetite–they feel better and have more energy. Some people even have the impression that their stomach has shrunk.

For these reasons, intermittent fasting is easy to follow long-term, and the feared yo-yo effect doesn’t occur.

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Why is intermittent fasting so different? How does it work?

Blocked energy storage

Storage rooms with closed doors.

To answer the question, let’s start with a different question. We all have huge amounts of stored energy, mostly in the form of body fat. Even lean people have enough fat reserves to survive for several weeks. So why are so many people hungry a few hours after their last meal and feel like they are starving?

The answer is that they are not able to access this energy storage. It’s blocked. Why is that?

External and internal energy sources

There are two ways to supply our body with energy: external and internal energy. When we eat, we are taking up external energy. When we fast, we have to rely on internal energy.

The problem is that it is not possible (or only to a very limited degree) to use both sources at the same time (1). That means that when we eat, our internal storage areas are blocked

So what happens when you constantly snack during the day with very short breaks in between? 

Correct: You reduce any opportunity to access your energy storage. The only time your body has a chance to burn excess storage is at night while you’re asleep–when you’re not eating.

Interestingly, this way of eating has been proposed to be healthy for many years. Low-calorie snacks eaten regularly throughout the day are supposed to help with weight loss. The idea behind it: 

  1. Consuming fewer calories than you expend results in weight loss.
  2. Regular meals are supposed to prevent a decrease in metabolic rate that is caused by calorie-reduced diets

Intermittent fasting research proves the second assumption wrong (2). Blocking our energy storage with regular meals does the opposite. When you consume less energy than your body needs, it has to decrease its energy expenditure

To a limited extent, the body can still access some of the stored energy. This is why people do lose some weight on calorie-reduced diets, especially in the beginning. But because regular meals make it so difficult to use stored energy, reduced caloric intake results in constant hunger. This is the number one reason why diets fail. With a lot of discipline, you can stand hunger for a while–but no one can take the hunger pangs forever.

To understand why regular meals block our energy storage, let’s take a short biochemical excursion. We’ll keep it simple and won’t get too detailed–promise. 

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Insulin–the fat-storage hormone

Orange syringe with liquid dropping out.

You probably know insulin as a blood sugar-lowering hormone that diabetes patients take to control their blood sugar. But insulin is also a storage hormone, especially a fat-storage hormone (3). When you eat something, your blood sugar rises. Shortly after, insulin is released. Insulin helps cells to remove sugar from the blood. As a consequence, blood sugar goes back to normal levels (4).

At the same time, insulin blocks fat storage. Why is that? Because high blood sugar is harmful, it is crucial that the cells are hungry for sugar and “eat up” as much as possible. If cells removed fat instead, blood sugar would remain high. For this reason, cells can only feed on fat once blood sugar and insulin are back to normal.

You can compare this to parents who don’t allow their kids to eat sweets before a meal. When they do, the kids are not hungry anymore and refuse to eat their vegetables.

Depending on what and how much you eat, it can take a few hours for insulin to return to normal levels. That means that with frequent snacking, insulin is constantly elevated throughout the day.

To make things worse, constantly elevated insulin levels lead to insulin resistance (5). That means that cells stop “listening” to insulin and refuse to take up sugar from the blood. As a consequence, much more insulin is necessary to control blood sugar. As you can imagine, high levels of a fat-storage hormone are somewhat counterproductive when it comes to weight loss.

Bottom line

Intermittent fasting is different from any other diet. In fact, intermittent fasting is not a diet, it is a way of eating that balances important hormones (especially insulin) and physiological processes that support weight loss. This is the reason why intermittent fasting works so well.


1. Stralfors P, Honnor RC. Insulin-induced dephosphorylation of hormone-sensitive lipase. Correlation with lipolysis and cAMP-dependent protein kinase activity. Eur J Biochem. Jun 15 1989;182(2):379-85. doi:10.1111/j.1432-1033.1989.tb14842.x

2. Welton S, Minty R, O'Driscoll T, et al. Intermittent fasting and weight loss: Systematic review. Can Fam Physician. Feb 2020;66(2):117-125.

3. Czech MP, Tencerova M, Pedersen DJ, Aouadi M. Insulin signalling mechanisms for triacylglycerol storage. Diabetologia. May 2013;56(5):949-64. doi:10.1007/s00125-013-2869-1

4. Wolever TM, Miller JB. Sugars and blood glucose control. Am J Clin Nutr. Jul 1995;62(1 Suppl):212S-221S; discussion 221S-227S. doi:10.1093/ajcn/62.1.212S

5. Petersen MC, Shulman GI. Mechanisms of Insulin Action and Insulin Resistance. Physiol Rev. Oct 1 2018;98(4):2133-2223. doi:10.1152/physrev.00063.2017

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