How Intermittent Fasting Affects Your Blood Sugar Levels

Marie-Luise Huber
7 min.
How Intermittent Fasting Affects Your Blood Sugar Levels
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In recent years, intermittent fasting has slowly become a mainstream diet. Many people are opting for longer intervals between meals in order to control their weight. Many are surprised to find that it is much easier to live with and much less restrictive than traditional diets such as keto, calorie counting and many others.
Due to the benefits of intermittent eating, which improves overall health, lowers disease risk and increases metabolic health, intermittent eating is also appealing to those who are not necessarily looking to lose weight.
In addition to weight loss, there are other surprising health benefits, such as lowering insulin and blood sugar levels.

In this article, we take a closer look:

  • whether intermittent fasting actually works.
  • what effects intermittent fasting can have on our bodies.
  • how to do intermittent fasting "correctly" and safely.
  • how to measure success.

The basic idea of intermittent fasting

What exactly is intermittent fasting?

Regardless of the specific design, intermittent fasting involves restricting the time window in which you eat. The most popular approaches are 16:8, 20:4 and 5:2. The numbers refer to the hours between "fasting and eating" in a day (16:8 and 20:4) or the days between "eating and fasting" in a week (5:2).

Where does intermittent fasting come from?

The practice of intermittent fasting was inspired by the search for solutions to a modern problem in prehistoric times. Thousands of years ago, food was not available in the abundance that many of us have become accustomed to today. Therefore, the human body has adapted to be able to continue functioning in the hours, days or even weeks it took our ancestors to find food.

When we don't eat, our blood sugar levels drop, which can be a problem as our brain relies on blood sugar 'fuel' to function. However, the human body has developed defense mechanisms to prevent this from affecting our cognitive and metabolic functions, allowing us to continue to seek (and eventually find) food in order to survive.

Fasting activates the body's stored alternative energy sources, such as fat, and triggers the production of ketone bodies to fuel the brain. In overweight people, fasting favors the reduction of insulin levels (a hormone that helps to transport glucose from the blood into the cells), which ultimately leads to weight loss. During fasting, the body is also more sensitive to certain hormones, which means that the body responds better and is better able to absorb nutrients from food when the fast is finally broken.

When do you start fasting?

The good news is that, whether you realize it or not, you're probably already reaping some of these benefits by practicing intermittent fasting while you sleep! If you eat your last meal a few hours before going to bed and your morning meal an hour or two after getting up, that gives you a fasting time of 12-13 hours.

However, intermittent fasting in this post is about more than just giving up late night snacks. Intermittent fasting here refers to the additional observance of fasting periods during which overweight or type 2 diabetics partially abstain from food during the day in order to lower insulin and blood sugar levels and support weight loss. And this is how it works.

How exactly does intermittent fasting work?

When should you eat and when not?

The most common model for intermittent fasting is 16:8, which means that you don't eat for 16 hours while maintaining an 8-hour window in which you consume all the food and (caloric) drinks you would normally eat throughout the day.

Many people interpret this to mean that they can indulge in any high-calorie snacks they like. Or they believe that 16:8 means that you should simply skip breakfast. Prof. Dr. Herzig, Director and Scientific Director of the Helmholtz Diabetes Center, points to studies that show that skipping breakfast is not the best way to practice intermittent fasting, but it does produce results:

"Eating early in the morning, having lunch and then eating the last meal in the late afternoon is better in terms of health parameters than skipping breakfast. However, everyone can ask themselves what they would prefer to do and whether it is easier for them in everyday life to skip breakfast or dinner." (Dr. Stefan Herzig)

For most people, replacing dinner with a late afternoon meal is more difficult than skipping breakfast because dinner is a social event shared with friends and family. It could be awkward if someone invites you to dinner and you just sit there with a glass of water, politely refusing to try any of the food. Prof. Dr. Herzig says that while it's not optimal to practice intermittent fasting if you start your 8-hour eating window later in the day so that you skip breakfast, it's"still beneficial."

How much should I eat during intermittent fasting?

However, intermittent fasting is not about skipping meals or reducing your calorie intake. The idea is that you still consume the same number of high-quality calories that your body needs each day, but in a smaller, fixed window of time, rather than eating and snacking throughout the day.

What should you eat during intermittent fasting? What should you pay attention to?

There is no standard protocol for intermittent fasting, to promote health and longevity. But many plans limit total calorie intake. This makes food quality a top priority.

We recommend planning for the most nutritious meals and snacks. This means mixing processed foods and focusing on fresh, healthy and colorful foods. Yes, you can eat cake, but it's much better as a snack after a high-fiber, high-protein, high-fat meal and not on an empty stomach while breaking your fast. You'll notice it in your blood sugar levels and feel better overall.

The first meal of the day counts

It depends on how you break your fast. 

A "breakfast" that is rich in carbohydrates can put you on a glucose roller coaster for the rest of the day. It really sets the tone. If your first meal contains fat and protein (before the carbs), your blood sugar will be more stable than if you start with a high-sugar meal that causes a glucose spike. Here are some recipes for blood sugar friendly breakfast ideas.

How long does the effect of intermittent fasting last?

When you stop intermittent fasting, your body returns to its original state almost immediately. This means that the benefits of intermittent fasting only come into play when you are actively practicing it. However, if you stop intermittent fasting and start again, the benefits will return very quickly.

This means that it is very easy to take a break from intermittent fasting, e.g. a weekend or a week, and then get back into it. As Prof. Dr. Herzig explains, "It's a very responsive thing that happens, and so the longer and the more continuously you do it, the better."

Is intermittent fasting suitable for everyone?

In addition to weight loss, intermittent fasting has been shown to be an effective way to reduce insulin resistance in people who are overweight or have type 2 diabetes. However, if you're already at a healthy weight and don't have type 2 diabetes, some studies have shown that intermittent fasting can actually lead to unhealthy metabolic consequences.
Although healthy participants in one study lost weight, intermittent fasting also led to increased insulin resistance, fasting glucose and lipids.

So there is limited evidence that intermittent fasting is more effective than a healthy, nutritionally balanced and calorie-conscious diet for healthy people within their desired weight range.

Should women do intermittent fasting?

As mentioned above, interval fasting has become very popular. Especially skipping breakfast and eating two more substantial meals. However, interval fasting is not the best diet for every woman.

When should women not do intermittent fasting?

Firstly, for women with hormonal disorders, especially PCOS, a good breakfast is very important to lower insulin and testosterone and improve ovulation. This is not only important for fertility, but also for long-term health.

Secondly, intermittent fasting can trigger eating disorders, as women may tend to overeat during the eating window. This deserves special attention, as studies have shown that up to 23 percent of women with PCOS are prone to eating disorders.

Intermittent fasting can also cause additional stress for the body during the menopause. Many women practise intermittent fasting to lose weight during the menopause. However, while the body is exposed to major hormonal fluctuations, intermittent fasting may not provide optimal support. 

Intermittent fasting is probably not a good idea for pregnant and breastfeeding women either. The natural calorie restriction caused by Intermittent fasting can have a negative impact on the development of the fetus. As the mother's nutrition is largely dependent on what she eats, a restricted diet during breastfeeding can lead to less nutritious milk and thus affect the development of the newborn.

What can this look like in real life?

Together with a hearty and rich breakfast, an interval of at least 12 hours between dinner and breakfast the next morning can improve insulin sensitivity.

A 12-hour interval should not be too difficult to achieve. For example, a woman could choose to fast between 7pm and 7am. She would then have to finish dinner before 7pm and wait until 7am for breakfast, but would sleep most of the time in between.  

The benefits of intermittent fasting

Lowering insulin & weight loss

Apart from the (optional) limited calorie intake, the point is that your insulin levels drop when you don't eat and your fat cells can then release their stored sugar to use as energy and keep blood sugar levels constant. You could say that your body flips a metabolic switch. 

This increases your insulin sensitivity and your body can respond better to the release of insulin. This means that less insulin is needed to transport the glucose in your blood into the cells where it is used as energy. 

Lowering insulin therefore helps you to lose weight. Too much insulin can hinder fat burning and therefore make weight loss more difficult. 

When you eat constantly, your body doesn't need to access energy stores in the form of fat because it has energy available in the form of glucose - its preferred fuel. You can find out more about this in our blog post on weight control and blood sugar.

Reduce inflammation

Eating less frequently can also help reduce inflammation, which is good because it also reduces insulin resistance. Here's why:
When you eat all the time, especially on a high-carb diet, your body is constantly releasing insulin, triggered by glucose uptake. Over time, the insulin builds up in the body and the cells can no longer use it as effectively as before - a phenomenon also known as insulin resistance. The cells become "numb" to the use of insulin, which is important for storing glucose. This in turn leads to higher blood sugar levels and inflammation.

So by limiting your calorie intake to a smaller window of time AND eating a healthier diet, you can reduce your risk of diseases such as cancer, diabetes, cardiovascular disease and Alzheimer's disease. 

Strengthens our circadian rhythm

Another benefit of interval fasting worth mentioning is that it boosts our natural circadian rhythm. Deep in everyone's brain there is a master clock that stimulates our body to perform certain activities, taking into account external data such as sunlight.

This so-called circadian rhythm is comparable to a smartphone with a whole day of alarms preset - getting up, having lunch, etc. - only much less annoying. Our circadian rhythm also includes a glucose/insulin timer. Sensitivity to insulin decreases throughout the day.

Our bodies take longer to process a piece of bread at 9 p.m. than at lunchtime. So if you eat dinner earlier in the day (for example, by sticking to a 4:8 schedule and eating only between 10 a.m. and 6 p.m.), the circadian effectiveness of insulin increases.

This circadian rhythm also explains why blood sugar levels go up and down during sleep. Even if you don't eat anything. In addition to the circadian rhythm, there are other factors that influence our blood sugar at night:

  1. Dreams and REM sleep: Non-REM sleep is associated with an increase in blood glucose levels, while REM sleep is associated with stable blood glucose levels. Thus, blood glucose levels may depend on the phase of sleep. 
  2. Meals eaten during the day: If these meals are very carb-heavy, it will lead to higher fluctuations during the day. That's why, at Hello Inside, we consider the quality of the food and the order of the meals crucial to avoid the blood sugar roller coaster that continues into the night.

Long-term effects

Intermittent fasting can be a viable, consistent way to increase insulin sensitivity and lower average blood sugar levels. By shifting our eating window to earlier in the day, we align our food intake with the time when our bodies are naturally most sensitive to insulin, lowering peak insulin and blood sugar levels.

Your blood sugar level can have a significant impact on how your body feels and functions. Therefore, a stable blood sugar level can be an important factor in your overall well-being and guide you in your own eating behaviors. So let's take one last look at some ways you can put IF to work for you.

Accompany intermittent fasting with a continuous glucose monitor (CGM)

If you want to start intermittent fasting, it is particularly important to monitor your blood sugar levels. This way you can make sure that intermittent fasting is working and monitor your metabolism. It is also important to pay attention to whether your blood sugar levels drop too much. If this is the case, you should change your intermittent fasting to stabilize the values. It is not only high blood sugar levels that have negative side effects. If your blood sugar levels are too low, this can lead to short-term symptoms such as dizziness, cravings, tiredness and concentration problems. As always, if in doubt, it is best to consult a doctor.

In the long term, intermittent fasting can not only support your metabolism, but can even help you lose weight, as described above.

Firstly, because you may consume fewer calories in the smaller time window. And secondly, because you will ideally stabilize your blood sugar and thus improve your insulin sensitivity and reduce cravings, for example.

 5 tips for getting started with intermittent fasting for a better blood sugar balance

If you haven't tried intermittent fasting yet:

  1. Plan healthy, nutritious meals in advance
    Although you may be tempted to indulge in your favorite snacks and tasty dishes once your fast is over, you should stick to a healthy diet of protein, fruit and vegetables.
    Choose a hearty meal to break your fast. (e.g. eggs, tofu, avocado, cheese). Make sure your meals are colorful and high in fiber so you get all the nutrients and stay full longer. Eat sugar and refined grains last or for dessert. Here you can find a Blood sugar-friendly shopping list.
  2. Drink enough
    Make sure you are well hydrated - especially during the fasting phases. This includes water, herbal teas and calorie-free flavored drinks.
  3. Move
    Let your body burn fat between meals. Don't snack. Be active throughout the day. Build muscle. Sport and exercise are important for a healthy metabolism and blood sugar balance. 
  4. Start with a modified schedule
    Start with a schedule that is realistic for you and your routine and then gradually increase the intensity and duration. You don't have to jump headfirst into the program. Consider a simple form of intermittent fasting that works for you. Test which fasting phases are easiest for you to stick to and which make you feel best.
  5. Listen to your body
    Keep an eye on how your body reacts to your change in diet by monitoring your blood glucose level with a continuous glucose monitor (CGM). Hello Inside allows you to track your blood glucose levels with a CGM over time so you can see if your lifestyle is supporting a healthy life.

Keep in mind that intermittent fasting may not be the right solution for you if you are already in a healthy weight range or don't have type 2 diabetes. 


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Marie-Luise Huber
Luise has spent the last 15 years studying nutrition and guiding people toward healthy lifestyles. Before becoming Head of Nutrition at Hello Inside, she helped parents plan the right lunches for their children. Luise also optimized food ingredients in Central and Eastern European countries. She has helped more than 1000 people of all ages on their weight loss journey. Her favorite tip for balancing blood sugar is exercise, as Luise loves to run, ski or bike. And she loves to bake (not always blood sugar friendly).
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