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One day fasting

What happens to our body during a one-day fast? Does one-day fasting help you lose weight? How does short-term fasting affect our health?

Fasting as a form of self-denial and humiliation before God by giving up food is an ancient and religious tradition.

Nowadays, fasting is mainly associated with weight loss – it’s all crash diets and intermittent fasting diets.

There is a lot of research on fasting and the possible benefits and risks of various food restrictions, respectively – this is a relatively well-researched field.


In this article, we will look at what happens to our bodies during short-term fasting.


Types of one-day fasting

A “one-day fasting” usually means fasting for 24 or 48 hours 2 to 3 times a month.

However, these days can be very different – both in terms of the duration of the fast and in terms of what each of us understands by fasting.

During the one-day fast, you can, for example:

  • Completely refuse food and drink only water.
  • Completely abandon food and drink both water and calorie-free drinks such as herbal teas, Zero Cola, etc.
  • Unloading days
  • Completely refuse to eat and drink only vegetable or fruit juices (it should be added here that fruit juices usually also contain a lot of sugar and therefore you can absorb even more calories with them than with “eating as usual”).
  • Eat only fresh or steamed vegetables and drink water and/or calorie-free drinks.
  • Eat and drink less than usual, for example – limiting the number of calories taken with food to 600 or 800 calories per day.
  • Exclude from the diet unhealthy food and drinks, for example – ultra-processed food, sweetened beverages, packaged juices, etc.

There is also an opinion that during the one-day fasting, one should give up not only food but also sex 😊.


What happens during fasting?

Whether a person is fasting or not, the body still needs energy. The body’s primary source of energy is sugar (glucose), which our bodies usually get from carbohydrates, including grains, dairy products, fruits, vegetables, beans … and, of course, sweets.

Glucose is stored in the liver and muscles and released into the bloodstream whenever the body needs it.

However, during fasting, this process changes because after about 8 – 12 hours of fasting (if you don’t eat anything), the glucose reserves in the liver are depleted, and the body enters a state called gluconeogenesis – the body goes into fasting mode.


Research shows that gluconeogenesis increases the number of calories burned because since carbohydrates are not taken in, the body has to produce glucose itself. During this phase, fat is mainly used for glucose production.

Read more about how our metabolism works HERE.


Eventually, this source of energy also runs out, and the body switches from fasting mode to starvation mode. During this phase, a person’s metabolism slows down, and the body begins to break down muscle tissue for energy.

Starvation begins when food is not consumed for several days in a row or when calorie intake is strictly limited for about a week or more.

In other words, fasting for 24 hours is usually safe unless you have any health problems.


Can fasting promote weight loss?

Fasting appears to help with weight loss. However, research shows that this is not the case for everyone.

If fasting is compared with other methods of weight loss – it does not show better results. In some studies, obese people who followed intermittent fasting diets lost slightly more weight than those who only reduced their daily calorie intake, but the difference was not statistically significant.

The effectiveness of fasting seems to have less to do with the physical effects it causes than with how it fits into a particular lifestyle because:

  • Several studies show that people who fast are more likely to give up trying to lose weight than those who follow more traditional diets such as calorie counting. The feeling of hunger is just hard to bear for a long time.
  • A second potential problem is that the results of fasting are reduced or even completely “erased” by overeating after the fasting period.
  • Third, fasting days can create a false sense of security – I have already fasted and “have saved”, and therefore now I can eat more.

How does short-term fasting affect our health?

It turns out that it is positive, as long as fasting is temporary and is not practised more often than 2 to 3 times a month:

  • Research shows that intermittent 24-hour fasting can improve cardiovascular health.
  • Some animal studies suggest that fasting can help fight certain types of cancer or even help improve/maintain a memory.

Water intake

Drinking water is very important for maintaining health even under normal conditions, but drinking water during fasting is especially important.

An adult woman of average height should drink at least 2 litres of water a day. Water can also help reduce hunger pangs.

If it is hard to drink clean water – drink herbal or other teas, and add flavour to the water with a slice of lemon, berries or mint leaves…

Read more about why you should drink water HERE.


Risks associated with fasting

Although short-term fasting is generally safe, in some cases, it can be risky.

Talk to your doctor before you start fasting if:

  • You have diabetes;
  • You have a history of any eating disorder;
  • You are taking medication that must be taken with food (before or after eating);
  • You are not yet 18 years old;
  • You are pregnant or breastfeeding.

How to break the fast?

Even if you fast for only 24 hours, your intestines are already empty and “stuffing” them quickly is not the best solution.

Therefore, you should return to your eating habits little by little.

Here are some ways to safely break your fast:

  • Drink water. I’m repeating myself, but this is important.
  • Eat in small portions, 5 to 6 times a day.
  • Chew each bite thoroughly (at least 30 times).
  • Choose foods that are easier to digest, such as porridge, cooked rather than fresh vegetables, bananas, etc.
  • Don’t experiment – trying new foods after fasting can make digestion difficult and cause nausea.

Key takeaways

Short-term fasting is generally safe and can be beneficial, including as a weight loss aid.


Fasting doesn’t help you lose weight faster, better, easier, or for longer than other traditional approaches, and it is harder to stick to in the long run.


If you are fasting for health reasons, consult your doctor or nutritionist and do not fast longer than necessary, as prolonged fasting can cause many complications.

There are many different types of fasting, from eliminating food to limiting the number of calories. Accordingly, everyone can find the most suitable for their lifestyle.

In combination with a balanced diet and a healthy lifestyle, short-term and infrequent fasting can benefit your health.

Alternate day fasting for weight loss in normal weight and overweight subjects

Intermittent fasting: the science of going without

Meal frequency and timing in health and disease

Meal timing during alternate day fasting: Impact on body weight and cardiovascular disease risk in obese adults

A Periodic Diet that Mimics Fasting Promotes Multi-System Regeneration, Enhanced Cognitive Performance, and Healthspan

Fasting-Mimicking Diet Reduces HO-1 to Promote T Cell-Mediated Tumor Cytotoxicity

Fasting: Molecular Mechanisms and Clinical Applications

Practicality of intermittent fasting in humans and its effect on oxidative stress and genes related to aging and metabolism

Dietary and physical activity adaptations to alternate day modified fasting: implications for optimal weight loss

Effect of Alternate-Day Fasting onWeight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults

24-Hour Water-Only Fasting Acutely Reduces Trimethylamine N-Oxide

Randomized cross-over trial of short-term water-only fasting: metabolic and cardiovascular consequences

Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults

Gluconeogenesis and energy expenditure after a high-protein, carbohydrate-free diet

Alternate day fasting and endurance exercise combine to reduce body weight and favorably alter plasma lipids in obese humans

Long-term effects of 2 energy-restricted diets differing in glycemic load on dietary adherence, body composition, and metabolism in CALERIE

Determinants of weight loss success with alternate day fasting

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