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Fasting or Intermittent fasting

Fasting is one of the newest trends in weight loss. What does it promise us and what happens to us during the fast?

Fasting, or intermittent fasting, is one of the world’s most popular health and fitness trends right now. Some claim it has helped them lose weight, improve their health and simplify their lifestyle.

Some studies show that short-term and intermittent fasting can help lose weight and improve brain and heart health, and in some studies, it is also linked to better cognitive function and digestion.

Fasting as an eating pattern is not new. People have been fasting – taking longer breaks between meals – for simple lack of food or religious reasons.

In this article, let’s look at what exactly fasting is. How does fasting affect our bodies? What benefits does it bring us and what are the possible side effects and health risks?


What is fasting or Intermittent Fasting?

Intermittent fasting is a pattern of eating that involves regular periods of fasting and eating according to a set schedule.

Intermittent fasting does not specify how much or which foods you should or should not eat, but only when you should eat.

The underlying idea is that if we eat only during a certain period each day, rather than all day, we are more likely to take in fewer calories.

Especially nowadays in the Internet and social networking age, when it is becoming more and more difficult to maintain a healthy weight.

In the past, when there was no internet, computers and smartphones, television programmes ended around 23.00 and people stopped eating because they went to bed.

Portions were also much smaller and people moved more.

Now, sedentary lifestyles are the mainstream, the Internet, television and other entertainment options are available 24/7 and this means that you can sit all day and most of the night and – eat.

Fasting is not, in fact, a diet, but an attempt to reduce calorie intake by limiting the periods of eating.


The most popular types of intermittent fasting

Eating regime 16/8

Fasting each day for 16 hours and eating whatever you like for 8 hours. During fasting you can drink water, sugar-free coffee and other zero-calorie drinks that may help reduce hunger.


Eating regime 5:2

Eat as before 5 days a week, but limit calorie intake to 500-600 Kcal for 2 days (women are advised to eat 500 calories, men 600).


Eating regime Eat/Don’t eat

24-hour fasting once or twice a week. Water, sugar-free coffee and other calorie-free drinks are allowed during the fast, but other types of food are not.


Eating regime Fasting every other day

Eat normally one day, the next day eat nothing, the next day eat, the next day eat nothing and so on. There are different versions of this method. Some of them allow take in about 500 calories on fasting days. Only water, sugar-free coffee and other calorie-free drinks are allowed during the fast.


Eating regime Soldiers’ diet

Eat small amounts of raw fruit and vegetables during the day and one huge meal at night. In other words, fast all day and eat once at night. This eating regime is quite similar to the Paleo diet – you eat mostly whole, unprocessed foods.


Eating regime Spontaneous skipping of meals

Occasional skipping of meals – essentially intermittent but spontaneous fasting.


What happens to us during fasting?

From around the 12th hour of fasting, the glycogen reserves in our muscles and liver run out. As glycogen reserves run out, insulin levels drop, signalling the body to switch to other resources for energy production.


Over the next 12-24 hours:

  • When glycogen stores run low, the body starts looking for an alternative source of energy – fatty acids begin to be converted into ketone bodies.
  • Cellular autophagy (cellular renewal processes during which accumulated old and non-functioning proteins are broken down and eliminated) is activated;

Within 24-48 hours:

  • Production of ketone bodies increases to provide the body with energy.
  • Human growth hormone levels increase, which promotes fat loss and cell renewal.

After 48-72 hours:

  • If no additional carbohydrates are consumed, within about 48 to 72 hours the body is already in a state of deep ketosis, getting energy mainly from fat;
  • The autophagic process is enhanced, promoting extensive cell renewal and regeneration;
  • Various metabolic changes continue to optimise the efficiency of the body’s energy use and preservation.

In other words, the longer we fast, the more the body tries to save energy. As a result, our metabolism changes very significantly.


The health benefits of intermittent fasting

Studies have shown that short-term intermittent fasting can:

  • Potentially contribute to weight loss (if fasting results in a calorie deficit).
  • Reduce risk factors for heart disease, e.g. – lower blood pressure.
  • Improve insulin sensitivity, which in turn improves blood sugar control.
  • Contribute to lower levels of oxidative stress markers.


  • Relax the digestive system, which can improve gut health and function.

Psychological and cognitive effects of fasting

  • Some people report increased mental clarity and concentration during periods of fasting (due to stable blood sugar levels and efficient use of energy in the brain).
  • Changes in mood and behaviour are also common (depending on individual differences in metabolism, psychological factors and general health).

Side effects of intermittent fasting

1 Feeling hungry

Constant hunger and cravings for food are highly distressing and are usually the most unpleasant side effects of fasting.

Studies comparing the feeling of hunger in people on different intermittent fasting regimes are very limited.

A 2022 study involved 34 participants who followed the 5:2 diet or a traditional calorie deficit and performed strength/resistance workouts. After 12 weeks, participants in both groups reported low levels of hunger and most participants had adhered to their eating regimes.

Another study looked at 112 obese adults who followed either a modified 5:2 diet or a traditional calorie deficit for 1 year. Weight loss and heart health benefits were similar between the two groups, but participants on the 5:2 diet felt hungrier on average.

A 2020 study of 1,422 people studied fasting regimes lasting between 4 and 21 days. Most participants only felt hungry during the first few days.

In other words – the feeling of hunger may diminish as the body adapts to the new conditions (lower calorie/energy intake).


2 Headaches and lightheadedness

Headaches and lightheadedness are common symptoms during fasting.

These symptoms can be caused by many physiological and environmental factors related to changes in eating habits and general lifestyle:

Fasting headaches usually occur in the front of the head and are generally described as dull, diffuse pain.

The headache may start a few hours after fasting or close to when the person usually eats. Pain is typically mild to moderate and is caused by:

  • Prolonged fasting can cause a drop in blood sugar (hypoglycaemia) and this can lead to headaches as glucose is the main source of energy for the brain.
  • Insufficient fluid intake can lead to dehydration, which is a common cause of headaches.
  • People who regularly consume caffeine may reduce their caffeine intake during fasting, which can also cause headaches.
  • Fasting can alter the balance of electrolytes (sodium, potassium, magnesium), which are essential for normal brain function.
  • Headaches can also be caused by psychological factors such as stress and hunger.

Lightheadedness (sometimes fainting) during fasting is also a common side effect, especially when standing up quickly (orthostatic hypotension).

There may also be general weakness and a feeling of unsteadiness or visual disturbances, for example, a person may see ‘spots’ (scotomas).

Causes are similar to those of fasting headaches:

  • Lightheadedness, like headaches, can be caused by low blood sugar. The brain needs a constant supply of glucose and a drop in glucose can cause lightheadedness and even fainting.
  • Lightheadedness can also be caused by insufficient water intake, which can lead to a decrease in blood volume and blood pressure.
  • Prolonged fasting can cause a drop in blood pressure, especially when standing up quickly (orthostatic hypotension).;
  • Electrolyte imbalance can interfere with normal cell function.
  • Low energy levels (due to reduced calorie intake) can also cause weakness and lightheadedness.

3 digestive disorders

Indigestion, diarrhoea, nausea and bloating may also occur during fasting.

Indigestion is characterised by symptoms such as:

  • A burning sensation or discomfort in the upper abdomen.
  • Heartburn (burning sensation in the chest caused by acid reflux).
  • Bloating (even when eating very little).

Causes include:

  • Even when the stomach is empty, it continues to produce acid, which can irritate the stomach lining.
  • The transition from fasting to periods of eating can interfere with the normal digestive process, causing discomfort.

Diarrhoea or watery stools, pain or cramps in the lower abdomen associated with bowel movements.


  • After prolonged fasting, large or fatty meals may overload the digestive system.
  • Following prolonged fasting, too rapid intake of large amounts of food can cause rapid changes in fluid and electrolyte levels.

Nausea as a general feeling of abdominal discomfort or vomiting (especially if food is consumed too quickly).


  • During prolonged fasting, the body goes into a state of ketosis, which in some people manifests as nausea.
  • Without food to neutralise stomach acid, it can irritate the stomach lining and cause nausea.

Increased gas production causes bloating and a feeling of tightness or discomfort in the abdomen.


  • After fasting, the digestive system may react sensitively to the sudden appearance of food.
  • After fasting, gas in the intestines can also be caused by foods previously processed easily.

4 Irritability and other mood changes

During periods of severe calorie restriction or fasting, blood sugar levels can drop and cause:

  • Irritability;
  • Anxiety;
  • Decrease in ability to concentrate.

Why does this happen?

  1. During fasting, blood sugar levels can drop and – because the brain is so dependent on glucose – low blood glucose levels can cause irritability, confusion and difficulty concentrating.
  2. Hormonal changes occur during fasting:
    • Fasting can increase levels of the stress hormone cortisol, which can contribute to feelings of stress, anxiety and irritability.
    • Fasting changes levels of hormones that regulate hunger. The hunger hormone ghrelin increases appetite and may affect mood, leading to irritability. Leptin (the hormone “responsible” for satiety) levels decrease also contributing to mood swings.
  3. Fasting can affect levels of neurotransmitters such as serotonin and dopamine, which are linked to mood regulation. Low levels of these neurotransmitters can lead to feelings of depression, irritability and anxiety.
  4. Psychological factors:
    • Stress caused by fasting, and in particular concerns about loss of control over the fasting process or its consequences.
    • Changes in eating routines can cause psychological distress and mood swings.

5 Fatigue and low energy levels

Research has shown that some people who practise intermittent fasting experience fatigue and low energy levels.

Low blood sugar levels can cause feelings of tiredness and weakness. Furthermore, intermittent fasting can cause sleep disturbances in some people, which can lead to feelings of tiredness during the day.


6 Bad breath

This is caused by insufficient saliva secretion and an increase in the acetone level in the breath:

  • Fasting makes the body use fat for energy. Acetone is a by-product of fat metabolism, so it increases in the blood and breath during fasting.
  • Dehydration can cause dry mouth.

7 Sleep disorders

Sleep disturbances, such as the inability to fall asleep or stay asleep, are considered one of the most common side effects associated with intermittent fasting.

Research also points to this.

Sleep disturbances are most common in the first days of intermittent fasting because the body excretes large amounts of salt and water in the urine, which can lead to dehydration and low sodium levels.


Some studies show that intermittent fasting does not affect the quality, duration of sleep or severity of insomnia.


Risks of intermittent fasting

1 Nutrient deficiencies (don’t confuse with calorie deficit)

By limiting your eating period, you will probably eat less and therefore take in fewer calories.


Whether you get all the nutrients your body needs depends not on when or how much you eat, but on what you eat.

In other words, if your diet is complete and nutrient-dense and you consume at least 1200 Kcal (women) to 1400 Kcal (men) on average per day, you are unlikely to have any problems.


If you eat mostly ultra-processed food and restrict your calorie intake too much – health problems are almost inevitable.


2 Metabolic slowdown

Metabolic slowdown during (prolonged) fasting is a well-documented phenomenon – the body goes into a lean mode (learns to survive on fewer calories). Both the basic metabolic rate (the amount of energy the body needs to maintain basic physiological functions at rest) and the thermic effect of food (the energy used to digest, absorb and metabolise food) decrease.

When you start eating normally, you will already need fewer calories.

In other words, even if you start eating normally again and take in the right number of calories for your weight and level of physical activity, there still will remain a surplus which is stored in reserve – converted into fat.


3 Dehydration

During the first few days of fasting, the body excretes large amounts of water and salt in the urine.


If you do not restore the lost fluids and electrolytes, dehydration can start.

Using laxatives during fasting increases the risk of fluid imbalance and dehydration.


People who practice intermittent fasting often also forget to drink or do not drink enough.


Drink water throughout the day and keep an eye on the colour of your urine – dark-coloured urine can be a sign of dehydration.

Read more about why and how much water you should drink HERE.


4 Binge eating    

All restrictive diets are associated with the risk of “bingeing”, which makes the previous fasting period pointless – especially in this case. For example, for the 6/18 diet the optimal eating window is between 8:00 and 14:00 – will you be able to subsist on water, teas and sugar-free coffee from 14:00 until bedtime?


5 Rapid fluctuations in sugar levels

For people with diabetes or low blood sugar, fasting can lead to rapid fluctuations in sugar levels, which in turn can be dangerous for their health.


The longer the fasting period, the more significant the risks to your health become.


Who should avoid intermittent fasting?

  • Pregnant women.
  • New mothers who are breastfeeding.
  • Growing children and adolescents.
  • Type 1 diabetics.
  • Anyone with a digestive system-related disease.
  • Anyone with a chronic disease of any kind.
  • All those who take medications daily that should not be taken on an empty stomach.
  • People with an overactive thyroid gland (hyperthyroidism).

Key takeaways

It is important to understand that weight loss is only possible with a calorie deficit – if you eat more than your body uses, you will gain weight no matter what you do, when, what and how you eat.

Like any weight loss diet, fasting focuses on consuming fewer calories per day.

In other words – if you can’t restrict yourself in any other way – eat for example only 6 hours a day and you will almost certainly consume fewer calories in those 6 hours than you did eating the whole day.


Taking in small amounts of calories/nutrients throughout the day – exactly when you need them – will definitely have a greater effect, because the amount of calories you take in is easier to control – you take in exactly as many calories, nutrients, minerals, vitamins, enzymes … as your body needs (no unspent calories to turn into fat).


If fasting as a method of calorie restriction appeals to you, I would recommend a 5:2 eating regime for weight loss, or a 6/18 regime (eating between 8 am and 2 pm) alternating every other week with a weight-maintaining 8/16 regime (eating between 8 am and 4 pm).

If you have an eating window in the first half of the day, the calories you consume will be more likely spent before you go to bed (when metabolism slows down and energy expenditure drops significantly).

In other words – you are less likely to have unspent calories that your body will turn into fat.


Although the advertisers of intermittent fasting do not restrict the type of food consumed, I would still recommend avoiding at least fast food, sausages and products containing palm oil.


Eat tasty, eat balanced and – stay healthy!

Association of Eating and Sleeping Intervals With Weight Change Over Time

Effects of Short-term Fasting on Ghrelin/GH/IGF-1 Axis in Healthy Humans: The Role of Ghrelin in the Thrifty Phenotype

Intermittent fasting: is there a role in the treatment of diabetes? A review of the literature and guide for primary care physicians

The Effects of Calorie Restriction on Autophagy: Role on Aging Intervention

Five Days Periodic Fasting Elevates Levels of Longevity Related Christensenella and Sirtuin Expression in Humans

Ketones and Human Performance

The Effect of Medium Chain Triglycerides on Time to Nutritional Ketosis and Symptoms of Keto-Induction in Healthy Adults

Physiological process of fat loss

Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects

Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes

Intermittent fasting and weight loss

Intermittent fasting for the prevention of cardiovascular disease

Health Effects of Alternate-Day Fasting in Adults: A Systematic Review and Meta-Analysis

Intermittent fasting for the prevention of cardiovascular disease

Fasting headache

Effects of Intermittent Fasting on Health, Aging, and Disease

Intermittent Fasting: Its Effects on Body Weight and Daily Living

Hypoglycemic symptoms in the absence of diabetes: Pilot evidence of clinical hypoglycemia in young women

The Psychological Effects of Short-Term Fasting in Healthy Women

Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting

Breath acetone as a marker of energy balance: an exploratory study in healthy humans

Halitosis: Current concepts on etiology, diagnosis and management

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