VERIFIED CONTENTAuthor: Aleksandra Żyłowska-Mharrab, dietitian, food technologist, educator

Intermittent dry fasting, which is fasting without food or drink for a specific period of time during the day, is rated as an extreme food choice with a high he alth risk. The most common argument is that intermittent dry fasting leads to dehydration. Is it really so? Does dry intermittent fasting dehydrate the body? How does it affect he alth and sports performance? Who shouldn't use dry intermittent fasting? We answer on the basis of scientific publications in reputable journals.

Dry fastingis a type of fasting in which the fasting person completely refrains from eating and drinking for a certain period of time. This practice is nothing new. It has already been described by Hippocrates. Christians believe that Jesus fasted without water or food for 40 days, and that dry fasting, intermittent for religious reasons every year during Ramadan, takes up most of the world's 1.5 billion Muslims.

Variants of dry intermittent fasting

The variants of dry fasting that have been analyzed in the research studies are:

  • 5-day Total Dry Fast- 5 days no food or drink,
  • Ramadan- approximately 30-day intermittent dry fasting, no food or drink from dawn to dusk,
  • Bahá'î- 19-day intermittent dry fasting, no food or drink from dawn to dusk.

Definitely most studies concern intermittent dry fasting during Ramadan, because a very large group of people in the full age range undertake it voluntarily every year. For this reason, it is dry intermittent fasting that will be the topic of this article based on the latest scientific research.

Intermittent dry fasting in the world of science and is widely believed to be harmful to he alth due to the lack of regular water supply to the body. Is Dry Intermittent Fasting Really Harmful to Your He alth? Or maybe on the contrary?

Does intermittent dry fasting dehydrate the body?

The risk of body deodorization is the argument most often raised by opponents of dry fasting. Numerous studies have investigated the determinants of dehydration in fasting peopleduring Ramadan, that is, intermittent dry fasting. Their results may surprise you.

Scientific research shows that intermittent dry fasting during Ramadan is associated with less water consumption per month. This causes changes in kidney function, so that water is reabsorbed into the body instead of being excreted out to maintain fluid balance.

This shows that there is an effective form of protection against dehydration during intermittent dry fasting by maximizing urine concentration as well as reducing urine output.

Chronic water shortage during the day (not drinking enough fluids after breaking fasting) is, however, a stressor for the kidneys. It has a negative effect on the kidneys' ability to concentrate urine and weakens the efficiency of the water-saving mechanism.

Hydration level is commonly measured in terms of serum osmolality and urine specific gravity. According to a research review published in the journal Nutrients in 2022, numerous tests have shown that serum osmolality was within the normal range during intermittent dry fasting due to homeostatic mechanisms.

Osmolality before fasting and after 30 days of fasting was similar. The water level in the body before and after Ramadan did not change. All this indicates that properly conducted intermittent dry fasting does not dehydrate the body. Of course, there are also scientific reports that showed symptoms of dehydration - an increase in serum osmolality, hematocrit and hemoglobin, and urine specific gravity.

Experts conclude that drinking the recommended daily amount of water (7-10 glasses) in the time from breaking the fast to restarting fasting protects the body against symptoms of dehydration.

However, failure to compensate for the necessary amount of fluid during intermittent dry fasting poses a risk of dehydration. The risk of dehydration is associated not only with fasting habits, but also before fasting - the amount of water you drink every day, the level of physical activity and diet. Moreover, it seems that the negative water balance that can be caused by dry intermittent fasting does not have any harmful long-term he alth effects among he althy people who engage in fasting.

To sum up, the body copes very well with the lack of water for several hours a day. However, to avoid dehydration, you should drink the usual amounts of water recommended for the day after breaking the fast. Otherwise, changes indicating dehydration are observed.

Intermittent dry fasting and glucose levels

According to scientific research, intermittent dry fasting according to the principle of no eating or drinking from sunrise to sunset may be harmful to people with type 2 diabetes and type 1 diabetes. Patients with diabetes should not undertake dry intermittent fasting according to this schedule if they have problems with glycemic control on a daily basis.

In diabetics, intermittent dry fasting may cause severe hyperglycemia and hypoglycemia. This is because when you eat a meal after a day of fasting, your blood glucose rises dramatically, and then you develop hyperglycemia at night due to late eating.

This pattern of increases and decreases in glycaemia is more common among diabetics undergoing drug therapy and in patients who are poorly controlled or not adhering to he althy lifestyle principles.

Contrary to the results for diabetics, most analyzes show that dry intermittent fasting in he althy people causes a significant decrease in fasting glucose and insulin levels, reduces the rise in glycemia after a meal, and increases the sensitivity of tissues to insulin OR does not cause any changes in the indexes carbohydrate economy.

So, among people who do not have diabetes, dry fasting is beneficial in regulating carbohydrate metabolism. It may also be beneficial in diabetics who are well controlled, physically active, and do not overeat with their first meal when interrupting fasting. Just the wrong size of the meal after the end of fasting was indicated by scientists as one of the main causes of hyperglycemia in diabetics.

Dry intermittent fasting and the lipid profile

The research reviews of the effects of dry intermittent fasting on the lipid profile are very promising and mostly consistent. Analyzes show that intermittent dry fasting increases blood levels of "good" HDL cholesterol and decreases "bad" LDL and total cholesterol. This is most likely due to less frequent and larger meals than in a typical eating system where numerous meals and snacks predominate.

During intermittent dry fasting, postprandial lipemia is higher and longer than usual, followed by long periods without digestion and absorption, which has a positive effect on lipid metabolism. Some studies also show a decrease in triglyceride levels as a result of the use of intermittent dry fasting. Others, however, do not record any changes to this parameter.

There are studies in which the results contradict most, pointing to an increase in LDL and a decrease in HDL. However, specialists emphasize that against the background of such a large numberstudies showing the opposite effect of dry intermittent fasting, most likely the main factor contributing to this effect is an unhe althy diet when interrupting fasting and abandoning current physical activity.

The impact of dry intermittent fasting on the risk of heart disease is rated as neutral or positive. Most studies have shown no reduction in blood pressure with fasting. However, fasting in Ramadan has been proven to have a beneficial effect on endothelial function, allowing better availability of nitric oxide, necessary for the proper functioning of the heart. Some studies found a decrease in the concentration of inflammatory markers.

It was shown that there was a significant reduction in CRP, pro-inflammatory cytokines, a decrease in IGF-1 and IL-2 levels, an increase in adiponectin, a decrease in TNF-α and a decrease in the concentration of amyloid A and protein carbonyl groups in the serum.

It cannot be stated unequivocally that dry intermittent fasting reduces the risk of heart disease, because the research is conducted on too small groups and there are no long-term observational studies that could confirm this relationship shown in short-term analyzes.

How does dry intermittent fasting affect athletic performance?

Without delving into natural physiological details, it seems to state that dry intermittent fasting - lack of water and food - reduces athletic performance. Many professional athletes are convinced of this. In a poll that asked 54 Olympics footballers about their subjective assessment of the effects of fasting during Ramadan on performance, 81.5% said fasting reduces their athletic ability.

One of the newest studies analyzing the impact of dry intermittent fasting on sports performance is a meta-analysis published in 2022 in Sports Medicine. It shows that:

  • dry intermittent fasting has very little effect on athletic performance during aerobic training, regardless of whether you are training in the morning (after you start fasting) or in the evening (before stopping it).
  • dry intermittent fasting lowers sprint peak power during evening training by an average of 16.8%. When training is performed earlier in the day, fasting has no negative impact on performance.
  • dry intermittent fasting reduces the sprint pace by 3-4%.

It turns out, therefore, that dry intermittent fasting does not have as much impact on athletic performance as it may seem. The greatest negative effects are experienced by athletes who perform anaerobic endurance training (e.g. sprint interspersed with jogging), when physical activity is performed in the evening, i.e.after many hours of fasting.

Aerobic capacity, strength, jump height and fatigue index in the presented meta-analysis did not change under the influence of intermittent dry fasting. Athletes appear to be able to participate in competitions on an empty stomach with little effect on physical function. However, this applies to trained, professional athletes. It cannot be concluded that beginners also react the same to dry intermittent fasting.

Side effects of dry intermittent fasting

Intermittent dry fasting requires a complete reorganization of meal times. The hours of sleep also change. Instead of eating during the day, meals are served late in the evening and very early in the morning, and sometimes in the middle of the night. Falling asleep with a full stomach appears. This means that during intermittent dry fasting, side effects such as:

  • Indigestion

Many people who use dry intermittent fasting forget about the he alth meaning of fasting. Lest it be harmful, the meal that interrupts fasting should not be heavy and difficult to digest, and it often is. This causes the stomach, which has not been working for many hours, shrunk after a day, and suddenly expands significantly and becomes burdened with lots of foods rich in fat and sugar.

People on intermittent dry fasting often go to bed with a full stomach. This eating pattern results in indigestion. To avoid these ailments, small and light meals are recommended while eating is permitted.

  • Constipation

Reducing water consumption during intermittent dry fasting very often causes constipation. Mechanisms that prevent dehydration during dry fasting days also contribute to reducing the volume of stools.

In order to prevent constipation, it is necessary to drink plenty of water during the break of fasting, combined with eating foods rich in fiber, preferably soluble, which in the presence of water swells and increases the volume of stools.

  • Headaches

A headache is a common side effect of dry fasting. It accompanies most fasting people for the first 3-5 days, but may also appear in the days that follow. The main cause of headache in the initial period of dry intermittent fasting is the process of adaptation to hunger and the "switchover" of metabolism from glucose based to fat based fat.

A disrupted sleep rhythm has a big impact on headaches. Sleep is often shortened by eating at night and in the morning. There are hunger and thirst during dry, intermittent fastingstressors, which also contribute to the occurrence of headaches.

Dehydration of the body is the main cause of frequent headaches in people who do not drink enough fluids (7-10 glasses) during the fasting hours.

Who shouldn't use dry intermittent fasting?

While scientific research published in reputable journals gives a clear picture that intermittent dry fasting is safe and beneficial for he althy people, people burdened with diseases (and in some physiological states) should not undertake it. Intermittent dry fasting can be dangerous and is not recommended:

  • diabetics with disease control problems,
  • people with kidney diseases, kidney stones ( although there are studies that show that even dialyzed people tolerate dry intermittent fasting well),
  • pregnant women,
  • elderly,
  • people with eating disorders and a history of eating disorders,
  • for children,
  • people taking their daily medications at fixed times in the company of meals.

Although intermittent fasting is much easier to use and better researched, in which the eating hours fall during the day and there is no fluid restriction, properly performed intermittent fasting also brings numerous he alth benefits.

Probably science will prove more and more with time, as the he alth potential of fasting of various kinds is enormous. Recent scientific research shows that dry intermittent fasting does not pose a risk of dehydration, as long as the fasting person maintains hydration during the break. It is also not dangerous for he althy people.

The use of dry intermittent fasting may seem like an extreme choice, but those who choose to do so need not fear negative side effects, as long as, of course, they refill their fluids at night and do not overeat.

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Diets