Physiological And Metabolic Impact Of Fasting On Healthy Subjects

Written by: Dr. Mohammad Ali Shafiee

Published on: February 25th, 2019

Disclaimer: The nature of this review is only to provide some evidence and to discuss the physiological effects of fasting on the healthy body. By no means does it replace medical advice from your family doctor. For medical advice, please consult your family doctor or your specialist.

Ramadan is the month of spirituality, self-control, discipline, and preparation. There are many health advantages in fasting. To name few of the important benefits of fasting, which include developing a strong willpower, patience, contentment, and a dignified human spirit, as well as fostering the ability to control one’s instincts for transcendental purposes. However, at the same time fasting could also have medical benefits. What better time than Ramadan to re-evaluate your diet, eating habits, and try to make better choices in addition to focus on a healthier and happier life.

Hygiene, cleanliness and health maintenance is are the keys to happiness and success in all areas, particularly food and edible hygiene, by which our body is directly affected. In this regard, Islam encourages Muslims to follow their food hygiene seriously in order to guarantee their health. Islam also always encourages Muslims to consider their health and to prioritize their health care needs. In this context, the Prophet Mohammad (Peace Be Upon Him, “PBUH”) said, “Appreciate your health and maximally benefit your wellbeing so as to not acquire diseases.”

Prophet Mohammad (PBUH) also strongly encouraged Muslims to try to adopt the best and healthiest lifestyle including; a balanced diet, regular exercise as well as a balanced mental, and physical fitness with spiritual states balance.

The nourishment and food that we consume directly affects our health. Thus, the Prophet of Islam (PBUH) encourages Muslims to seriously maintain a healthy lifestyle, including a balanced diet, regular physical and mental exercise should be adopted to find a balance between one’s material and spiritual needs.

The general belief is that good nutrition and healthy eating habits equates healthiness. But this belief is not completely true, so in this regard, Prophet (PBUH) said: ” صُومُو تَصِّحو ” “Fast to stay healthy”. In other words, fast to flourish your health.
Imam Ali (AS) in Nahj al-Balagha sermon stage 234, implies that fasting causes the physical and psychological relaxation of different organs of the body.

«وَ مُجاهَدَةِ الصِّيامِ فِى الاَْيّامِ الْمَفْرُوضاتِ، تَسْكيناً لاَِطْرافِهِمْ، وَ تَخْشيعاً لاَِبْصارِهِمْ، وَ تَذْليلاً لِنُفُوسِهِمْ، وَ تَخْفيضاً لِقُلُوبِهِمْ، وَ اِذْهاباً لِلْخُيَلاءِ عَنْهُمْ، لِما فى ذلِكَ مِنْ تَعْفيرِ عِتاقِ الْوُجُوهِ، بِالتُّرابِ تَواضُعاً، وَالْتِصاقِ كَرائِمِ الْجَوارِحِ بِالاَْرْضِ تَصاغُراً، وَ لُحُوقِ الْبُطُونِ بِالْمُتُونِ مِنَ الصِّيامِ تَذَلُّلاً; مَعَ ما فِى الزَّكاةِ مِنْ صَرْفِ ثَمَراتِ الاَْرْضِ وَ غَيْرِ ذلِكَ، اِلى اَهْلِ الْمَسْكَنَةِ وَالْفَقْرِ.

Imam Ali (AS) also said: Five things that eliminate phlegm and forgetfulness but increases memory are: (1) brushing your teeth, (2) fasting, (3) Qur’an recitation, (4) honey, and (5) milk.

Scientific analysis and logical breakdown of fasting:

From a scientific point of view, fasting is the avoidance of eating and drinking during the day (basically the elimination of the midday meal with no water intake), when the human body is particularly metabolically active. The question is how not eating and not drinking during the day can be beneficial to health?

In regard to this question, if we search for a similar situation in nature, we realize that naturally primitive and wild animals avoid eating and drinking in the event of illness until recovery. In some studies, when scientists force-fed these animals, they had worse outcomes than otherwise. In comparable studies on stroke patients, forced-feeding caused an increase mortality. These evidences suggest that temporarily avoiding eating and drinking can help recovery.

In other circumstances, when we carefully study the lives of our primitive ancestors, due to inadequate natural resources and the danger of predators, they did not have constant access to food and water (unlike nowadays). Instead, they had intermittent periods of no access to food and water alternately.

Periods of no access to food resulted in consuming stored carbohydrates followed by the mobilization of stored fat for metabolism. Many scientists believe these periods were the main reason for their very low prevalence of obesity, diabetes and hypertension.

The useful predictable Metabolic effects of fasting:

Hours after eating, following the metabolization of recently consumed nutrient, the body initially uses stored carbohydrates as fuel. But then, lowering blood sugar reduces insulin secretion from the pancreas. This allows the mobilization of fat from fat tissue into the liver and lipolysis, generating free fatty acids and ketones.

Contrary to general belief, carbohydrates (glucose) are not the preferred fuel for body and brain, but due to our non-stop eating habits, they have become the common and regular fuel for the body and brain. However, Free Fatty Acids are the preferred fuel for most of body organs. Since free fatty acids cannot cross the blood brain barrier, ketones are the preferred fuel for the brain and nervous system. In other words, as long as ketones are available in the blood, nervous system and brain does not consume glucose.

Eventually, after depleting the stored fat, body will start burning and metabolizing our muscle protein as the main fuel. Fasting during Ramadan is planned carefully by our mighty Lord who is well aware of our body physiology in a way that prevents this switch in Iftar. Thus a gradual but progressive transition from using carbohydrates to using fat as the main energy source allows for the metabolism of unwanted fat, and prevents muscle protein breakdown as fuel.

A common question that comes to mind is, whether fasting causing causes nutritional deficiencies and malnutrition?

To answer this question, many proper studies have been conducted in different countries and numerous articles has been published in prestigious journals in the world. In these studies, the unique diet of those who fast before, during, and after Ramadan were compared. These studies confirmed an increase in consumption of nutrients and energy during Ramadan, as well as a dramatic increase in consumption of fruits and vegetables.

In this respect, Iftar has a very important role and therefore constitutes 65% of total energy consumption, 74% of consumed fat, 71% protein and 56% carbohydrate consumption. In this context, following Islamic suggestion, every Muslim is trying to improve their nutritional status consuming higher-quality meals that are high in nutrients to accomplish their religious obligation in the best possible way.
The effects of Ramadan Fasting on the central nervous system and mental health have been under great consideration. The general belief is that Fasting results in a decreased attention span. However, a prospective study done in youth in Qatar, which was published in Journal of the Academy of Nutrition and Dietetics, showed mild decrease in attention span but surprisingly a significant increase in working memory as well as strategic planning capacity. Furthermore, a significant improvement in cognitive function and overall memory capacity after Ramadan were observed.

Furthermore, Fasting provides some rest to the stomach. In some studies, appetite reduction following Ramadan fasting is reported to be likely due to increased fat metabolism and ketone production, increasing leptin and beta-endorphins and peptides.

Ramadan Fasting during pregnancy?

In Bradford England, Ramadan fasting during pregnancy was evaluated in a study. More than 40 percent of pregnant Muslim women in the UK fast for at least one day during Ramadan. In this case, no side effects were associated with a fasting mother’s baby. Fasting did not increase the risk of preclampsia or gestational diabetes. More pregnant women in the first trimester and second fasted. Fasting in the third quarter combined with a low calorie intake was associated with low birth weight births, and is not recommended.

Other potentially beneficial metabolic effects of fasting:

In another study in Turkey, Ramadan fasting was associated with a transient insulin sensitivity, followed by compensatory increases in pancreatic beta cells along with significant performance improvements in insulin sensitivity and decreased insulin resistance. Furthermore, an improved body mass index with some degree of weight loss was observed. Also, possible mechanisms have been described prophylactic against diabetes.

In several other studies in Turkey, Pakistan, Tunisia, and Iran, physiological changes during Ramadan fasting is investigated. Fasting has no significant impact on the body’s metabolism, or daily average serum level. A brief drop in blood glucose was observed, but a significant increase in cholesterol, HDL (good cholesterol) and Apo protein A1 were also observed, respectively. Additionally, a significant reduction in cholesterol, LDL (cholesterol useless), with a slight reduction in total cholesterol and triglycerides were observed, which could be useful for the cardiovascular system.

Visceral fat versus subcutaneous fat

Stored fat in body are composed of two different types of fat.

1. Subcutaneous fat is directly under the skin, and it is accessible fat for metabolic usage.
2. Visceral fat resides around the organs, including the heart, bowels, etc. This type of fat is metabolically active and can have negative effects.

In a study in Turkey, changes in abdominal fat distribution in Ramadan were assessed. In this study, 38 subjects were fasting during Ramadan. Subcutaneous fat and abdominal visceral fat before and after the fast of Ramadan were measured using a CT scan. In this study, no significant difference was found in the amount of total body fat, waist circumference, and BMI. But there was a significant reduction in visceral fat.

Water homeostasis during fasting:

Why do we need water? The human body truly needs water to lose heat generated during work, exercise, metabolism, digestion, etc. Fasting during Ramadan has been designed in a very delicate way, hence avoiding drinking and eating have been suggested simultaneously. Avoiding eating during the day reduces the amount heat generated from digestion and metabolism. Furthermore, Islam seriously instructs us to avoid heavy work and exercise. This will allow gradual direct heat loss from the skin and can reduce the amount of water loss by one fifth. Therefore, there could be only mild negative water balance and there is no evidence in literature that this could cause harm to the body.

Our study at St. Michael’s Hospital on fasting:

In order to assess the risk of calcium phosphate precipitation during fasting, a research study on seventeen healthy subjects at St. Michael hospital was done. These subjects avoid eating and drinking for 18 hours, but their urinary samples were collected as frequently as possible. As expected, their urine flow rate had reduced down to 60 percent. However, urinary sodium and calcium excretion had reduced significantly, therefore urinary calcium concentration was meaningfully reduced. At the same time, their urinary phosphate excretion was reduced, but its concentration remained almost constant. However, their urinary pH was significantly lower. Nevertheless, divalent phosphate (HPO4 the type of phosphate that preferentially can bind with calcium) concentration reduced significantly, due to changes in urinary pH. Urinary citrate, even with a urine volume reduction, which prevents the formation of calcium phosphate precipitate, remained constant. Therefore, we concluded that the risk of calcium precipitate reduced meaningfully during 18 hours fasting.

Fasting and its effects on society as a whole:

As it is rightly expected from the holy month of Ramadan, Muslims are encouraged to increase their kindness for others, modesty, spirituality, and sacrifice. According to a multitude of studies in different Islamic countries, fewer crimes have been reported during Ramadan. For instance, in Saudi Arabia, the rate of road accidents in the month of Ramadan has been significantly lower than that of the other months. A similar study in Jordan also states that there are fewer reports of suicide in Ramadan than in any other month. There is also a significant decrease in the number of patient hospitalizations who are suffering from cardiovascular, digestive, and renal issues.

What type of food should be consumed in Iftar?

It is very important to consider that there are a lot of different tastes and individuals habit have different preferred food regimens. We are here to provide you with general recommendations that could potentially be useful.
Due to iftar’s own unique nature, it is better to split your iftar into two portions. Firstly, it is important to compensate for the negative water balance and electrolytes, and quickly replenish your glucose levels by eating a light carbohydrate containing food or beverage. Two or three hours after the initial meal, it is recommended to start on a high nutrient meal that comprises of essential carbohydrates, fats, and proteins. One important thing to keep in mind is that overeating will cause more harm than good, as it causes fatigue and digestive system problems.

What type of food is better to eat during Suhur?

Suhur is the most important phase for everyone who wants to fast, because it helps maintain your blood sugar levels, and helps prevent thirst during the day. It is quite obvious that the earlier you wake up, the more your digestive system is ready for food. Complex carbohydrates are gradually digested and absorbed, and they can provide a source of energy for the body 12-18 hours after they have stopped eating. The examples of complex carbohydrates are pure grain, cereals containing a lot of fibre, bran, barley and vegetables. Also, foods full of unsaturated fatty acids such as walnuts and other nuts, are essential. Foods rich in protein, such as milk, egg, yoghurt, and meat.

Drinking fluids, such as water, in moderation is quite important. Drinking excessively would cause the excretion of your electrolytes, which is detrimental. Low consumption of salts can help the body to retain its water, but high levels of salt intake could cause thirst during the day. Consumption of watery fruits such as watermelon can serve as another source of water absorption for the hours following Suhur.
In the case of illness: In the case of illness, you should definitely consult with your family physician or your specialist. If fasting worsens your disease or impedes your recovery, it is not recommended, and even forbidden. It is important to keep in mind that we are fasting for the sake of God, it’s not merely putting yourself in difficult situations to boost your spirituality.

In Quran Surah Baghara, Ayah 184 Allah Sobhanallah says:

اَيَّامًا مَّعْدُودَاتٍ فَمَن كَانَ مِنكُم مَّرِيضًا أَوْ عَلَى سَفَرٍ فَعِدَّةٌ مِّنْ أَيَّامٍ أُخَرَ وَعَلَى الَّذِينَ يُطِيقُونَهُ فِدْيَةٌ طَعَامُ مِسْكِينٍ فَمَن تَطَوَّعَ خَيْرًا فَهُوَ خَيْرٌ لَّهُ وَأَن تَصُومُواْ خَيْرٌ لَّكُمْ إِن كُنتُمْ تَعْلَمُونَ

“[Fasting for] a limited number of days. So whoever among you is ill or on a journey [during them] – then an equal number of days [are to be made up]. And upon those who are able [to fast, but with hardship] – a ransom [as substitute] of feeding a poor person [each day]. And whoever volunteers excess – it is better for him. But to fast is best for you, if you only knew.” [2:184]

At the end of Ramadan, as your stomach is smaller and your will is stronger, there is a unique opportunity to create a very balanced and healthy lifestyle. By fasting, we can start to begin to wisely manage our eating habits. Furthermore, we can improve our discipline, organization, and even our integrity. Fasting could also help us strengthen our spiritual journey, and reach to the ultimate goal. If it’s done in sincerity, it can have very impactful advantages for the maintenance and improvement of our health. However, if fasting is not done in moderation, and one is on the extreme side of the spectrum, eating unhealthy food and not taking care to prevent hunger, you may damage your organs and cause harm to yourself.

We should also attempt to form a systematic healthy guideline in the form of a brochure to educate the general public on this important issue. We should also support the creative processes; volunteer in research programs that explain the physiological role of fasting that could influence our quality of life and help us to increase awareness, which would be spiritually rewarding.


Dr. Mohammad Ali Shafiee is a General Internal Medicine (MB, FRCP) Lecturer, Teacher at Department of Medicine, at the University of Toronto.


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