Fasting is the abstinence from consuming food and/or beverages for different periods of time. There are many different types of fasting: during the holy month of Ramadan (absence of water and food intake from sunrise until sunset), intermittent fasting (short term fasting- less than 24 hours; long term fasting- longer than 24 hours), alternate day fasting, prolonged fasting (for more than 3 days) (1).


During the holy month of Ramadan, Muslims around the world abstain from water and food from sunrise to sunset.

Fasting is one of the Five Pillars of Islam, which form the basis of how Muslims live their lives. The other pillars are faith, prayer, charity and making pilgrimage in the holy city of Mecca in Saudi Arabia.

Ramadan is a time for reflection, prayer and spending time with family and friends. Fasting is important during Ramadan as it encourages Muslims to meditate on their faith, and cultivate a stronger spiritual connection with God. Some make a special effort to reach out to their communities and the needy. It is common to have a meal (known as suhoor) just before dawn and another (known as the iftar) directly after sunset.

What happens when we eat?

Every time we consume calories, an insulin response in our body is triggered and blood glucose levels rise, staying up for around 90 minutes to a couple hours. Insulin is tasked with carrying glucose to the cells and storing it in the liver and muscles, like glycogen. If blood glucose level is still high even after glycogen is replenished in the muscles and liver, the remaining glucose will be stored as fat. Therefore, when insulin levels increase, the body goes into fat-storing mode.

All foods raise insulin to some degree. In particular, refined carbohydrates (white flour, white bread, white rice, pastries, sodas, snacks, pasta, sweets, breakfast cereals and added sugars) tend to significantly raise insulin compared to other healthier fats (avocado, cheese, dark chocolate, whole eggs, fatty fish, nuts, olive oil, coconut oil, full fat yogurt), but insulin still goes up in both cases.

Fasting is one of the mechanisms developed to lower blood glucose and decrease fat stores. Eating triggers the liver’s natural response to break up and digest food. (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14).

What happens when we fast?

  1. Postabsorptive Phase: Occurs 6 to 24 hours after fasting. Blood sugar and insulin levels fall. The liver breaks down glycogen for energy, stores last for approx. 24 to 36 hours                                  (1).
  • Gluconeogenesis: Occurs 24 hours to two days after fasting. Glycogen stores have run out. The liver produces new glucose from amino acids in a process called gluconeogenesis. In non-diabetics, glucose levels fall but stay within the normal range (1).
  • Ketosis  : Occurs 2 to 3 days after fasting. Low insulin levels stimulate lipolysis, the breakdown of fat for energy. Triglycerides, the main constituents of natural fats and oils, are broken into the glycerol backbone and three fatty acid chains. The glycerol is used for gluconeogenesis, so amino acids can be reserved for protein synthesis. Fatty acids are used directly for energy by most tissues of the body, excluding the brain. The body uses fatty acids to produce ketone bodies, which are capable of crossing the blood-brain barrier and are used by the brain for energy. After four days of fasting, approximately 75% of the energy used by the brain is provided by ketones. The two major types of ketones produced are beta-hydroxybutyrate and acetoacetate, which can increase up to time 70 times during fasting (1).
  • Protein Conservation Phase: Occurs 5 days after fasting. High levels of growth hormone maintain muscle mass and lean tissues. The energy for basic metabolism is almost entirely supplied               by fatty acids and ketones. Blood glucose is maintained by gluconeogenesis. Increased norepinephrine levels prevent any decrease in metabolic rate. There is a normal amount of protein turnover, but it is not being used for energy (1).

Benefits of fasting

With the absence of glucose and fats from meals, fasting flips a metabolic “switch” liberating fat stores via fatty acid oxidation (fat burning, in other words) and ketone production while overall prioritizing the protection of lean muscle mass and function.

Fasting therefore provides a mechanism that not only improves overall body composition but also triggers the activation of biochemical processes and signaling pathways of aging and disease. Fasting also stimulates hormesis, a compensatory defense response following exposure to a mild stressor, in this case, short term food deprivation. Hormesis triggers a vast array of protective mechanisms that not only repair cell damage but also provide protection from subsequent exposure to mote devastating stressors (2).

  1. Fasting decreases insulin levels which helps utilize stored body fat

Fasting is a mechanism used to lower blood glucose levels and decrease stored fat levels in the body. A study in which 10 people with type 2 diabetes showed that short-term intermittent fasting significantly decreased blood sugar levels. Another review found that both intermittent fasting and alternate-day fasting were just as effective as limiting calorie intake when it comes to reducing insulin resistance. Decreasing insulin resistance can increase your body’s sensitivity to insulin, allowing it to transport glucose to cells more efficiently. Coupled with the potential for lowering blood sugar, fasting can help keep blood sugar levels steady, preventing spikes and crashes (14)(15).

  • Fasting decreases inflammation

Inflammation is part of the body’s complex biological response of tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. It is a protective response involving immune cells, blood vessels, and molecular mediators. Research shows that inflammation may be involved in the development of chronic conditions, such as heart disease, cancer and rheumatoid arthritis (16). Some studies have found that fasting can help decrease inflammation, with one study in particular monitoring 50 healthy adults and concluding that intermittent fasting for one month significantly decreased their levels of inflammatory markers. Another small study discovered the same effect when subjects fasted for 12 hours a day for one month (16)(17)(18).

  • Fasting improves heart health

Through improving blood pressure, triglycerides and cholesterol levels, fasting lessens the potential for coronary heart disease, arrhythmia, heart valve disease, as well as cardiomyopathy, aortic aneurysm, and aortic valve problems. One small study showed that with eight weeks of alternate-day fasting, levels of “bad” LDL cholesterol and blood triglycerides lowered by 25% and 32% respectively. Another study of 110 obese adults showed that fasting for three weeks under medical supervision significantly decreased blood pressure, blood triglycerides, total cholesterol and “bad” LDL cholesterol. Furthermore, in a study of 4,629 individuals engaged in fasting revealed that subjects had a lower risk of coronary artery disease, as well as a significantly lower risk of diabetes, which is a major factor for heart disease (19)(20)(21).

  • Fasting boosts metabolism and helps with decreasing body fat

The metabolism defines all chemical reactions involved in maintaining the living state of the cells and organism. It can be conveniently divided into two categories:

a) catabolism,the breakdown of molecules for energy production

b) anabolism, the synthesis of all compounds needed by the cells

The metabolism is closely linked to nutrition. Energy formation is one of the vital components of metabolism. The pathways of the metabolism rely on nutrients breaking down in order to produce energy, that is then required by the body to synthesize new proteins and nucleic acids (DNA and RNA) for the building, upkeep, and repair of tissues, as well as for the efficient functioning of the body.

Some research has also found that short-term fasting may boost the metabolism by increasing norepinephrine levels.

Norepinephrine is an excitatory neurotransmitter, which means it stimulates activity in the brain, boosting the function of different cells to keep brain and body running efficiently.

One study showed that whole-day fasting could reduce body weight by up to 9% over a period of 12 to 24 weeks. While another found that intermittent fasting over 3 to 12 weeks was as effective in inducing weight loss as continuous calorie restriction. It decreased body weight and fat mass by up to 8% and 16% respectively. Fasting was found to be more effective than calorie restriction at promoting fat loss while simultaneously preserving muscle tissue (22)(23)(24).

  • Fasting increases Human Growth Hormone (HGH) secretion

The Human Growth Hormone (HGH) is produced by the pituitary gland and plays a key role in growth, body composition, cell repair, and metabolism. HGH also boosts muscle growth, strength, and exercise performance and aids in recovery from injury and disease.

Maintaining levels is especially important during weight loss, injury recovery, and athletic training (25)(26)(27)(28)(29)(30)(31).

Intermittent fasting can help optimize HGH levels in two main ways. It can help decrease body fat, which directly affects HGH production. It also maintains lower insulin levels for most of the day, as insulin is released upon eating. Research suggests that insulin spikes can disrupt natural HGH production. One study observed large differences in HGH levels on days where fasting occurred compared with regular eating days. Shorter 12 to 16 hour fasts likely help as well, though more research is needed to compare their effects with full-day fasts (32)(33)(34)(35)(36)(37).

Autophagy is an intracellular system that involves the self-destruction of the cell through the degradation of its cellular components. During fasting, it can be used to generate energy in order to feed other, surviving cells while damaged ones are cleared away during the process (1). Apoptosis causes the cell to destroy itself after the damage has occurred. It is one of the ways that cellular organisms protect themselves from cancer (38)(39)(40)(41).

Metabolic flexibility, regular exercise and fasting

Metabolic flexibility describes the ability of an organism to respond or adapt according to changes in metabolism or energy as well as other prevailing conditions (in this case, fasting) or activity, such as exercise.  

Metabolic flexibility has tissue specificity, meaning that it responds differently to fast or fed conditions, as well as day time or night time  (42). Skeletal muscle and adipose tissue play a crucial role in energy metabolism; regardless of tissue, metabolic flexibility is driven by cellular and organelle processes, perhaps most pertinently in the mitochondria. Exercise is a physiological condition requiring metabolic flexibility in order to match fuel availability with the metabolic machinery to meet enormous increases in energy demands. Muscle plasticity is a term that characterize muscle’s ability to respond to a variety of stimuli, including metabolic flexibility (43).

Metabolic flexibility improves with regular exercise. Exercise, both aerobic and anaerobic, can alter fuel storage and availability. The duration and intensity can each profoundly influence energy demand, thereby affecting energy stores and catabolic pathways in very different ways. Enhanced metabolic flexibility improves insulin sensitivity, which allows the cells of the body to use blood glucose more effectively; reducing blood sugar, increasing muscle glycogen storage and improving exercise performance and endurance.  

Exercise can promote higher rates of fatty acid oxidation during acute exercise and at rest. It also induces mitochondrial biogenesis, increasing mitochondrial mass and thereby allowing greater glucose intake for the muscles. Additionally, mitochondria content and function have been reported to explain improvements in both insulin sensitivity as well as an increased capacity for fatty acid oxidation.

Intermittent fasting enhances metabolic flexibility because it stimulates the body in the fasting and feeding phases to rely on different sources of energy. During sleeping/post-absorptive conditions, the skeletal muscle switches from higher rates of fatty acid oxidation to greater oxidation, storage of glucose and reduced fatty acid oxidation after feeding.

Adipose tissue shifts from higher rates of lipolysis during fasting to suppressing lipolysis and fat storage during feeding. From rest to exercise, skeletal muscle increases the rates of both fatty acid and glucose oxidation to support higher energy demands, while lipolysis in adipose tissue is drastically enhanced (44)(45)(46)(47)(48)(49).

Metabolically flexible individuals tend to maintain a healthy weight without trying too hard, because the body does not have to constantly maintain food-seeking mechanisms. When the body is metabolically flexible, it does not store extra fat to prepare for a constant need for energy. It burns whatever energy source is available.

Tips for fasting during Ramadan (50)

  1. Balanced nutrient intake

Ensure that the foods consumed during Suhoor and Iftar are balanced in their nutrients to fuel and replenish the body appropriately. This will include foods from all major food groups such as carbohydrates from wholegrain sources, proteins, low-fat dairy and fruits and vegetables. Choose slow-energy releasing carbohydrates which sustain body energy throughout the day. Slow-energy releasing, or low-glycemic index foods, include grains, such as barley, wheat, oats, beans, lentils and long grain basmati rice. Aside from meat, diversify protein consumption by looking for other sources such as cheese, Laban, yoghurt, eggs or beans (e.g. foul meddames, lentils or baked beans) as it promotes satiation during fasting hours. Additionally, choose healthier fats such as avocadoes, olive oil, raw nuts and fish.

  • Hydration

Drink lots of fluids, ideally water and sugar-free drinks, before meals to help hydrate your body faster. Try to increase your water intake further by consuming foods high in water content, such as watermelon and salads. 8-10 glasses are recommended during and between Iftar and Suhoor.

  • Maintaining electrolytes

Essential minerals dissolved in fluid. Electrolytes, found in blood, sweat and urine play a role in conducting nervous impulses, contracting muscles, keeping body hydrated and regulating your body’s pH levels. Electrolytes balance can be maintained in the Suhoor period by eating fruits like bananas and kiwis, which will help keep your body hydrated throughout the day.

  • Timing

Break your fast with something light (e.g. 1 to 3 dates with a glass of water), and follow up with a healthy meal 1 to 2 hours later. Food should be chewed slowly in order to avoid overeating. Try to have Suhoor just before sunrise, instead of at midnight. This will spread out your energy intake more evenly.

  • Volume

Monitor the amount of food consumed during Suhoor and Iftar. Meal size should be reduced as stomach size will inevitably shrink throughout the month of fasting. Larger meals can also cause discomfort and indigestion.

  • Stay active

During fasting, exercise should be conducted at moderate intensity. 2 to 3 hours after Iftar is ample, as the food has had time to settle and the body has fully rehydrated. For early risers, before morning Suhoor is also sufficient. This way, you can utilize the excess energy from the previous night’s meal, yet still have an empty stomach. You can hydrate while you exercise and once you are done you can eat again to refuel.

  1. Salty food

Foods such as pickles, olives and salty cheeses will increase your thirst during fasting hours

  • Excess sugar

 Keep sugary foods such as Loqaimat, Sago and custard to a minimum.

  • Fats

Fatty meats, fried food and pastries are low in nutritional value and can lead to bloating and fatigue the next day.

  • Decreasing caffeine consumption

Try to do this gradually, in order to avoid headaches during the day

Types of fasting (51)(52)(53)(54)(55)

  1. 12h fasting– suitable for beginners as fasting hours window can be incorporated in night sleep.
  2. 5:2 fasting– includes eating normally for 5 days without counting calories and then other 2 days eat 500 (women) or 600 (man) calories. It can be used as initial fasting option for fasting beginners as it will be easier for person to sustain low number of fasting days.  Training program should be carefully planned during fasting days.
  3. 16:8 fasting include 16 hours of fasting and 8h of eating window. Recommended fasting time for man is 16h and for women 12-14h. This method of fasting can be effective for those who did have any results from 12h fasting.
  4. Alternate day fasting– can be extreme form of fasting for beginners and people with certain medical conditions (e.g. type 2 diabetes). There are several variations of the alternate fasting plan, which involves fasting every other day. Alternate day fasting can include a complete avoidance of solid food or allowing up to 500 calories.
  5. A weekly 24h fast (“Eat-Stop-Eat” diet) involves fasting for 24h, for example from breakfast to breakfast or lunch to lunch. Water, tea or other calorie free drinks are allowed during fasting hours. On non-fasting days person should have normal eating pattern. This type of fasting can be challenging as it may cause fatigue, headaches or irritability. Before transitioning to 24h fasting it can be beneficial to try 12h or 16h fasting first.

The following tips may help people stay on track and maximize the benefits of intermittent fasting:

  1. Staying hydrated. Drink lots of water and calorie-free drinks, such as herbal teas, throughout the day.
  2. Resting and relaxing. Avoid strenuous activities on fasting days, although light exercise such as yoga may be beneficial.
  3. Making every calorie count. If the chosen plan allows some calories during fasting periods, select nutrient-dense foods that are rich in protein, fiber, and healthful fats. Examples include beans, lentils, eggs, fish, nuts, and avocado.
  4. Eating high-volume foods. Select filling yet low-calorie foods, which include popcorn, raw vegetables, and fruits with high water content, such as grapes and melon.
  5. Increasing the taste without the calories. Season meals generously with garlic, herbs, spices, or vinegar. These foods are extremely low in calories yet are full of flavor, which may help to reduce feelings of hunger.
  6. Choosing nutrient-dense foods after the fasting period. Eating foods that are high in fiber, vitamins, minerals, and other nutrients helps to keep blood sugar levels steady and prevent nutrient deficiencies. A balanced diet will also contribute to weight loss and overall health.


  1. Fung, Moore (2017) The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting
  2. Rhonda Patrick (2017)
  3. Basu et al (2006) Dietary Factors That Promote or Retard Inflammation
  4. Menendez et al (2005) Oleic acid, the main monounsaturated fatty acid of olive oil, suppresses Her-2/neu (erbB-2) expression and synergistically enhances the growth inhibitory effects of trastuzumab (Herceptin) in breast cancer cells with Her-2/neu oncogene amplification.
  5. Fulgoni (2013) Avocado consumption is associated with better diet quality and nutrient intake, and lower metabolic syndrome risk in US adults: results from the National Health and Nutrition Examination Survey (NHANES) 2001-2008.
  6. Mozaffarian et al. (2010) Trans-palmitoleic acid, metabolic risk factors, and new-onset diabetes in U.S. adults: a cohort study.
  7. Taubert (2007) Effects of Low Habitual Cocoa Intake on Blood Pressure and Bioactive Nitric Oxide
  8. Vander et al. (2005) Short-term effect of eggs on satiety in overweight and obese subjects.
  9. Wang et al. (2006) n-3 Fatty acids from fish or fish-oil supplements, but not alpha-linolenic acid, benefit cardiovascular disease outcomes in primary- and secondary-prevention studies: a systematic review.
  10. Jackson, Hu (2014) Long-term associations of nut consumption with body weight and obesity
  11. Bauchamp et al. (2005) Phytochemistry: ibuprofen-like activity in extra-virgin olive oil.
  12. Liu (2008) Medium-chain triglyceride (MCT) ketogenic therapy.
  13. Morelli (2014) Yogurt, living cultures, and gut health.
  14. Arnason et al. (2017) Effects of intermittent fasting on health markers in those with type 2 diabetes: A pilot study
  15. Barnosky (2014) Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings
  16. 16) Hunter (2012) The inflammation theory of disease
  17. Faris et al. (2012) Intermittent fasting during Ramadan attenuates proinflammatory cytokines and immune cells in healthy subjects.
  18. Askungar et al. (2007) Interleukin-6, C-reactive protein and biochemical parameters during prolonged intermittent fasting
  19. 19 Bhutani et al. (2010) Improvements in coronary heart disease risk indicators by alternate-day fasting involve adipose tissue modulations.
  20. Beleslin et al. (2007) The effects of three-week fasting diet on blood pressure, lipid profile and glucoregulation in extremely obese patients
  21. Horne et al. (2008) Usefulness of Routine Periodic Fasting to Lower Risk of Coronary Artery Disease among Patients Undergoing Coronary Angiography
  22. Zauner et al. (2000) Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine.
  23. Tinsley et al. (2015) Effects of intermittent fasting on body composition and clinical health markers in humans.
  24. Varady (2011) Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss?
  25. Clasey et al. (2001) Abdominal visceral fat and fasting insulin are important predictors of 24-hour GH release independent of age, gender, and other physiological factors.
  26. Rudman et al. (1990) Effects of human growth hormone in men over 60 years old.
  27. Moller et al. (1991) Effects of growth hormone on glucose metabolism.
  28. Willmore (1991) Catabolic illness. Strategies for enhancing recovery.
  29. Isaksson et al. (1992) Growth hormone stimulates longitudinal bone growth directly
  30. Godfrey et al. (2003) The exercise-induced growth hormone response in athletes
  31. Liu et al. (2008) Systematic review: The effects of growth hormone on athletic performance.
  32. Klemper et al. (2012) Intermittent fasting combined with calorie restriction is effective for weight loss and cardio-protection in obese women.
  33. Mansell et al (1990) Enhanced thermogenic response to epinephrine after 48-h starvation in humans.
  34. Zauner et al (2000) Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine.
  35. Lanzi et al. (1999) Elevated insulin levels contribute to the reduced growth hormone (GH) response to GH-releasing hormone in obese subjects.
  36. Greenwood et al. (1966) The plasma sugar, free fatty acid, cortisol, and growth hormone response to insulin.
  37. Ho et al. (1988) Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man.
  38. Alirezaei et al (2010) Short-term fasting induces profound neuronal autophagy
  39. Kim, Lemasters (2011) Mitochondrial degradation by autophagy (mitophagy) in GFP-LC3 transgenic hepatocytes during nutrient deprivation.
  40. Mathew et al (2009) Autophagy suppresses tumorigenesis through elimination of p62.
  41. Wolfe et al (2013) Autophagy failure in Alzheimer’s disease and the role of defective lysosomal acidification.
  42. Kelly et al. (1999) Metabolic flexibility was also inferred to have tissue specificity in response to nocturnal and diurnal fasted and fed conditions
  43. Pette (1980) Plasticity of Muscle
  44. James et al. (1984), Kraegen EW, Chisholm DJ. Effect of exercise training on whole-body insulin sensitivity and responsiveness
  45. Sherman et al. (1981) Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance
  46. Karlsson, Saltin (1971) Diet, muscle glycogen, and endurance performance. Journal of Applied Physiology
  47. Coen et al. (2015) Exercise and Weight Loss Improve Muscle Mitochondrial Respiration, Lipid Partitioning, and Insulin Sensitivity After Gastric Bypass Surgery
  48. Jong-Yeon et al. (2002) Long- and medium-chain fatty acid oxidation is increased in exercise-trained human skeletal muscle. 
  49. Goodpaster, Sparks (2017) Metabolic flexibility in health and disease
  50. Ramadan, healthy tips (2020), Abu Dhabi Public Health Center
  51. Roger Coller (2020) Intermittent fasting: the science of going without
  52. Harvie et al.  (2011) The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women
  53. Harvie et al. (2016) Intermittent energy restriction induces changes in breast gene expression and systemic metabolism
  54. Varady et al. (2013) Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial