Can you do without? - The latest on Fasting

With the festive season behind us some of us may be carrying a few more pounds than we were during our summer holidays. The run up to Christmas can be a minefield of overindulgence. According to the British Dietetics Association the average person consumes 6,000 calories (kcal) on Christmas day itself and it’s not uncommon for people to average an extra 500kcal per day throughout the holiday period, amounting to a potential weight gain of 5lbs by the time January rolls around.(1) And it doesn’t take a market researcher to realise ‘tis the season’ for the weight-loss industry, whose busy elves are working ‘round the clock’ to convert those extra Christmas kilos and new year’s resolutions into new (and returning) customers. Yet with more than 60% of the British population classed as overweight,(2) it’s not just a seasonal business opportunity. It’s estimated that more than half the British population make a conscious attempt to lose weight each year(3) and there seems to be no end of diet programmes to choose from.(4) But with those statistics and the obesity epidemic showing no signs of slowing down(5) it would be easy to conclude that most diets don’t work over the long-term and that in the end, hunger always win.

Throughout our evolution we spent much of our time and energy searching for food in order to survive. Fortunately, since food wasn’t always available, we developed metabolisms that allowed us to store excess energy as fat to keep us alive during times of scarcity. Our ancient ancestors hunted and gathered their food but today the opposite is true and most of us have far more “energy in reserve” than needed.(2) It has been estimated that the majority of humans can survive for 30 or more days in the absence of any food(6) yet most of us shudder at the thought of missing even one meal.

Fasting for weight loss

The one thing most weight loss programmes have in common is some form of calorie restriction - the principle of creating a calorie deficit by taking in less energy than the body needs. In these conditions our body bridges the energy gap by tapping its reserves (i.e. fat). In dietary terms the ultimate form of calorie restriction is ‘fasting’ – to eat little or no food for an extended period of time. Many of us may conjure up images of total deprivation at the mere mention of the word but its something we all do every night for roughly 10-12 hours, the period of time between dinner and breakfast the following day.

Types of fasting

There are different types of fasts that, for the most part, can all be distinguished by either intensity, frequency or duration.

At the starting gate is the act of extending the ‘overnight fast’ which is sometimes called time-restricted feeding (TRF), condensing the number of hours in the day when you allow yourself to eat. You consume the same number of daily calories, just over a shorter timeframe. For example, eating dinner at 6pm and not eating again until 10am is a 16-hour fast giving you an 8-hour window to eat. Although limited studies on humans exist, mice fed in this manner had considerably lower body fat (12% vs 40%) despite eating the same amount of food. They also had better liver function and cholesterol profiles.(7)

At the more extreme end is the ‘absolute fast’, a water-only fast lasting anywhere from 1 to 2 days and done as often as once or twice a month – to 2 weeks or longer and done once or twice a year. These sorts of fasts are generally practiced by more experienced fasters to detoxify and heal the body or as part of a longer-term anti-aging strategy.(8)

But it seems the hardest part of fasting is the period of time between 18 to 48 hours without food, when your metabolism is trying to adapt from carb-burning to fat-burning. Although humans are well adapted to run on fat, most of us are bona fide sugar burners. With our high-carbohydrate diets and regular meals and snacks, we rarely let the glucose tank get low enough to have to burn fat for fuel. And so adapting to becoming a fat-burner can carry with it a few unpleasant symptoms, including light-headedness, irritability, brain-fog, headaches and even nausea, not to mention the obvious... Hunger. But these symptoms don’t last and experienced fasters claim ‘the more you fast, the easier it gets’.

But it’s these interim symptoms that make fasting a real challenge and can lead us to throwing in the towel. Many have turned to shorter intermittent fasts (IF) such as alternate day fasting (ADF) where you only eat every other day, or the popular “5:2” diet where you eat normally 5 days a week and consume very few calories (500-600 kcal) on the other two days. This lighter form of fasting is gaining ground amongst dieters who claim it’s easier to adhere to than absolute fasting as you’re allowed a small amount of food on your fasting day and the psychological affect of knowing you’re going to eat normally again the next day seems to help people stay on track. Studies also show its every bit as effective at reducing body fat and even more effective at preserving muscle mass compared with daily calorie restriction.(10)

Longest Recorded Fast

How long a person can go without food ultimately depends on how much body fat they have accumulated and, as long as key nutritional requirements are met, there are some examples of extremely long fasts indeed. The world’s longest recorded fast was carried out by a 27 year old male from Dundee Scotland named Angus who in 1965 went 382 days without eating. He was under medical supervision and received vitamin and mineral supplementation throughout. At the start of the fast he weighed 456lbs and when he finally broke fast, he had achieved his target weight of 180lbs, losing a whopping 276lbs in total (0.72lbs/day) which for the most part he was able to maintain, weighing just 196lbs 5 years on. This record has held since breakfast, June 30th 1966.(9)

Fasting in practice - Tips for first-time fasters

Whether you’re going for an absolute fast or an intermittent fast, going without food past the point of being hungry can be testing both physically and mentally. Here are a few tips to make your experience more manageable:(11)

  • Going in with the right attitude is key. Remind yourself why you’re doing it and embrace the feeling of being a little hungry, knowing it’s only temporary.
  • If you can, choose fast days that don’t interfere with social obligations.
  • Try to drink more water than usual as we get a lot of water from the foods we eat.
  • Dress warmly as some fasting newbies report feeling chillier on fast days.
  • Drink herbal teas or zero-calorie coffee or tea (i.e. no milk) to help maintain productivity. Caffeine may also mildly suppress appetite.
  • Keeping yourself busy can distract you when it counts and help get past any hunger pangs.
  • If you work in an office plan regular breaks to get up from your desk and move around. Try 5-minute movement breaks every hour.
  • Go for a brisk walk during your lunch break - As well as getting away from people eating, exercise can help suppress hunger.(12)
  • Find a ‘fast’ friend - the mutual support and joint commitment can make all the difference.
  • Break your fast by eating light, and slowly, avoiding things that are hard to digest like beans and legumes.

The biology of fasting

Glucose is the preferred energy source for most of the cells in our body and dietary carbohydrates are the quickest source. A person weighing 70kg needs only about 4g (1 teaspoon) of circulating glucose in the blood at any given time(13) and our body has ways of maintaining glucose levels whether we eat carbohydrates or not. After eating our blood glucose levels rise and our body gets busy storing it in our muscles and liver as glycogen. Any surplus gets converted into fat and stored in our fat cells (adipose tissue).

So what happens in our bodies when we go without food? First, our blood glucose levels drop slightly and our brain, sensing this dip, triggers changes in our biochemistry to bring blood sugar levels back up. The pancreas responds by reducing the secretion of insulin, a hormone responsible for getting glucose out of the blood and into the cells where it’s used for energy. At the same time different cells in the pancreas secrete glucagon, a hormone which tells the liver to convert its stored glycogen back into glucose through a process called glycogenolysis. But when glycogen levels dwindle (usually after about 16-18 hours without eating) changes in our metabolism occur in order to recruit energy from other sources, namely fat. Our adipose tissue releases stored fat into the bloodstream in the form of glycerol and fatty acids. The liver converts the glycerol into glucose through a process known as gluconeogenesis. The fatty acids are directly oxidized for energy by our muscle cells and are also converted by the liver into ketone bodies - chemicals that the body makes out of fat when glucose is not available to provide an alternative fuel source for our cells, particularly important for the brain and central nervous system, which are major consumers of glucose and are unable to utilise fatty acids directly. Muscle tissue can also be broken down to release amino acids, some of which the liver can use to make glucose.

A recently discovered gene called FGF21 appears to play a key role in regulating the hormones to help the body adapt from a fed state to a fasting state. Levels of FGF21 hormone rise substantially during fasting and work by increasing the release of triglycerides from adipose tissue (lipolysis) and initiating the conversion of fatty acids into ketone bodies (ketogenesis). FGF21 also plays a role in brain “torpor”, the reduction of body movement and temperature in order to conserve energy.(14)

Religion and Fasting 

Have you ever wondered why virtually every religious text encourages some sort of fasting ritual? From Lent to Ramadan to Yom Kippur, the cultural traditions of fasting can be an important part of religious practice. In Judaism, Yom Kippur is recognised as the most solemn day of the Jewish calendar. It’s an absolute fast where nothing crosses the lips for 25 ½ hours as part of atonement. In the Hindu faith, Karva Chauth is an annual fasting ritual that is a celebration of a wife’s love and devotion to her husband. It involves going without food for a whole day and the woman has to see the moon before she can break fast with her husband feeding her the first morsel of food. Muslims around the world abstain from food and drink from dusk to dawn during Ramadan, a month-long celebration of religious devotion. But a fasting ritual doesn’t have to be about total abstinence or even food. In the Christian faith, the giving of oneself over Lent, the 40 days up to Easter, is a sign of solidarity with Jesus who retreated into the wilderness and fasted as preparation for his ministry. And the tradition of giving up meat on Friday is practiced to show solidarity with Jesus on the day he was crucified. It can be a humbling process to go without food and the things we love; a certain dethroning of ourselves to feel compassion for those less fortunate. But whether these religious fasts are for spiritual nourishment, sharpening the mind, healing the body or even teaching us control over our fleshly desires, ancient wisdom has a way of being passed down through the ages and there are side benefits to the spiritual practice of fasting that have their roots firmly in science.(15)

The healing power of fasting

The act of digesting, assimilating and eliminating food requires considerable energy - around a third of the total daily energy budget of a fairly sedentary person.(8) When fighting illness fasting allows the body to put that energy to use elsewhere, namely the immune system and the process of healing. Advocates of fasting claim the human body has an innate ability to heal itself if the [healing] process is not interfered with. The list of diseases reportedly helped by fasting is long and includes conditions such as epilepsy,(16) metabolic syndrome,(17) rheumatoid arthritis,(18) eczema,(19) cancer,(20) and many others.

  • Heart health – Intermittent fasting has been shown to reduce prevalence of coronary artery disease(21) and reduce blood pressure and resting heart rate while increasing resistance to stress.(22,23)
  • Immune health – research has shown that prolonged fasting (PF) for between 2-5 days has a more profound effect on the immune system than IF. PF kick-starts our stems cells to regenerate new white blood cells boosting our ability to fight disease.(6)
  • Brain health - Fasting enhances cognitive function, learning and memory by promoting the growth of new nerve cells.(6,23) And according to Mark Mattson, Professor of Neuroscience at John Hopkins University, this makes a lot of sense from an evolutionary standpoint. “If you were one of our early ancestors living in the wild and you went without food for a while, you would need to keep your mind active so you can figure out how to find food,” says Professor Mattson.(23)

Many of the healing processes mentioned above are enabled via a cellular process known as autophagy (meaning “self-eating”) that is increased by fasting. It is this mechanism by which cells recycle themselves and its a vital cellular function for health and longevity.(24)


There are some health conditions where fasting is not advised or extra caution should be used before considering a fast. Here are a few:

  • Eating disorders - Fasting is not recommended for those who are underweight or for those with eating disorders such as anorexia and bulimia. These conditions can cause deficiencies in electrolytes and a wide range of nutrients making fasting potentially dangerous.(25)
  • Children – There is conflicting opinion as to what age it is safe for a child to fast. Some advocate from 18-20. The NHS position is that IF from the age of puberty is not harmful but it does depend on the child’s nutrition levels and general health. Fasting of any kind is not advised before the age of seven or eight.(26)
  • Pregnancy and nursing mothers – Fasting is not advised during pregnancy because of the fetal requirements for nutrition. Studies on fasting and pregnancy during Ramadan show a link with lower birth weight babies and a higher chance of mental and other disabilities in adulthood.(27) For nursing mothers, short-term fasts (up to 25 hours) are considered safe and show no significant change to breast milk composition however dehydration should be avoided.(28)
  • Diabetes – As diabetes is a disease of blood sugar control that varies by individual, fasting depends on your condition. The NHS advise that if you need insulin to manage your condition (type 1 diabetes) you should not consider fasting. However, if you have type 2 diabetes and manage your condition through diet or tablets, it is possible to fast, although your GP may need to adjust your medication.(26)
  • Thyroid conditions – The thyroid gland controls metabolism and when it’s underactive it can lead to weight gain. Fasting as a strategy for weight loss can slow down thyroid function,(29) not helpful if your thyroid is already underactive. There is also anecdotal evidence that intermittent fasting may disrupt blood sugar regulation,(30) which also is not helpful for thyroid function.(31) That said, limited research suggests that intermittent fasting of less than 24 hours is safe for those with hypothyroidism and mild-to-moderate hyperthyroidism.(32,33) As one loses weight however it may be necessary to modify your medication and medical supervision is advised.
  • Underlying medical conditions or prescribed medications – if you have any other underlying medical condition, or are on a prescribed medication, consult with your GP before embarking on any fasting regime.


1) BDA (2014). Getting stuff this Christmas! 

2) HSCIC (2016). Statistics on Obesity, Physical Activity and Diet. 

3) Mintel (2014). Dieting in 2014? You’re not alone – 29 million brits have tried to lose weight in the last year. 

4) NHS Choices (2016). Top diets review for 2016. 

5) NHS Choices (2015). Britain: ‘the fat man of europe’. 

6) Longo VD & Mattson MP (2014). Fasting: molecular mechanisms and clinical applications. Cell Metabolism; 19:181-92.

7) Chaix A et al (2014). Time-restricted feeding is a preventative and therapeutic intervention against diverse nutritional challenges. Cell metabolism; 20(6):991-1005.

8) Moser I & Soloman S (2010). Fasting. In: How and when to be your own doctor. Toronto, LNT Press. p57-107.

9) Stewart WK & Fleming LW (1973). Features of a successful therapeutic fast of 382 days' duration. Post Graduate Medical Journal. 49:203-9.  

10) Varady KA (2011). Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss? Obesity Reviews; 12:e593-e601.

11) (2016) Getting Started. 

12) Martins C et al (2007). Effects of exercise on gut peptides, energy intake and appetite. J Endocrinol; 193:251-8.

13) Wasserman DH (2009). Four grams of glucose. Am J Physiol Endocrinol Metab; 296(1):e11–21.

14) Reitman ML (2007). FGF21: A Missing Link in the Biology of Fasting. Cell Metabolism; 5(6):405-7.

15) BBC World Service (2013). Fasting. The why factor. 

16) Freeman JM & Vining EPG (1999). Seizures decrease rapidly after fasting. preliminary studies of a ketogenic diet. Arch Pediatr Adolesc Med; 153:946-9.

17) Wan R et al (2010). Cardioprotective effect of intermittent fasting is associated with an elevation of adiponectin levels in rats. J. Nutr. Biochem; 21:413–7.

18) Muller H et al (2001). Fasting followed by vegetarian diet in patients with rheumatoid arthritis: a systematic review. Scand. J. Rheumatol; 30:1–10. 

19) Lithell H et al (1983). A fasting and vegetarian diet treatment trial on chronic inflammatory disorders. Acta Dermato-venereologica; 63(5):397-403.

20) Lee C et al (2012). Fasting cycles retard growth of tumors and sensitize a range of cancer cell types to chemotherapy. Sci. Transl. Med; 4(124):124-7.

21) Horne BD et al (2012). Relation of routine, periodic fasting to risk of diabetes mellitus, and coronary artery disease in patients undergoing coronary angiography. Am J Cardiol; 109(11):1558-62.

22) Castello L et al (2010). Alternate-day fasting protects the rat heart against age-induced inflammation and fibrosis by inhibiting oxidative damage and NF-kB activation. Free Radic Biol Med; 48:47–54. 

23) BBC Radio 4 (2013). Fasting, old and new. The food programme. 

24) Jing K & Lim K (2012). Why is autophagy important in human diseases? Experimental & molecular medicine; 44(2):69-72.

25) Gaby A (2011). Anorexia nervosa. & Bulimia nervosa. In: Nutritional Medicine. Concord, Fritz Perlberg, p1009 & p1034. 

26) NHS Choices (2014) Ramadan health facts.

27) Almond D & Mazumder B (2008). Health capital and the prenatal environment: the effect of maternal fasting during pregnancy. National bureau of economic research; Working Paper 14428.

28) Bener A et al (2001). Fasting during the holy month of Ramadan does not change the composition of breast milk. Nutrition Research; 21(6):859-64.

29) Spencer CA et al (1983). Dynamics of serum thyrotropin and thyroid hormone changes in fasting. J Clin Endocrinol Metab; 56(5):883-8.

30) Kresser C (2011). To (intermittent) fast or not to fast; that is the question.  

31) Kalra S et al (2014). The hypoglycemic side of hypothyroidism. Indian J Endocrinol Metab; 18(1):1–3.

32) Raza SA et al (2012). Thyroid diseases and Ramadan. Indian J Endocrinol Metab; (4):522–524.

33) Azizi F (2015). Islamic Fasting and Thyroid Hormones. Int J Endocrinol Metab; 13(2):e29248.