Published on 16th December 2016


As January begins, so does the festive season hangover, with some party-goers nursing fragile livers and many of us feeling somewhat cuddlier than before. In a search for better ways to shed the spare tyre, Mike Murphy and Louise Wates look at the growing popularity of fasting

It doesn’t take a market researcher to realise ‘tis the season for big business to be jolly, as the weight loss industry’s busy elves work round the clock to persuade us to convert all those extra Christmas pounds into pounds Sterling on slimming products and services. It’s estimated that the average person consumes 6,000 calories (kcal) on Christmas day alone — and it’s not uncommon 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 

To undo the damage, many are turning to the Fast Diet, or 5:2 diet, which was famously popularised by television presenter, doctor and writer Michael Mosley, and which involves low calorie intake for two days of the week (between 500-600 calories) and normal consumption for the rest of the week. But the 5:2 is only a small part of the growing interest in the concept of going without. 

While we can’t survive without water for more than about three days, it has been estimated that most humans can survive for 30 or more days without food.2 And although we may shudder at the thought of missing just one meal, all of us — unless we are grazers, nibblers and midnight snackers — carry out a natural fast overnight, from the time we finish our evening meal to when we eat breakfast; something that perhaps makes the idea of fasting seem less daunting after all!

Longest recorded fast

How long a person can go without food ultimately depends on how much body fat they have accumulated. 
The world’s longest recorded fast was carried out by a 27-year-old man from Dundee, Scotland named Angus who in 1965 went 382 days without eating. 

Angus was under medical supervision and received vitamin and mineral supplementation throughout. At the start of the fast he weighed 456lbs (206.8kgs)and when he finally ended it, he had achieved his target weight of 180lbs, losing a whopping 276lbs (125kgs) in total (0.72lbs (0.36kgs)/day) which for the most part he was able to maintain, weighing just 196lbs (88.9kgs) five years on. This record has held since breakfast, June 30th 1966.

It appears that 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” 

Types of fasts

Fasts can be distinguished by intensity, frequency or duration. At the starting gate is the act of extending the overnight fast, sometimes called time-restricted feeding (TRF), during which meals are eaten over fewer hours. The same number of daily calories are consumed, just over a shorter time frame; for example, eating dinner at 6pm and not eating again until 10am makes a 16-hour fast with an eight-hour window to eat. Although limited studies on humans exist, mice fed in this manner have been found to have considerably lower body fat (12 per cent vs. 40 per cent) despite eating the same amount of food. They have also been found to have better liver function and cholesterol profiles.3

Nutritional therapist and author of The Gut Makeover, Jeannette Hyde, says that she included a 12-hour overnight fast in her book after reading two studies4,5 in which groups of mice were given access to the same quantities of food, but the mice that were only allowed to eat in a 12-hour window, as opposed to all the time, extracted fewer calories from the food they ate (found by stool examinations). Those mice also had a more healthy, flourishing, diverse microbiome. “Caloric extraction and hunger hormones are thought to be linked to the health of our microbiome, so having a 12-hour fast may be a good way to manipulate that,” says Hyde. “I find the 12-hour fast a really neat way for people to lose weight as part of an overall gut restoration plan, and longer term, a simple and effective way to keep their weight stable.”

Hyde says that fasting also makes sense from the perspective of autophagy (which literally means self-eating), when the body deals with the destruction and renewal of cells. “This is where the body goes into housekeeping mode overnight, clearing out the junk and repairing itself.”

At the more extreme end is the ‘absolute fast’, which lasts anywhere from one to two days, and is carried out as often as once or twice a month, to two weeks or longer and undertaken once or twice a year. These sorts of fasts are generally practised by more experienced fasters to ‘detoxify’ and ‘heal’ the body or as part of a longer term anti-ageing strategy.6 However, these are harder to stick to than overnight fasting when we are (generally) asleep. 

Professor Valter Longo, the University of Southern California’s (USC) Edna Jones Professor in Gerontology, Professor in Biological Science, and director of the USC Longevity Institute, has researched fasting for many years. He told Optimum Nutrition that there are “definitely” benefits to the overnight fast in terms of weight and health. “However, the 16-hour fast may be too difficult for most people,” he says. “Better to stay with the 12-hour and have a more gradual effect which can be maintained for life.”

Being able to stick to a fast is a major issue and in Longo’s experience, even cancer patients — who, research shows, could experience benefits from fasting — find it difficult to keep to a water-only fast, and end up cheating. But regardless of this fact, Longo does not recommend longer fasts unless under medical supervision, and says that it should preferably involve using diets that have been clinically tested.

Pretend fasting

Longo’s own research has involved developing a fasting mimicking diet (FMD), which was designed to produce the biological effects of fasting, but with a carefully-constructed menu to provide some nutrient intake and to stave off the worse effects of hunger.

Initial research on yeast and mice found that FMD could extend life and improve a variety of health markers. A follow-up study on humans also showed that FMD decreased IGF-I (a hormone associated with ageing and cancer susceptibility), and reduced biomarkers associated with diabetes and cardiovascular disease — despite subjects eating their normal diet on non-FMD days and without a reduction in calories overall.8 So the concept of fasting may be moving away from the nil-by-mouth rule, as research is showing that it may be possible to achieve health benefits without having to endure extreme deprivation. 
“We have abandoned fasting and moved to fasting mimicking diets (FMDs), since water-only fasting should only be done in a clinic and has potential side effects,” says Longo. “The potential of FMD is very high since we have shown that it has effects on longevity, autoimmunities, regeneration, and risk factors for diabetes, cancer, and cardiovascular diseases.”

Longo puts his own research into practice by following the five-day FMD twice a year. He also says that he usually doesn’t eat lunch but has a 100 kcal snack instead. While he says that there are potential benefits to the 5:2 diet, Longo also says that it needs to be understood more. “It [fasting] is also not only about calorie restriction but about dietary composition.” In other words, the balance of nutrients when following a low-calorie day is just as important as the reduction of calories. 

The exact composition of the FMD diet, however, remains a secret. Having been patented, FMD has since been turned into a diet for purchase and is sold as ProLon® — the name being derived from pro-longevity. (In interviews elsewhere, Longo has said that he has committed his shares in the manufacturing company, L-Nutra, to a non-profit organisation called Create Cures, which is focused on further research.) And if anyone is thinking of copying it, the mimicking diet may not be easy to mimic itself. According to initial press reviews in 2015, the nutrient composition of ProLon® alters each day, but it is generally low-protein, low-sugar and reasonably high in healthy fat. However, while available in the US and Italy, it is yet to hit the shelves in the UK. Longo is also writing a book, which he hopes will be out soon. Any profits, as with his shares in L-Nutra, he says, will go to his non-profit foundation to continue research. 

Of course, none of us will ever know if any kind of fasting will make us live longer. But in terms of improved health and reduced risk of disease, the potentials are interesting. 
It appears that the hardest part of fasting is the period of time between 18 to 48 hours without food, when the 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’.

It’s these interim symptoms that make fasting a real challenge and can lead us to throwing in the towel. This is possibly why the 5:2 diet is gaining ground amongst dieters who claim it’s easier to follow than absolute fasting — because a small amount of food is allowed on fasting days. Also, the psychological effect of knowing that food can be eaten as normal the next day seems to help people stay on track. Studies also show it’s every bit as effective at reducing body fat and even more effective at preserving muscle mass compared with daily calorie restriction.9

The biology basics

The body enters a fasting state about eight hours after eating, when the gut has finished absorbing nutrients from our last meal.16 Blood glucose levels drop slightly and the brain, sensing this dip, triggers changes 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, 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 draw energy from other sources; namely fat. Our adipose tissue releases stored fat into the bloodstream in the form of glycerol and fatty acids, and 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. This is particularly important for the brain and central nervous system, which are major consumers of glucose but 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.

The healing process

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.6 It is thought that when fighting illness, fasting allows the body to put that energy to use elsewhere, namely the immune system and the process of healing. Some advocates of fasting claim that 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,10 metabolic syndrome,11 rheumatoid arthritis,12 eczema,13 cancer,14 and many others — although studies and evidence vary.

Heart health:

Intermittent fasting has been shown to reduce prevalence of coronary artery disease15 and reduce blood pressure and resting heart rate while increasing resistance to stress.17

Immune health:

Research has shown that prolonged fasting (PF) for between 2-5 days has a more profound effect on the immune system than intermittent fasting. PF kick-starts our stems cells to regenerate new white blood cells, boosting our ability to fight disease.2

Brain health:

Fasting enhances cognitive function, learning and memory by promoting the growth of new nerve cells.2,18 Speaking to the BBC’s Food Programme, Mark Mattson, Professor of Neuroscience at John Hopkins University, said that 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,” he says.18

Many of the healing processes mentioned above are enabled via autophagy, which is increased by fasting. It is this mechanism by which cells recycle themselves — a vital cellular function for health and longevity.19
However, despite the hype, fasting is not for everyone and it is not advised when there are some pre-existing conditions. Professor Longo’s view is clear. Fasting, he says, can be “dangerous”, especially in combination with different types of drugs such as insulin, metformin etc., which is why it should only be done under medical supervision.

Red flags

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.20


There is conflicting opinion as to what age it is safe for a child to fast. Some advocate from 18 to 20-years old. The NHS position is that intermittent fasting from 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.21

Pregnancy and nursing mothers:

Fasting is not advised during pregnancy because of the foetal 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.22 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.23


As diabetes is a disease of blood sugar control that varies by individual, fasting depends on your condition. The NHS advises 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.21

Thyroid conditions:

The thyroid gland controls metabolism and when it’s under-active it can lead to weight gain. Fasting as a strategy for weight loss can slow down thyroid function24 — not helpful if your thyroid is already under-active. There is also anecdotal evidence that intermittent fasting may disrupt blood sugar regulation,25 which also is not helpful for thyroid function.26 That said, limited research suggests that intermittent fasting of less than 24 hours is safe for those with hypothyroidism and mild-to-moderate hyperthyroidism.27,28 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.



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