In recent years, various intermittent fasting plans have become popular with people seeking to lose weight or improve their health. The most popular regimens generally involve very low or no calorie intake on certain days per week, then eating normally on non-fasting days.
Alternate Day Fasting
Professor Krista Varady created the Every-Other-Day Diet, based on her groundbreaking research into ‘alternate-day modified fasting’ at the University of Illinois in Chicago. Proponents describe it as ‘the diet that lets you eat all you want (half the time) and keep the weight off!’
The plan involves alternate ‘fast’ and ‘feast’ days. Fasting days consist of a single 500 calorie meal at lunchtime. But then there is no restriction on what, when or how much is eaten on feasting days. The two key attractions are:
- The promise that ‘you’ll lose weight and improve your health – while eating anything you want and all you want, every other day’.
- Where most diets include a daunting set of rules to be obeyed, here there is only one rule: eat no more than 500 calories on Diet Day, eat anything you want and as much as you want on Feast Day. That’s it. No counting calories, carbs, fat or protein. No avoiding any particular food; all foods are allowed. No complex meal preparations and plans.
Two Days Per Week Fasting
Developed by popular UK TV medico Dr Michael Mosley, the Fast Diet involves fasting for two days per week. People maintain their usual eating routines for the other five days. Dr Mosley sums up: ‘If we were to distil the Fast Diet into a single soundbite, it would all come down to 5:2. That’s five days of normal eating, with little thought to calorie control and a slice of pie for pudding if that’s what you want. Then, on the other two days, you reduce your calorie intake to 500 calories for women and 600 calories for men.’
Proponents claim that since you are only fasting for two days of your choice each week – and eating normally on the other five days – there is always something new and tasty on the horizon. In short, it’s easy to comply with a regime that only asks you to restrict your calorie intake occasionally. It ‘recalibrates the diet equation, and stacks the odds in your favour’.
Daily Intermittent Fasting
Daily intermittent fasting limits eating to a certain number of hours each day. The 16:8 Diet is an increasingly popular method, which involves fasting for 16 hours per day, leaving an eight-hour window for eating.
There are various forms of this plan, with the most popular advocating that the ‘fasting’ phase should last through the night and during the morning hours. Ideally the ‘fast’ should then be broken around midday, with the last food for the evening being consumed around 7pm or 8pm. Alternately for those with a personal preference for later daily routines, the food window may be between 2pm and 10pm.
Periodic Fasting
Medical News Today has reported on a study suggesting periodic fasting – defined as ‘one day of water-only fasting a week’ – may reduce the risk of diabetes among people at high risk for the condition.
Another study, conducted by Dr Valter Longo at the University of Southern California found longer periods of fasting (two to four days) may even ‘reboot’ the immune system, clearing out old immune cells and regenerating new ones – a process they say could protect against cell damage caused by factors such as ageing and chemotherapy.
Potential health benefits
A comprehensive survey by US newsletter Medical News Today found advocates of intermittent fasting ‘say the following benefits can be achieved’:
Weight loss
Since the body is unable to draw its energy from food during fasting, it dips into glucose that is stored in the liver and muscles. This begins around eight hours after the last meal is consumed. When the stored glucose has been used up, the body then begins to burn fat as a source of energy, which can result in weight loss.
Reduce cholesterol
As well as aiding weight loss, Dr Razeen Mahroof at the University of Oxford in the UK explains the use of fat for energy can help preserve muscle and reduce cholesterol levels.
Regenerate immune cells
The study by Dr Longo noted above suggests prolonged fasting may also be effective for regenerating immune cells. ‘When you starve, the system tries to save energy, and one of the things it can do to save energy is to recycle a lot of the immune cells that are not needed, especially those that may be damaged,’ he explains.
What is more, the team found cancer patients who fasted for three days prior to chemotherapy were protected against immune system damage that can be caused by the treatment, which they attribute to immune cell regeneration.
‘The good news is that the body got rid of the parts of the system that might be damaged or old, the inefficient parts, during the fasting,’ says Dr Longo. ‘Now, if you start with a system heavily damaged by chemotherapy or ageing, fasting cycles can generate, literally, a new immune system.’
Longevity
Some researchers report years of animal studies have shown a link between restriction of calories, fewer diseases and longer life. Scientists have studied the mechanisms behind those benefits and their translation to humans.
Insulin-like growth factor-1 (IGF-1) is a hormone linked to certain diseases that affect lifespan, such as cancer and type 2 diabetes. Some experts report eating increases IGF-1 production. Fasting may be a way to decrease IGF-1 levels, which could potentially lower the risk of chronic diseases and extend lifespan.
Cancer
Studies have reported that restricting calories decreases IGF-1 levels, which results in slower tumour development. A very small study in people with cancer found that fasting reduced some of the side effects of chemotherapy, including fatigue, nausea, vomiting and diarrhoea.
Due to potential negative effects, long-term calorie restriction is not recommended for people with cancer. Short-term calorie restriction, such as intermittent fasting, may be an option for this group of people.
Neurological diseases
Intermittent fasting may also impact cognition. In a study of mice with genes for Alzheimer’s disease, intermittent fasting improved performance on measures of cognitive decline associated with ageing.
Blood sugar
Some studies have shown that intermittent fasting improves insulin sensitivity more than traditional diets, but others have not found the same advantage. Researchers have also reported intermittent fasting and traditional diets lead to comparable decreases in haemoglobin A1c.
Potential health risks
According to the UK’s National Health Service, there are numerous health risks associated with intermittent fasting.
People who fast commonly experience dehydration, largely because their body is not getting any fluid from food. If you are used to having breakfast, lunch, dinner
and snacks in between, fasting periods can be a major challenge. As such, fasting can increase stress levels and disrupt sleep. Dehydration, hunger or lack of sleep during a fasting period can also lead to headaches.
Fasting can also cause heartburn; lack of food leads to a reduction in stomach acid, which digests food and destroys bacteria. But smelling food or even thinking about it during fasting periods can trigger the brain into telling the stomach to produce more acid, leading to heartburn.
According to 5:2 Diet advocate Dr Michael Mosley, intermittent fasting is not recommended for people with the following conditions:
- Being underweight
- Eating disorders
- Type 1 diabetes
- Type 2 diabetes that is controlled by medication
- Pregnancy (or women breastfeeding)
- Recent surgery
- Mental heath conditions
- Fever or illness
- Conditions where Warfarin is prescribed.
Negative feelings and behaviours reported by Dr John Berardi in his
book Experiments with Intermittent Fasting include:
- Changes in mood
- Extreme hunger
- Low energy
- Obsessive thoughts about food
- Binge eating behaviour
And Medical News Today noted people interested in trying intermittent fasting ‘should consider whether or not it will work with their lifestyle. Fasting stresses the body, so it may not be beneficial for people already dealing with significant stressors’.
