What is Intermittent Fasting?

December 28, 2017
Intermittent Fasting (IF) has been a weight loss approach that has generated a lot of media attention in recent years and of course, the 5:2 diet books are still some of the best selling diet books on Amazon.  

However the practice is actually ancient and has been practiced as a procedure in many religious ceremonies for centuries.   Plato claimed he fasted for greater physical and mental efficiency, and Mark Twain said: “A little starvation can really do more for the average sick man than can the best medicines and the best doctors.”
There is no real clear definition of what IF actually is.  In essence it is an umbrella term for various diets that cycle between a period of fasting and non-fasting.   One thing is for sure, IF used in conjunction with calorie restriction is another acceptable and tested method which can aid weight loss http://onlinelibrary.wiley.com/doi/10.1111/j.1467-789X.2011.00873.x/full
There really are unlimited intermittent fasting protocols but two of the most popular modern methods can can be grouped into 2 categories: whole-day fasting and time-restricted feeding.

1) Whole-day fasting involves regular one-day fasts.
  • The strictest form would be Alternate Day Fasting (ADF). This involves a 24-hour fast followed by a 24-hour non-fasting period.
  • The 5:2 diet allows the consumption of 500–600 calories on 2 fasting days.  Sometimes the fasting days are back to back, sometimes not.  This pattern of eating is often referred to as the 5:2 diet because you eat normally for five days of the week and cut your calories to about 25 percent of normal intake on two nonconsecutive days of the week. Men consume just 600 calories on their two weekly fast days, while women are limited to 500 calories. Not surprisingly, those calories must be spent wisely on fast days, or you could blow half of them on a flavoured latte.
2) Time-restricted feeding (TRF)
This involves eating only during a certain number of hours each day.  A common form of TRF involves fasting for 16 hours each day and only eating during the remaining 8 hours, typically on the same schedule each day.  A more lax version of this would be twelve hours of fasting and a twelve-hour eating window, or a stricter form would be to eat one meal per day, which would involve around 23 hours of fasting per day.

There are differing ideals on what can be consumed during the fasting periods. Some would say only water, others would allow tea or coffee (without milk or sugar) or zero-calories drinks with artificial sweeteners. Yet others would allow "modified fasting" with limited caloric intake (e.g. 20% of normal) during fasted periods rather than none at all.  Thus you can see IF is really variable in it’s structure.

Michael Mosley, a journalist and trained Doctor from the Royal Free hospital in London, who helped write the book “The Fast Diet” explains there are no specific rules for when you eat or meal frequency.

Theoretically, the longer amount of time without eating the better, the diet authors say, so dieters would ideally have just one 500 to 600 calorie meal that day or two meals several hours apart. However, Mosley also suggests the best approach is one that the dieter will actually stick to, so if it's easier to spread those 500 to 600 calories on several low-calorie snacks throughout the day, go for it.
On fast days, aim to meet your calorie count with about 50 grams of good protein, such as steamed white fish, skinless chicken and plant-based protein like tofu, nuts, seeds, legumes or eggs. Fruits and vegetables will play a big role on fast days given that many of them have low glycemic loads, few calories and plenty of fibre and nutrients.  On the other five days of the week, there's no calorie cap, and no food is off-limits. This freedom isn't permission to binge and make up for your two fast days, but it does mean you shouldn't feel guilty about eating a slice of cake.

A 2014 review described that intermittent fasting has not been studied in children, the elderly, or the underweight, and could be harmful in these populations. It also suggested that people choosing to fast for periods of time greater than 24 hours should be monitored by a physician. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3946160/


The review concluded that fasting is unlikely to have much effect on conditions other than obesity, such as aging or other chronic condition, unless combined with moderate calorie restriction and a plant-based diet such or even the Mediterranean Diet.  However, such is the flexibility of IF that one can theoretically choose their preferred diet, be it vegetarian or Mediterranean and eat this way using principles of IF.

The review concluded that fasting is unlikely to have much effect on conditions other than obesity, such as aging or other chronic condition, unless combined with moderate calorie restriction and a plant-based diet such or even the Mediterranean Diet.  

Advantages – There are all sorts of claims around IF, things such as improved blood sugar regulation, improved cognition and brain health, superior weight loss and even enhanced longevity.  Professor Valter Longo has carried out research on mice which does show interesting results for many of the above claims 
The diet is not so much a diet, but a structured way of eating your calories.  For certain mindsets and people, this can work very well and IF has been used successfully as a weight loss tool for many people.
There is also no obvious food restrcitions, as long as your stay within your windows and calorie guidelines, and ensure you are eating nutrient dense foods, you can essentially enjoy any foods you wish.
Disadvantages – IF does not really appear to have any clear magical qualities that we can claim yet beyond helping provide a method for some people (not all) to help sustain a caloric net benefit.
The NHS suggest certain populations (children, elderly and underweight) should not try the method and those that do fast for longer than 24 hours should do so under the guidance of a physician.
The allowance of all foods on non fast days have led to reports of binge eating on foods with low nutritional density.
According to PhD Nutritional researcher Alan Aragon, the human research shows no special health effects of IF compared to more linear caloric deficit diets.
To Sum It Up
If you prefer to practice some variant of IF and are successfully making or maintaining progress on it, then good for you, stick with it. If you prefer a more conventional or linear dietary pattern with a higher meal frequency and are doing well on it - good for you, stick with that, because the claims of IF's supposed *superiority* do not have strong scientific support.
In the end, IF is just one approach, among many effective ones, for improving health, performance, and body composition.

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