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Intermittent Fasting - Complete Guide

Would you skip breakfast if it gave you the chance to live longer?

I skipped breakfast this morning, skipped my 11am snack and have been doing so for the last three years.

My daily eating consists of two meals, the first is eaten at 12pm and the last just before 8pm. I then go 16 hours before eating again. It is an approach to eating called intermittent fasting.

Within my first 6 months of intermittent fasting I decreased my body fat percentage by 2%, increased lean muscle mass and decreased my 5k time from 24 minutes to 19. I trained the same amount, I ate less, yet I got faster, stronger and leaner.

How is this possible?

For most of history, humans haven't been eating three meals per day with additional snacks in between. Rather, people evolved to survive and prosper though long periods of food shortage. The advancement of farming and food-on-demand is only a recent prospect and it has drastically changed our eating habits. We now spend most of our time in front of computers and eat as we please. Eating in this way is not what our bodies are used to but we can change. How long would it take to form a new habit?

A 1982 study monitoring showed that a five day fast managed to increase Human Growth Hormone (HGH) by 300%. As fat starts to replace glucose as the main energy source for the body the need to burn carbs decreases and the need to catabolise protein is reduced. Another study showed a 5-fold increase in HGH production in response to a 2 day fast.

Sugars and fats are the body's primary sources of energy. When the body has depleted its glucose stores the body will adjust by using fat. This is the way we survived when food sources were not as consistent. A couple of thousand years ago it is highly likely that humans would have to go without food for a few days, multiple times per year. Our bodies are adapted to food consumption based on this inconsistency.

What is Intermittent Fasting?

The three most common forms of intermittent fasting are:

Alternate-day fasting - Eating one meal that contains around 25% of daily calorie needs 3 times per week. e.g. Monday-Wednesday-Friday. On non-fasting days, caloric intake is unrestricted.

Whole-day fasting - Similar to alternate-day fasting but restricted to one to two days per week. Complete fasting or consuming up to 25% of daily caloric intake on 1/2 days per week with no calorie restrictions on the other 5/6 days. The 5:2 diet is an example of this. It encourages 2 days of fasting with limited calorie intake and 5 days where calorie intake is not tracked.

Time-restricted feeding - Encourages feeding within a fixed window each day typically between 8-10 hours. Eg Eating only between 12-8pm and fasting the rest of the day.

The method I refer to here is time-restricted feeding, or less abstractly known as intermittent fasting. It separates itself from "diet plans" in that it sustainable for long periods without feeling hungry. It is more of an adjustment in how many hours you eat for. You can eat the foods you want, you just eat two meals instead of three.

Why do People Fast Intermittently?

Intermittent fasting is an schedule adjustment technique that is most commonly used to lose weight. However, this is not the only health benefit. It is possible to gain muscle mass whilst reducing body fat percentage.

Many people will struggle to get enough calories in in an 8 hour window and feel full eating less than their normal daily calories.

How Does Intermittent Fasting Work?

Intermittent fasting makes intuitive sense. When we eat food, it is broken down in our digestive systems and turns up as a variety of molecules in our blood. Our bodies use the sugar from broken down carbohydrates for energy, a process that occurs rapidly for refines carbs and sugars. When we do not use all of the energy provided by food we store it as fat. Sugar cannot enter our cells on its own, our bodies produce insulin to bring the sugar into our fat cells for storage.

In between meals our insulin levels decrease and fat cells can release the sugar they have been storing. We lose fat when our insulin levels go down and this is the basis of intermittent fasting - to allow our bodies enter a state where they are using fat as our fuel source meaning we lose weight.

We start to burn fat around 12 hours after eating, this is when we enter the fasted state. For this reason it is rare for people without actively intermittently fasting. It is also why people who start fasting lose fat without changing their diet. Intermittent fasting puts the body in a state of fat burning that you would never normally reach.

Cooking is a passion of mine. Cooking one less meal per day is still something I appreciate. Anything that makes life simpler or reduces decision making, especially in the mornings, is greatly appreciated.

Benefits of Intermittent Fasting

Examples of Intermittent Fasting


How long should I fast for to receive the benefits associated?

Your body will start burning fat after around 12 hours. For benefits associated with fat loss 14-16 hours is recommended.

When doing a 16:8 fast can I eat whatever I want?

Losing weight is a simple matter of energy in vs. energy out. Weight loss occurs when the body is in a caloric deficit. Intermittent fasting aids weight loss by decreasing the window for eating, making it difficult to graze all day.

Is intermittent fasting just used for weight loss..? Or can I build muscle at the same time?

While many people use intermittent fasting a vehicle to lose weight, building muscle is very possible. People will often lose weight while fasting and think they have also gained muscle. You see, because they look more muscly, you would be inclined to think you gained muscle. What has happened is losing fat has made your muscle look more defined. Muscle gain occurs when you have you have a caloric surplus meaning muscle that gets broken down during exercise can be rebuilt in a larger, stronger form. Caloric surplus is entirely possible during 16:8 fasting and I have gains a lot of muscle while fasting.

When should I exercise when using the 16:8 method?

I exercise every morning as soon as I wake up. This is part of a habit ladder i have built to ensure mornings are as productive as possible. Exercising and fasting can be a little tricky as it will make you more hungry. So if you're still getting used to fasting it can make lasting the whole 16 hours more difficult. To begin with, try exercising during the feed period, then move to exercising just before breaking your fast so you can eat straight after working out. This way you will be occupied in the last hour of your fast and as soon as you finish your workout and your body needs nutrients you can eat. Exercising in the the feed or fast periods are both fine, it's just a matter of personal preference.

Will intermittent fasting slow down my metabolism?

We are commonly told that eating little and often is the best method for weight loss. The logic goes something like, "consuming food increases metabolism so eating more food will increase my metabolism more". Whilst true, it is also true that the metabolic increase is proportional to the number of calories consumed so whether you consume your food in large meals or lots or little ones, the metabolic effect will be the same.

The positive side effects of Intermittent Fasting are phenomenal for a lot of people, but as with all advice involving health you should contact your medical practitioner before starting.

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