Intermittent Fasting for Trail Runners

What is Intermittent Fasting?

Intermittent fasting is simply timing your food consumption so that you have a period where no calories are consumed.

The point of intermittent fasting is to give your body a period when it must rely on stored energy rather than getting it from frequently consumed meals or snacks. It’s a recent development and quite unnatural, with respect to our evolutionary history, to be in a constantly fed state. Until quite recently, food was scarce, and our days were spent ensuring that we had enough food to survive. Now we have such an abundance of food, that instead we must spend our time inventing new ways to burn off the energy we consume.

When people fast, they slowly burn through the glucose stored in their liver. Under normal conditions, there’s roughly 700 calories of stored glucose in the liver. This equates to roughly 1.5 – 2 hours of moderate intensity running. Running out of blood glucose leads to hitting the infamous “wall”, also known as “bonking”. Without exercise, it takes 10 to 12 hours to use the liver’s energy stores. Once you’ve run out, fats are used for energy.

This process is called “metabolic switching,” and the constant eating pattern favoured by people in modern societies doesn’t allow their bodies to run through their liver’s energy stores and make the switch to fat-burning.

Intermittent fasting is commonly touted as a weight loss technique but it doesn’t necessarily need to be practised for the purpose of losing weight, as it has many other benefits.

Benefits of Intermittent Fasting

Some of the benefits of intermittent fasting include:

  • Weight loss
  • Body fat loss
  • Improvement in your body’s ability to burn fat as a fuel source
  • Lower and more stable blood sugar levels
  • Reversal of type 2 diabetes
  • Increase in mental concentration
  • Increase in overall energy
  • Increase in growth hormones
  • Improvement in your cholesterol levels
  • Reduction for the risk of Alzheimer’s
  • Longer life
  • Reduction in inflammation

One of the most notable effects of intermittent fasting for me is that I feel less urgency to eat, even towards the end of a fasting period. This is particularly good for making healthier food choices and avoiding junk foods that you might eat in “emergency” situations.

Also, I’ve never really been a huge fan of eating breakfast, so intermittent fasting suits me quite well. It may not be for everyone though.

Styles of Intermittent Fasting

There are a number of ways to do intermittent fasting. These are a few of the most common:

Daily time-restricted feeding gives you a narrow window during which you can eat, usually 6 to 8 hours each day. One popular formula is 16:8, where you fast for 16 hours and have a window of 8 hours in which you eat. This is typically achieved by ceasing food consumption by about 8 or 9pm and then remaining fasted until lunchtime the following day. This has the added benefit of avoiding some of the junk food consumption that is common in the evening.

5:2 intermittent fasting requires that people only eat one moderate-sized meal (500-600 calories) on two days each week.

The warrior diet is a slightly modified version of a daily time-restricted feeding diet. It involves eating small amounts of raw fruits and vegetables during the day and eating one large meal in the evening.

There are many other variations too. The primary aim is to have periods where you’re taking in limited calories, and your liver’s energy stores are depleted in order to encourage the burning of fat for energy.

How to get Started with Intermittent Fasting

Decide how long you want your fasting a eating periods to be, and what time of day is most practical for you to begin fasting. As a guide, you may want to aim to stop eating at 8pm, fast for 16 hours, and have your first meal of the day at midday.

Add these steps one at a time until you can do them all:

  1. Don’t eat after dinner.
  2. Don’t snack before your first meal of the day. This includes drinks that contain energy.
  3. Delay breakfast by 1 hour.
  4. Do step 3, increasing by 1 hour at a time, until your first meal of the day is at lunch time.

Be sure to drink plenty of water while fasting.

At first, you’ll probably feel very hungry and it may take a fair degree of will power to resist snacking. As you progress and your body gets used to the condition of not being constantly fed, you’ll find the feeling of hunger become more manageable. It will totally change your perspective on hunger and eating.

It’s not uncommon to feel tired, get a headache, and generally feel “out of sorts” when beginning to fast. It’s normal and the side effects typically go away as your body gets used to fasting. As you become more experienced, most people feel great after the negative symptoms have passed, and a sense of calm, well-being, and heightened concentration takes over. Sometimes the negative side effects can be due to electrolyte imbalances, so it may be beneficial to consider supplements.

Running while Fasting

Once your body is accustomed to physical activity in a fasted state, you shouldn’t have too much trouble maintaining a regular training schedule. Keep in mind though, that although your body may be getting better at burning fat for energy, it will always be getting that energy from a combination of glucose and fat. That ratio depends on a lot of things, and you can extend your endurance by using glucose at a slower rate, but once you run out, there’s no getting around it. So if you’re running for an extended period, you will need to eat on the go.

You can choose to run during your non-fasted period thereby avoiding the problem. However, many runners who practise intermittent fasting prefer to do their daily run in a fasted state. This logic behind the practise is that it forces your body to burn fat. The more you do it, the better your body should become at using fat for fuelling your runs, theoretically. This is called being “fat adapted”.

Pay attention to intensity when beginning to run while fasted. There is some science to suggest that performance may be reduced, even for individuals that are fat adapted. So, don’t expect to do your high intensity workouts to the same level as you would have in a non-fasted state. However, for low and medium intensity runs, you may find that there is little difference between fasted and non-fasted performance.

Further reading

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