Intermittent Fasting isn’t a diet per se, it’s a pattern of eating, a way to time your meals to leverage a few interesting things about human metabolism and cognition. In the research world we call it ’intermittent energy restriction’ and this sums it up nicely - you’re restricting your energy intake to certain times.
Why Intermittent Fast?
The most common reason for Intermittent Fasting is to improve body composition by losing body fat. There are other reasons though, many people and studies report increased focus and productivity. Lifestyle is also a consideration, if you’re daily fasting and not eating a morning meal (i.e. breakfast) it can free up some time which is useful if you’re rushing to work or on the school run.
How does Intermittent Fasting work?
At any part of the day, you are either in a fed or fasted state. As you might expect the fed state is after a meal when your body is working to digest, absorb and use the food you have eaten. This fed state lasts around four to five hours. During this time your body will preferentially use carbohydrates in the food you have eaten for energy, rather than rely on stored body fat. This is because the carbohydrates are abundant, i.e you’ve just eaten them, and because your insulin levels are higher after a meal - this hormone makes it harder to use fat as fuel.
About 12 hours after the meal you enter a fasted state (4-12 hours after a meal is actually called the post-absorptive state). In this fasted state your body is using the stores of energy in the body to keep everything going and relies heavily on stored body fat for energy.
With our normal pattern of eating, say 3 meals day, it’s easy to see one is rarely, if ever, in a true fasted state. This is one of the reasons Intermittent Fasting is so popular and often effective - a simple shift in meal timing can shift one towards fat burning.
But this isn’t all, daily Intermittent Fasting can also make a big contribution to reducing the ‘calories in’ side of the energy balance equation, i.e. help you to eat less in any given day. If you think about it this is quite counter-intuitive, one would expect that by fasting for say 18 hours a day you’d be inclined eat more in the other 6 hours. Studies have shown this not to be the case - people that fast for 18 hours eat less in their other meals than those that don’t fast. Bonus!
How to Intermittent Fast?
The two most popular ways to implement Intermittent Fasting are daily or weekly.
Daily is really very simple, you aim to enter the fasted state every day by fasting for 16-18 hours a day and eating your meals in the other 6-8 hours. Effectively this means having your evening meal at say 7pm then your next meal the following day at say 1pm - that’s an 18 hour fast.
Weekly is the method popularised by 5:2 - eat your normal meal pattern on 5 days, on 2 consecutive days you “fast” allowing yourself just 500-600 on these days.
Both of these methods leverage the same science and thinking, which is best is really down to the individual, which will work best with your lifestyle.
By Form’s Head of Nutrition, Dr Adam Collins. Dr Collins is an MSc, PhD in Nutrition with research interests in exercise intensity and energy balance, intermittent fasting and timing of food around exercise.