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Intermittent Fasting 101

Posted by Brad Dennis, Ph.D. on

Intermittent Fasting 101

Intermittent fasting has become one of the most popular diet trends worldwide. But what is it? And is it safe?

What is Intermittent Fasting?

Fasting refers to abstaining from eating food for a period of time. And while modern society has ingrained it in us that we should never feel hungry and that having at least three meals a day is the norm, humans have been fasting for various reasons, both voluntarily and (unfortunately) involuntarily, for thousands of years. Many modern religious traditions incorporate fasting as part of their spiritual practices.

Our bodies have evolved to be able to go without food for hours and even, when necessary, days. In fact, fasting can be seen as far more normal than most of our modern eating habits.

In general, intermittent fasting refers to restricting the time one eats within each 24-hour period. Done correctly, intermittent fasting can help the body burn fat, and studies have given evidence that there may be other health benefits, including reducing the risk of type 2 diabetes, insulin resistance, and reducing the risk of cardiovascular disease.

What Happens When We Fast?

After several hours without food, the body will exhaust its stores of sugar and it will start to burn fat. Science calls this metabolic switching. Most of us, because we are constantly eating throughout the day, sometimes right up until we go to bed, never go through this switch – we are constantly providing our bodies with fresh calories to use instead of allowing it to start consuming our stored fat.

How to Intermittently Fast Properly

Please check with your doctor before trying intermittent fasting, in case it’s contraindicated with a preexisting health condition or with a medication you are taking.

16/8 Fasting

By far the most popular intermittent fasting method is the 16/8. It’s the easiest to implement and the least disruptive to your daily life. With 16/8 intermittent fasting, you split each 24-hour period between an 8-hour window in which you are allowed to eat, and the 16 hours in which you are fasting.

Choose an eight-hour window that works for your schedule. Set it up so the bulk of the time you’re fasting is when you are sleeping; for many, that might mean choosing starting their eight hours in the mid-morning (between 9-12pm) and ending in the late afternoon/early evening (between 5-8pm). But if your schedule is different because you work a 3rd or 4th shift, your best 8-hour window will look much different. When choosing your 16/8, try to keep the following in mind:

  • Your typical hunger cycle. If you’re starving right when you wake up, then forcing yourself to fast for 3-4 hours before your eating window begins is not a good idea. Alternatively, if you find you’re hungriest at the end of the day, then starting your eating window later is your best bet.
  • Your typical eating/meal times. When do you eat your meals? Do you ever skip meals? If so, which one do you most often skip (without resulting in a major disruption, such as heavy snacking, headaches, feeling weak/dizzy)?
  • Your daily activity cycle. When are you most active? Your 8-hour window should be during that time.
  • Your normal sleep cycle. Again, make sure as much of your fasting window as possible is happening when you’re sleeping. This lets you avoid any uncomfortableness associated with feeling hungry (something that many of us simply aren’t used to feeling). 

5:2 / The Fast Diet

Another very popular method of intermittent fasting, the 5:2 refers to five days where you eat normally, and then 2 days where you have a very restricted diet that only allows for a total of 500-600 calories per day. The two days should be non-consecutive; for example, you can choose to fast on Sunday and Wednesday, but not on Thursday and Friday.

The Warrior Diet

This doesn’t involve a true fast at all; instead, it involves heavily restricting your calorie consumption for most of the day, then having one large meal in the evening. Basically, it means you “undereat” for 20 hours a day, then “overeat” during the remaining 4-hour window.

The key aspect of this method is in restricting yourself to the approved foods during the day: small amounts of nuts, vegetables, and fruits are acceptable, as are hardboiled eggs and dairy.

Alternate Day Fasting (ADF) and Eat-Stop-Eat

These two are much more restrictive (and thus more disruptive) methods of intermittent fasting.

Eat-Stop-Eat is similar to the 5:2 method. The difference is that instead of severely restricting calories on two days, with Eat-Stop-Eat, you actually fast for those two 24-hour periods. And as with 5:2, it’s important for those two days to be non-consecutive. You never want to fast two days in a row.

Alternate Day Fasting takes it another step further. With ADF, you fast every other day, which will result with you fasting for a total of 3 days one week, and 4 the next.

Who Shouldn’t Fast?

When it comes to deciding whether or not someone should try fasting, common sense applies. People who are pregnant or who think they might be pregnant should not fast, nor should anyone who is currently breastfeeding a child. Children and adolescents should also not be fasting on a regular basis. Anyone with a chronic health condition, illness, or disease should not fast. Anyone who has a history of disordered eating, or who currently has an eating disorder should not fast. Again – talk to your doctor before you attempt any fasting method, to be sure it’s right for you.

Should you decide to try fasting, if at any point you start to feel unwell or unsafe, stop the fast immediately. And never consider it a failure; some bodies simply aren’t able to tolerate.

Last Thoughts

Just remember: all methods of intermittent fasting allow you to drink water and coffee/tea (as long as no sugar or milk/non-dairy are added to them) during your fasting periods and in fact, it’s important to keep drinking fluids, as they can help curb hunger pangs and keep you from feeling dizzy or unwell if you’re unused to fasting. Zero-calorie carbonated water is also fine, but zero-calorie sodas are not. It’s important that no sugars of any kind be in the liquids you choose to drink during the fasting period. So waters like La Croix and Bubly are OK, but sodas like Coke Zero are not.

When you break your fast, eat normally. There are no food restrictions. The one thing to avoid is over-eating; doing so will negate any beneficial effects received from the fast. And be aware – if your normal eating involves fast food, you are not likely to see much benefit from fasting.


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