Intermittent fasting is trending in the health world right now, and you probably know how I feel about health trends.  (If you don’t, here’s the short version: people are constantly looking for something “magic” and “cutting-edge” to reduce the effort or pain it takes to be healthy.  There is no such thing.  The diet industry is a multi-billion dollar industry because the real answer is to eat a balance of whole foods, including many plants, healthy fats, and clean proteins, reduce sugar, move your body, and manage stress.  But people don’t want that answer … they want something new).

That said, I’m all about using scientific advances to improve our health, so if something new does advance in the research, I’ll be all over it.  I’m not closed minded … I just find that most “diets” and trends aren’t worth their weight in kale.

What is intermittent fasting?

So, back to intermittent fasting.  What is it, anyway?  Intermittent fasting is simply extending the period of the day during which you are not eating.  All of us do it every day, because we’re not eating overnight as we sleep.  But for many Americans who eat a late-night snack before bed and pour sugar in their coffee or eat breakfast as soon as they wake up, the fasting period may be only 6-8 hours.  Intermittent fasting encourages people to extend this fasting window from 12 to 20 hours (meaning your “feeding window” is 4 to 12 hours during the day).

The most popular variety of intermittent fasting is called 16:8, meaning that you fast for 16 hours and eat for 8 hours (let’s say, eating from noon to 8pm, or 8am to 4pm).  Other popular varieties include the 5:2 diet (eating normally 5 days per week, and restricting to 500-600 calories per day on 2 days per week), eat-stop-eat (a 24-hour fast, once per week), and The Warrior Diet (fasting through the day and eating one large meal at night).

All of this sounds complicated, right?  But let’s think about it further. 200 years ago, intermittent fasting was completely normal!  No one had a stopwatch out, but without modern electricity, it was likely rare to eat after sundown or before sunrise.  And if we go back even further, to the hunter-gatherer days, even a day or two without food was likely common.  So, this isn’t really “new” after all … it is just human nature.  But the stresses of living in today’s world make it more complicated (so, jump ahead to the “cautions” section if you’re about ready to dive in!).

How to do it

• Choose a fasting window (I recommend starting with 12 hours, so if you finish eating at 8pm, you can start eating again at 8am).
• During your fasting window, you can consume water, herbal tea, and black coffee.
• During your feeding window, you can technically eat whatever you want, although I encourage you to eat a diet high in vegetables, healthy fats, and lean proteins. Cramming four servings of McDonald’s into a 4-hour feeding window is sure to eliminate any potential health benefits from the fast.

Benefits of Intermittent Fasting

There are several benefits to intermittent fasting.  Here are some of them:

• Improved body composition / reduced body fat. Going without food for an extended period of time forces your body to rely upon stored body fat as fuel, which is a good thing for most people.  Having to burn body fat during the fast also makes you more insulin sensitive (meaning that you can handle sugar better and are less likely to succumb to the “sugar roller coaster” and, eventually, the risk of type 2 diabetes).  Many studies show that fasting may not decrease overall body weight in most subjects, but does reduce body fat and change body composition. Interestingly, most of these studies are done on men, which we’ll discuss later.
• Increased insulin sensitivity. Basically, this means that your body is able to use the insulin it produces to keep your blood sugar stable (more here).  Decreased insulin sensitivity over the long term leads to conditions like Type 2 diabetes.  Studies show that fasting improves insulin sensitivity, but again, most of these studies are done on men.  And of the few studies done on women, fasting may even decrease insulin sensitivity.
• Improved digestion. Your gut needs to rest just like every other part of your body, and if it’s constantly digesting food, it doesn’t get to do so.  Many people see improved digestion when implementing a 12-hour fast.
• Longevity / disease prevention benefits. Research is still emerging, but there are some pretty promising results for intermittent fasting in patients with cancer, heart disease, Alzheimer’s, Parkinson’s, and stroke.  Intermittent fasting has also been linked (in rat studies, for now) to increased longevity.

THIS IS IMPORTANT!!  Contraindications / Cautions to Intermittent Fasting

Unfortunately, there are also several downsides to intermittent fasting.  Here are several contraindications and cautions:

• Females are less likely to be successful while fasting. Females are more finely attuned to stresses like caloric deficits, because a female’s body prioritizes fertility above all else.  Because the body sees fasting as a stressor, a female’s body can often “panic” during a fast, resulting in a shift towards more fat storage.  Your body is trying to protect you, but if you have gained fat while fasting, this is what is happening.  This article has a lot more on the differences.
• Hormonal imbalance can be exacerbated while fasting. While intermittent fasting can improve male hormone balance, and tends to have no impact on some females, if you are female and already have imbalanced hormones, intermittent fasting is likely to make it worse.  Too much calorie restriction or fasting for too long can lead to amenorrhea (lack of period) in females, and studies in rats show that fasting leads to everything from decreased ovary size to “reproductive shutdown”.
• For those doing intense exercise, fasting can be tricky. Research shows that fasted exercise is great for promoting fat burning, so as long as your blood sugar is stable, you don’t need to eat before a workout of an hour or so.  However, when doing fasted exercise, anabolism (the ability to build muscle) is impaired if you do not refuel after the workout (immediate refueling is not necessary if the workout isn’t fasted).  This means that if you’re doing a fasted work out at 5:30am for an hour, you’ll need to fuel shortly thereafter (let’s say, by 7:30am).  This means, if you’re going for a 12-hour fasting window, you need to be finished eating by 7:30pm.
• If you are under a lot of stress (environmental, mental, or physical), intermittent fasting adds to this stress load. Your body accumulates all types of stress together, making up your total stress load.  If it’s a crazy time at work, or you’re exercising more than is normal for your body, or you’re fighting off a sickness or chronic disease, or you’re going through an emotionally taxing time, adding another stressor (fasting) increases your likelihood of adrenal burnout and chronic stress overload.  This is not a good thing.
• Those with history of disordered eating should not impose artificial limitations like a fasting window. In general, if you have a history of disordered eating, I encourage you to listen to your body and focus on fueling well, rather than imposing artificial limits like fasting windows.

 The Bottom Line: Is Intermittent Fasting Healthy?

• Intermittent fasting is not a magic bullet … for anything.
• That said, intermittent fasting has some great health benefits for digestion, longevity, and several health conditions.
• For most people, achieving a 12-hour fast daily is doable and healthy.
• If you have hormone imbalance, cortisol issues / adrenal fatigue, high stress, history of disordered eating, engage in daily endurance exercise, and / or are a pre-menopausal female, I wouldn’t recommend fasting beyond 12 hours daily.
• If none of the above apply, and you want to experiment with more, just be sure you’re getting in plenty of nutrients and choosing well-rounded meals during your shortened feeding window!
• LISTEN TO YOUR BODY. If we really listen, it knows the answer.  If you feel amazing doing intermittent fasting, go for it.  If you feel tired, cranky, are performing poorly in exercise, are gaining body fat, or have any other negative issues, then stop!

Now it’s your turn … Have you tried intermittent fasting?  What pattern of eating works best for you?