Is Fasting Healthy? Weighing the Pros and Cons of Intermittent Fasting

Intermittent fasting has recently surged in popularity. Hugh Jackson, Justin Theroux, Beyoncé, and Nicole Kidman are among the celebrities who swear by fasting for weight loss and its other benefits.

“Advocates for taking periodic breaks from eating — for up to 24 hours once or twice a week — tout it as an effective and research-backed means of losing weight and improving health,” says Robert Collier in the Canadian Medical Association Journal. “That message has been reaching more and more ears of late.”

Collier wrote that in 2013. He probably had no idea how hot a topic intermittent fasting would become five years later.

What is Fasting?

As its name suggests, intermittent fasting (sometimes just called fasting) involves eating and not eating.

“Intermittent fasting simply means fasting overnight,” says Mark Hyman, MD, in Food: What the Heck Should I Eat?. “It’s what we always did as cavemen and cavewomen: Eat dinner before dark, and then don’t eat again until the next morning… [Fasting] gives your body a chance to repair, heal, clean up metabolic waste in your body and brain, and more. And it stimulates weight loss.”

When you don’t eat – in other words, when you fast – your body doesn’t shut down. Instead, it shifts its fuel source from glucose (either from incoming food or as a small amount of stored glycogen) to ketones, which are derived from fat. In other words, with intermittent fasting, your body uses fat (as ketones) for fuel.

The benefits of intermittent fasting are vast and impressive. Research shows fasting can improve metabolic diseases such as type 2 diabetes while preserving muscle mass and function. Fasting can optimize your immune system and create better brain health. It can even improve gut health and help you sleep better. (I’ve discussed many other fasting benefits here.)

But let’s be honest: You’re probably fasting for weight loss. Plenty of research shows that fasting can indeed help you lose weight, providing metabolic benefits including fat burning without restricting calories.

Equally important, intermittent fasting can create weight loss and fat loss while improving insulin sensitivity in overweight people.

As I mentioned earlier, when you’re fasting, your body eventually shifts from glucose to ketones. Your body literally burns stored fat for fuel, which makes fasting ideal for weight loss.

You also consume probably eat fewer calories when you fast. Theoretically, you might eat more calories during your first meal after fasting, but studies show overall you still create a caloric deficit.

One looked at how people ate after a 36-hour fast. Researchers found while they ate slightly more calories following the fast, they consumed almost fewer 2,000 calories over the two-day period.

Of course, your mileage will vary, but that’s an impressive way of restricting calories without actually counting them. And that makes intermittent fasting ideal for weight loss.

Is Fasting Healthy?

For most people, intermittent fasting is perfectly healthy. In fact, fasting can make you healthier: One study found fasting could reduce the oxidative stress that results when free radicals overtake your body’s antioxidant defense. Fasting can also lower inflammation, which (like oxidative stress) contributes to many diseases.

Despite its current popularity, fasting is hardly new. Many religions have observed this way of eating (or not eating) for thousands of years, and healthcare professionals have used fasting as a therapeutic tool to manage weight and reduce disease risk for centuries

“Fasting is as old as humankind, far older than any other dietary technique,” says Jason Fung, MD, in The Complete Guide to Fasting. “Although fasting has been practiced for millennia, it has been largely forgotten as a dietary therapy.” (Fung argues that intermittent fasting is bad for the food business, so marketers instead persuade people that they should constantly eat.)

Women and men respond to intermittent fasting differently. I discuss how to modify fasting and warning signs that you shouldn’t fast and ways to modify fasting in this article.

Of course, no plan or strategy works for everyone. Fung says the following people should not fast:

• Those who are severely malnourished or underweight
• Children under eighteen years of age
• Pregnant women
• Breastfeeding women

Fung recommends talking with your healthcare practitioner with the following conditions:

• Gout
• People taking medications
• People with type 1 or type 2 diabetes
• People with gastroesophageal reflux disease

I would include a few additional demographics and caveats here:

• Anyone with a thyroid or adrenal issue should confer with a healthcare professional before they undergo intermittent fasting.
• If you have any sort of eating disorder such as anorexia or bulimia, you should not fast.
• If you have any concerns about potential problems, proceed with caution with fasting. (That goes for any eating or diet plan.)

How to Fast

If you’re convinced intermittent fasting might help you lose weight or provide other benefits, you’ll want to find a plan that works for you in the short- and long-term.

If you’ve never fasted, I recommend eating a big dinner, closing up the kitchen for the evening, and then pushing breakfast back as far as possible the following morning. That alone creates a 12 – 14 hour (or more) fasting window, and hopefully, you’ll be sleeping during most of it.

Over time, you can gently stretch out that fasting window to 16, 18, or even 24 hours. One animal study found that fasting for 24 hours could support gut health and slow down the aging process.

You can do this in increments: Eat a big breakfast and lunch, then stop eating for 24 hours. You resume eating the following day, meaning you never go a day without eating. I do not recommend anyone fast beyond 24 hours without a healthcare professional’s approval and guidance.

You don’t have to fast every day. You can do alternate-day fasting, fast a few days every week, or even schedule in a 14-hour fast a few times a week. There are no definitive guidelines except that you don’t eat while you’re fasting.  

Even then, what you consume during fasting hours can vary. Some practitioners recommend water fasting, which means that you only consume clean, filtered water during your fasting hours. (Bonus: Water can also curb your appetite.) Other experts also allow black coffee and tea when you fast.

“Most definitions of fasting allow non-caloric drinks only,” says Fung. “This means that water, tea, and black coffee are all allowed during fasting, but sugar, honey, fructose, agave nectar, and other sugars are prohibited.”

Regardless of how long you maintain your fast, please don’t take unnecessary health risks. If you fast and suddenly feel faint or otherwise suffer adverse side effects, stop immediately and eat something.

“The most important thing to always remember is that if you are feeling poorly in any way, whether you are a man or a woman, you must stop fasting immediately and contact your health-care provider,” says Fung. Good advice.

7 Strategies to Optimize Intermittent Fasting

Intermittent fasting can be a powerful tool for weight loss and so much more. Despite how some experts and the media position it, however, fasting is simply a tool to reach your health goals. Fasting is not a panacea, nor is it for everyone.

If you want to try fasting, these seven strategies can help you maximize its benefits and make your effort more effective and sustainable over time.

Start slow and gradually increase.

Diving into a 24 hour fast immediately can become a full-blown disaster. Instead, start slowly with a smaller fasting window (such as the aforementioned after-dinner-to-breakfast fasting period). Play with that window. Gradually increase it. Don’t dive into the deep end before you’re comfortable with the shallow pool. And give it a fair try: Commit at least 30 days to fast before you fully determine whether it works for you. (Worth repeating: If you notice any adverse effects while fasting, please eating something and/ or talk with your healthcare professional.)

Keep a food journal.

Starting intermittent fasting is like starting anything: At the beginning, you want training wheels to keep you on track. That’s where a food journal can help. Writing down everything you eat and tracking the hours you eat and don’t eat can repay dividends as you learn to maintain fasting and get all its benefits. One study among almost 1,700 people found writing down everything you eat could double weight loss. You can even download apps on your phone to time how many hours you fast daily.

Eat healthy during your eating hours.

Fasting for 18 or even 24 hours (and getting the health benefits) doesn’t give you permission to deep-dive into a deep-dish pizza during the hours you do eat. Why would you want to work so hard to abstain and undo that hard work with processed, sugary junk foods? Combine fasting with our Core or Advanced Plan and you’ve got a stellar plan that helps you lose weight while stabilizing your blood sugar so fasting hours become easier.

Don’t talk about it or overthink it.

You’ll likely want to tell your friends, family, and coworkers about your newfound fasting schedule, especially when you start seeing its benefits. That could backfire: You might encounter someone telling you that you’re starving yourself, that intermittent fasting is a dangerous fad, or that you’ll regain the weight you lose on this plan. Naysayers abound, and in a weak moment, they can talk you down. That’s why in the beginning, you want to find a plan that works for you, commit to it, and don’t over-analyze it.

Consistency is key… until you get stuck.

In the beginning, you’ll want to keep a consistent eating and fasting schedule until you get the hang of things. Inevitably, plateaus occur and you might encounter weight loss resistance. That’s when you’ll want to play around with scheduling (and use your food journal) as guidance. Perhaps you’ll switch to a 24 hour fast two days a week, eat normally a few days every week, or otherwise mix things up until you’ve continued to reach your health goals. Working with a healthcare professional can help you customize your fasting strategies to reach your goals.

Maintain a stellar lifestyle protocol.

What you eat and don’t eat ultimately becomes an important piece of your healthcare puzzle. Just as important: Getting at least eight hours of stellar sleep nightly, managing stress levels, maintaining a healthy social and spiritual life, getting chiropractic care, and taking the right nutrients all to support your fasting efforts to lose weight and cultivate amazing health. When you maintain other good habits, you’ll find fasting becomes easier and creates more lasting benefits.

Use the obstacles as opportunities for self-growth.

Like learning to meditate or going to the gym, intermittent fasting can feel difficult in the beginning. The biggest obstacle people fear about fasting is hunger. Staying busy and staying hydrated while you’re fasting can help here. Over time, most people adjust and even feel more focused and energetic during fasting hours. Along the way, they discover some interesting truths about fasting. What we consider hunger, for instance, might actually be boredom. We might be eating to fill a void like loneliness. And the illusion of hunger isn’t always hunger. Sometimes, we think we should be eating (whether or not we’re actually hungry) and get hangry if we don’t. If you allow it, intermittent fasting can reveal deeper, fascinating truths about how you think and feel.

I discuss more hacks to optimize fasting and make it easier here. Be patient, stick with it, and be aware of warning signs that tell you to stop fasting.  

Newer studies will continue to evaluate intermittent fasting’s impact on weight loss, disease, and its other potential benefits. While I think most of this research will conclude positively, we must also remain aware of potential drawbacks with fasting for certain demographics.

Even though research evolves and metabolic and hormonal fluctuations underlie fasting, you don’t need to know any science or follow the latest studies to get its benefits. You simply don’t eat during your fasting hours. That could make intermittent fasting the simplest diet plan, and also one of the most effective.