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Fact or Fiction? Assessing 8 Common Intermittent Fasting Myths

Americans have been looking for the most effective weight loss method for decades, with new trends emerging as quickly as they’re refuted. Recently, intermittent fasting has been the buzz phrase. In fact, according to the International Food Information Council’s 2020 Food and Health Survey, intermittent fasting overtook “clean eating” as the most popular diet in the U.S., in 2020. Despite this, many remain skeptical about intermittent fasting’s legitimacy.

But before we assess this, let’s define the various types of intermittent fasting.

  • There’s the 5:2 approach, where you fast for two nonconsecutive days a week; for example, Monday and Wednesday.
  • There’s alternate-day fasting, which has you flip-flop between eating normally one day, and skipping meals the next.
  • There’s time-restricted eating, which is the most popular type of intermittent fasting. Here, you eat only during a specified time period; for example, from noon to 8 p.m (fasting from 8 p.m. to noon the following day). The most common restriction ratios are 16:8 fast, where you fast 16 hours and eat for eight hours; and the 14:10 fast, where you fast 14 hours and eat for 10.

Of course, any popular diet will be accompanied by myths and misconceptions. Intermittent fasting, especially since it goes against our “food is everywhere, all the time” culture, is not immune to this. That’s why we asked for clarification from experts. Below are their reactions to the misconceptions about intermittent fasting, and advice for how to healthfully approach this diet trend.

Myth #1: Intermittent fasting is best for weight loss.

The Truth: While intermittent fasting has been shown to be as effective for weight loss as other diets, “current research doesn’t show that it’s better than other calorie-restriction plans,” says Angela Fitch, MD, co-director of Massachusetts General Hospital Weight Center in Boston, faculty at Harvard Medical School, and chair of Jenny Craig’s science advisory board. However, it does have some advantages over other diets, one being that you don’t have to completely change your eating habits. This is because intermittent fasting is more about when you’re eating versus what you’re eating.

Krista Varady, PhD, professor of nutrition at the University of Illinois in Chicago, suggests that those who struggle with calorie-restriction and calorie-counting might be well-served by intermittent fasting. Varady, who frequently busts fasting myths on her Instagram account (@drkristavarady), has led several calorie-restriction studies that show a) people often get burned out after a month or two of counting calories; and b) these same cohorts have greater success with intermittent fasting. Another benefit of weight loss from intermittent fasting is that it can support in lowering blood pressure and improving insulin sensitivity; in turn allowing for better management of blood sugar levels and, thus, reduced risk of developing type 2 diabetes.

Myth #2: You can’t get enough nutrients if you fast.

The Truth: Getting ample nutrition can be harder when you’re fasting, especially if your diet is loaded with junk food. That doesn’t mean, however, that it can’t be done. “It’s harder only because you’re collapsing the window in which you’re eating,” says dietitian Alix Turoff, MS, RD, CDN, who’s also a certified personal trainer in Brooklyn, New York. “If you do fasting correctly and focus on healthy, nutritious foods, you should be able to eat a balanced diet, consuming all of the essential nutrients that your body needs.”

There is one exception, albeit a rare one. “If you’re an Olympic athlete like Michael Phelps who needs to eat thousands of calories a day, fasting could be tough,” says Turoff.

Myth #3: You can’t exercise when you’re fasting.

The Truth: Your body does get energy from the food you eat, but it’s not out of the question to exercise while fasting, especially if eating beforehand causes gastrointestinal issues when you’re physically active. “I know many runners who like to run when they’re fasted because they don’t get cramps,” Varady says. In fact, exercising during your fasting window may actually give you energy, especially if you’re doing alternate-day fasting. In studies Varady has conducted, people who combined alternate-day fasting with exercise usually pick fasting days to exercise. “They say they actually get a boost of energy from fasting,” she says.

“If you’re training for an event like a marathon, it might be wise to wait until after the event to fast so that your body always has enough fuel, especially for longer training sessions,” notes Turoff. If you do want to try to make it work during your training, just make sure that your feeding window aligns with your workouts and you schedule your longer workouts for later in the day.

Myth #4: Fasting will extend your life.

The Truth: You’ve probably heard that fasting can boost your longevity, and there is some research to back up this claim. Take, for instance, University of Southern California researcher Valter Longo, PhD, who’s promoting a type of fasting in which you limit your eating to a 12-hour window every day (doesn’t seem too hard, right?). A study in the journal Cell that he co-authored suggests that eating mostly plant-based foods while following this “fasting-mimicking” diet can yield numerous benefits, including weight loss and a longer life. But don’t get your hopes up — at least not yet. “We currently don’t have human data on longevity to say one way or another,” Dr. Fitch says. “Most of the [existing] data is theoretical based on animal models.” That’s because human longevity studies are costly and require at least 10 years of close observation and review. These studies are currently underway, though, so keep an eye out.

Myth #5: You’ll lose muscle if you’re doing intermittent fasting.

The Truth: “If you’re getting enough protein, known as the building block of muscle, in your diet, this shouldn’t be an issue,” says Dr. Fitch. (The Recommended Dietary Allowance (RDA) of protein is 0.36 grams of daily protein per pound of body weight.)

Turoff, however, advises getting 0.7 to 1 gram of daily protein per pound of body weight for most people. That’s because the RDA recommendation represents the bare minimum for avoiding deficiency, which doesn’t take into account special populations like seniors or people trying to lose weight.

Make sure, too, to consumer enough carbohydrates. If you don’t, your body may eventually break down muscle for fuel.

Myth #6: Fasting gives you brain fog.

The Truth: This might depend more on you than the diet itself. “When you fast, your body makes ketones as it burns fat, and many people experience increased mental focus with ketone bodies in their blood,” Dr. Fitch says. In fact, better concentration is one of the reasons fasting may have become so popular with high-level executives in Silicon Valley. Of course, there is a small adjustment period. “You might feel tired and brain fogged for the first 10 days,” says Varody, “but that should disappear after you adjust.”

Myth #7: Skipping breakfast is unhealthy.

The Truth: No doubt you’ve heard that breakfast is the most important meal of the day, and for certain populations — like school-age children — this is true. “Breakfast improves their concentration and focus,” Turoff says. This is not the case, however, for all populations. Research on the precise benefits of breakfast has been inconclusive. While some studies show that people who eat breakfast have a lower risk of obesity and weight gain, other studies suggest eating breakfast has no impact.

“Those who practice intermittent fasting, especially if they follow a 16:8 fast, often choose to eat between 12 p.m. and 8 p.m. so they can enjoy dinner with family and friends,” Varady says. This can be challenging, though, for people who wake up early, which is why Varady suggests sipping tea and coffee with a little sugar and a bit of cream in the morning to hold you over. The key is paying attention to your body. “As long as you’re not completely starving and counting the hours until lunch, you should be fine skipping breakfast,” says Turoff.

Myth #8: Fasting can mess up your metabolism.

The Truth: There’s a common misconception that fasting can mess up your metabolism, but that’s not accurate. For starters, understand that whenever you lose weight, your metabolic rate will naturally decline. “Weight loss causes your body to slow its energy burning to conserve energy and prevent further weight loss,” says Dr. Fitch. “Your body’s simply trying to defend or protect against weight loss, which is one reason it’s so hard to lose weight and keep it off.”

Also, about 75 percent of weight loss comes from fat, with the other 25 percent from muscle. This is true regardless of the diet you follow. “Your calorie needs will drop by 100 to 200 calories per day,” says Varody, “but that’s no different for intermittent fasting than any other diet.” Eating enough protein and doing strength training can help stanch this loss.

Even so, there are studies that support the claim that short-term fasting (like time-restricted eating, where you’re fasting for only half the day), can change how your insulin levels affect your body fat regulation. Exactly how that impacts your metabolism is something researchers are still figuring out. Dr. Fitch adds that there’s also a possible rise in the neurotransmitter norepinephrine, which may promote fat burning; as well as in increase in the number and health of your mitochondria, which will allow your body to burn more energy.

The Bottom Line

There’s nothing magical about fasting. While it works for some, it might not work for others. The same holds true for virtually any diet. “Fasting likely has human health benefits and allowing your body a rest from eating, especially at night, can be beneficial,” Fitch says. “But if it’s hard for you and leads to an increase in your stress level, then it’s not right for you.” Instead, do a type of natural fasting that nutrition experts have been advocating for years: Close your kitchen after dinner and don’t open it until your first meal the next day.

A version of this article appeared in our partner magazine The Complete Guide to Intermittent Fasting For Beginners.

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