Does Intermittent Fasting Really Work?

There’s a new trend in dieting these days—not eating. Yup, “fasting” became a popular trend in the U.K. with The Fast Diet by Dr. Michael Mosley and has trickled over to the states in several variations, including The 8-Hour Diet by David Zinczenko, consulting editorial director at American Media, Inc. And while fasting itself is certainly not a novel concept (people have been doing it for religious reasons for hundreds of years), “intermittent fasting” as a weight loss method seems to be the new trend. But is it safe? And does it really work? We talked to fitness and nutrition expert JJ Virgin, bariatric surgeon Dr. Marina Kurian and Dr. John M. Berardi of Precision Nutrition to find out.

While there are different levels of intermittent fasting (IF) diets, two of the most buzzed about are The Fast Diet and The 8-Hour Diet. The Fast Diet, sometimes referred to the 5:2 diet, encourages people to eat normally five days per week, and trim calorie intake down to 500 to 600 calories total for two non-consecutive days. In terms of weight loss, participants can expect to shed about one to two pounds per week, says Dr. Mosley on his website. The 8-Hour Diet, on the other hand, limits the window of time for calorie consumption to eight hours per day, which is supposed to make the body burn fat and calories more efficiently. According to MensHealth.com (the brand author Zinczenko used to work for), The 8-Hour Diet works on a cellular level and triggers the energy centers of the body’s cells (mitochondria) to selectively burn fat for energy. The diet is also said to reduce the amount of cancer-causing cell damage caused by the typical American diet, the site says.

Both diets claim that, in addition to helping participants lose weight (and keep it off), they’ll also help regulate blood sugar, possibly prevent diabetes, slow the ageing process, and prevent or minimize risk of heart disease. So far, these claims have only been supported by small human studies or animal studies. Our experts also noted that there isn’t much scientific research in support of IF diets as effective, long-term weight loss plans.

Still, that doesn’t necessarily mean they’re meritless, assuming you’re a healthy individual with realistic weight loss goals, some experts suggest. The positive aspect of these weight loss plans is that they challenge people to get in touch with hunger levels, says JJ Virgin, nutrition and fitness expert, author of The Virgin Diet. “Usually we’re eating because we are tired, thirsty or bored. Ask yourself why you’re eating again,” Virgin says.

Learning the difference between when you think you’re hungry and when you’re actually hungry was one of the biggest takeaways for John M. Berardi, MD, Chief Science Officer of Precision Nutrition and author of Experiments with Intermittent Fasting (based on his own experiments with various intermittent fasting plans over the course of six months).

“Trial fasting is a great way to practice managing hunger,” says Dr. Berardi. “It’s an essential skill for anyone who wants to get in shape and stay healthy and fit.” Though Dr. Berardi started off at a healthy weight, he lost 20 pounds in six months and reduced his body fat from 10 percent to four percent while maintaining most of his lean muscle mass, he says.

But is it dangerous?

While people should always check with their doctor before starting any weight loss plan, there are certain individuals who should steer clear of this type of diet, says Marina Kurian, MD, assistant professor of bariatric surgery at NYU Langone Medical Center. Those with insulin resistance issues, diabetics, pregnant women and people with low blood sugar should not try these plans, she says, as they could be dangerous to their health.

Virgin says this diet might also be problematic for women who are under chronic stress and have adrenal issues. “You’ll be stressing your body out more by following an intermittent fasting plan, which can lead to insomnia and possibly fertility issues,” Virgin says. Research has linked very low-calorie diets to chronic psychological stress and cortisol production, which has been shown to interfere with weight loss. The bottom line? If following this diet stresses you out or interferes with your sleep patterns, it could also impede weight loss.

Intermittent fasting could also be problematic for individuals with bingeing issues, or anyone who might have trouble controlling how much they’re eating on the non-fasting days. “Don’t let five of those days [or eight hours] be free-for-alls,” says Virgin. Both diets encourage eating wholesome, nutritious foods as often as possible during the times you’re not fasting.

Do fasting and exercise play nice?

As long as you’re adequately hydrated, Dr. Kurian says light to moderate exercise on lower-calorie days is generally safe, assuming you feel up to it. But if you don’t have the energy for exercise (or feel light-headed or dizzy), it may be best to save your strength for higher-calorie days. Plus, because exercise causes the body to burn through its glycogen stores, it’s important you’re able to refuel with a balanced meal or snack (including protein, carbs and fat), optimally within 30 minutes of working out.

If you already follow a regular exercise routine, it may be best to save your most intense workouts for days when you’re not fasting. After all, the goal should be to make the most of your workout session and that’s more likely to happen when you’re properly fueled with quality calories, Virgin says.

Bottom line: Is it worth trying?

As far as results go, regular fasting isn’t objectively better for losing body fat, Dr. Berardi says. While he says his IF experiments worked quite well, the intermittent fasting approach (bigger meals, less frequently) didn’t produce better fat loss than a more conventional diet approach (smaller meals, more frequently) might have.

And though it’s not dangerous to follow if you’re a healthy individual, Dr. Kurian says, she wouldn’t recommend this weight loss plan to her patients.

“You have to know what you’re capable of doing for a diet,” says Dr. Kurian. Instead, she recommends people aim to lose about a pound a week for long-term sustainable weight loss.

“Ask yourself if you can do this for life. It’s important to follow a weight loss plan you can maintain long term,” Dr. Kurian says. It’s also key to have a plan in place for the maintenance phase, after you’ve hit your weight loss goal. Otherwise, you’ll be more likely to regain the weight assuming following these fasting plans isn’t feasible for the rest of your life, she says.

Intermittent fasting can work, but it’s not for everyone—nor does it need to be, Dr. Berardi says. “In the end, IF is just one approach, among many effective ones, for improving health, performance and body composition.”

To learn more about the intermittent fasting diets discussed in this article, visit The 8-Hour Diet and The Fast Diet.

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