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Intermittent Fasting: Is It Safe After Bariatric Surgery?

A young woman checking her watch while eating a salad and working on her computer from home.

The concept of intermittent fasting tends to be divisive. It’s either heralded as the latest surefire fitness trend or maligned as a too-good-to-be-true potential health hazard. Such dueling schools of thought make it hard to know if intermittent fasting is worth trying — and how to go about it.

We asked Julie Parrott, MS, RD, LDN, a Clinical Dietitian Specialist here at Penn Medicine, to walk us through the good, the bad, and the unknown of intermittent fasting, including whether or not it’s a recommended practice for bariatric surgery patients.

“Emerging research indicates that there may be some benefit to intermittent fasting if done properly,” said Parrott, who is a member of the metabolic and bariatric surgery team. However, she added, “these early studies are somewhat controversial due to the vast number of intermittent fasting methods and lack of long-term data.”

Before you decide if intermittent fasting is a safe and effective diet plan for you — and, to be clear, you should consult with your team’s registered dietitian (RD) before making that decision — here’s a quick primer on exactly what it is and how it’s practiced.

What is Intermittent Fasting?

Broadly defined, intermittent fasting is the practice of setting routine times for fasting and eating, scheduling these times during the course of a day or spreading them out over a week. It’s one of a few diet plans that doesn’t limit what you can eat, only when.

One method of intermittent fasting “involves a fasting day where food intake is either completely restricted or partially reduced,” limiting daily energy requirements by 60 percent or even 85 percent, Parrott explained. This fasting day is followed by another 24-hour “feeding day,” during which food is consumed “ad libitum” (at will).

There are several other variations on intermittent fasting, and they’re often referred to in terms of ratios. For example, 5:2 intermittent fasting calls for 5 days of normal eating and 2 days of consuming 500 calories or fewer. (These fasting days don’t have to happen back-to-back.) 

There is also the 16:8 method, which requires 16 hours of fasting and 8 hours of eating at will. Those 16 hours of fasting may include time spent asleep, so most of us already have a headstart on this approach to intermittent fasting. 

Another approach is “fasting for 10 to 12 hours per day and eating a modest meal or a few snacks,” said Parrott, who also cautioned that “the studies around intermittent fasting are still in early stages,” which means there could be some associated dangers.

“The primary danger of fasting is hypoglycemia, or low blood sugar,” she said. “People can also become dehydrated. Additionally, we don’t fully understand yet what effect the timing of food throughout the day has on a person’s health status, not just weight.”

That being said, there are some potential benefits to a careful intermittent fasting regimen, including increasing lifespan and staving off “obesity-related diseases such as cardiovascular disease, diabetes and certain types of cancer,” Parrott said.

“However,” she added, “there are mixed results for weight loss and maintenance.”

Is Intermittent Fasting Safe Following Bariatric Surgery?

Weight loss surgery not only helps you lose excess body weight, also reduces your risk for developing heart disease, and has the potential to eliminate sleep apnea, high blood pressure and diabetes. 

But be forewarned: The positive results of bariatric surgery could be compromised by practicing intermittent fasting. During your post-surgery life, it’s important that your body remains nourished with vitamins, minerals, fluids and nutrients.

“Intermittent fasting is not a recommended practice for bariatric surgery patients since dietary intake is surgically restricted,” Parrott said. “Patients who have bariatric surgery are instructed to eat regularly-timed meals and reduced quantities (with emphasis on increased protein).”

For bariatric patients, a prolonged fast — or even a poorly managed short-term fast — could increase the risk of dehydration, poor caloric intake and vomiting.

The goal for life after bariatric surgery is “to create long-lasting behavioral changes, in order to maintain weight loss and health-related outcomes,” Parrott said. “Since dietary intake is already limited due to bariatric surgery, it is not a good idea to begin any type of diet, including intermittent fasting.”

It is of the utmost importance for bariatric patients “to eat healthy, quantity-controlled meals in a regularly scheduled manner throughout the day,” she said.

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Learn about bariatric surgery and get the support you need to continue on your weight-loss journey. We offer workouts, recipes and tips from Bariatric Surgery program team members, and stories from patients like you.

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