Intermittent fasting: What nutritionists and dietitians think about it

Elena Sheppard
·5 min read
What do experts think about intermittent fasting? (Photo: Getty Images)
What do experts think about intermittent fasting? (Photo: Getty Images)

Diet trends ebb and flow, but intermittent fasting has been holding steady in popularity for a few years running. Celebrities like Jennifer Aniston, Chris Pratt and Vanessa Hudgens swear by it, but what exactly is it? And how does it work? Most importantly, is it healthy?

In a nutshell, intermittent fasting is a system of eating that determines windows of time when people eat, which in turn leads to eating fewer calories. Over the years, research has shown that intermittent fasting improves metabolic health and accelerates weight loss, but much of the data collected has been from animals, or in short human trials, and thus the benefit for people is still a bit of a question mark. Yahoo Life spoke with a handful of nutritionists, dietitians and doctors to get their expert takes on intermittent fasting and learn more about whether this popular diet is safe.

What is intermittent fasting?

Intermittent fasting is a weight loss strategy that cycles between periods of eating and fasting, and typically involves choosing certain hours of the day when calories are not ingested. “Intermittent fasting doesn’t tell you what to eat, but it dictates when you eat,” says dietitian and nutrition consultant Alix Turoff. “For some people, intermittent fasting may make it easier to create a calorie deficit without feeling restricted. But intermittent fasting will not result in weight loss if you’re not aware of how much you’re eating during the time you’re consuming food.”

One popular routine is the 5:2 method of intermittent fasting which means that on two days of the week, a person caps their caloric intake at 500; while the other days of the week they eat normally. Supermodel Gisele Bündchen follows this method.

There is also time-restricted eating, which sees people choosing a certain number of hours per day that they do not eat. For instance, the 16:8 method which means people fast for 16 hours of a day and eat all their meals during an eight-hour window; or the 14:10 method which means people can eat for 14 hours of a day and then fast for 10.

What are the benefits?

According to Dr. Ethan Weiss, a cardiologist at the University of California, San Francisco, who has been practicing intermittent fasting for about seven years, intermittent fasting doesn’t have dramatic effects with regards to weight loss. Research released in September 2020, and conducted by Dr. Weiss and his co-authors, found that 16:8 intermittent fasting provided almost no weight loss benefit and that much of the weight loss in participants in the study came from lean mass (which includes muscle) and not body fat.

“I became a little bit of an evangelist for [intermittent fasting],” Dr. Weiss tells Yahoo Life of his own experience with the diet. “I decided that before I became too evangelical I probably oughta see if it works in people. That’s how we designed this study.” What they found is that intermittent fasting does “not much at all.”

Dr. Weiss did say that he was interested in what further research on intermittent fasting would reveal and whether or not other types of intermittent fasting would result in weight loss or better metabolic health. “Those studies need to get done,” he says.

Others, like Dr. Angela Fitch, Associate Director of the Weight Center at Massachusetts General Hospital, still adheres to intermittent fasting. “In general it is good for all of us to fast for at least 12 hours overnight. This was basically the way we ate for centuries,” she says. “I try to fast for at least 12 hours by cutting down my eating at night. There is also research to support it is not as healthy for us to eat at night.”

Dr. Fitch also mentioned research that is currently being done on intermittent fasting. “We think it helps decrease cellular aging and improved blood glucose regulation. It regulates certain beneficial neurotransmitters and decreases inflammation,” she says. “It also may increase stem cell production and increase apoptosis (cellular killing) or abnormal cells.” But, she adds, “We really don’t know [the benefits] with the scientific rigor we need yet to universally recommend it.”

One definite benefit of intermittent fasting? “It is free,” Dr. Fitch notes. “It doesn’t cost anything financially. So for many people, it can be done on a budget.”

Is it healthy and safe?

As Dr. Fitch says, intermittent fasting is not for everyone. “People who are pregnant, people who have an eating disorder or history of an eating disorder should not. Patients with serious health conditions like type 1 diabetes, heart failure and are on other medications like insulin, diuretics, etc. need to be sure they consult a physician to do it safely,” she says of intermittent fasting. “Whatever we do in life we have to balance our ability to live our best life with our happiest one and do what we can to maximize our health.”

Turoff is quick to note that intermittent fasting isn’t a magic solution. “For some people, intermittent fasting can help them achieve a caloric deficit more easily. For some other people, stressing about intermittent fasting can actually make matters worse,” the dietician says. “At the end of the day, the most important factor to consider when deciding if a way of eating is right for you is: how easily can I adhere to this? If you’re worried about breaking your fast with toothpaste or find yourself struggling through your fasting window, jump ship!”

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