Here’s what experts say about the rewards — and risks — of intermittent fasting

“So, what do you do about eating?”

My annual physical was going well, and my doctor was inquiring about my diet. “A little big of everything in moderation?” I said, shrugging; then, I countered, “What do you do about eating?”

“Well,” the doctor replied, “I practice intermittent fasting.”

I’d heard the hype over the years, about how fasting can help maintain a healthy weight, and potentially stave off everything from Alzheimer’s disease to sleep apnea to cancer. But it was the sight of my energetic, razor-sharp doctor — who is my age but doesn’t look anywhere near it — that made the most compelling case I’d ever seen for fasting. 

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As someone who has never once uttered the phrase, “I forgot to eat,” who carries granola bars around specifically to stave off hunger rage, I figured myself as unlikely a candidate for meal skipping as you would ever find. And the information out there about intermittent fasting seemed so confusing, so contradictory, I wasn’t even sure where to get started. Do you restrict certain types of food? Do you only eat at certain times of day? Do you not eat at all some days? Most importantly of all, though, I wanted to know: what’s actually in it for me?

As it turns out, quite a lot… maybe.

“Intermittent fasting isn’t about what you eat, it’s about when,” says Elizabeth Ward, a Boston area registered dietitian and nutrition consultant. “With no calorie restrictions or special foods to make or buy, IF (intermittent fasting) is more of a lifestyle than a prescriptive diet.”

How one goes about that, however, can be flexible.

“There are several types of IF, including time-restricted eating, and going with no, or very little, food for entire days,” Ward continues. “On the 16:8 plan, only calorie-free beverages are allowed for 16 hours and you eat during an eight-hour period of your choosing. The 5:2 plan consists of eating as usual on five days of the week and consuming 25% of your daily calories (about 500 for women and 600 for men) on the other days. Alternate day fasting (ADF), allows for calorie-free beverages on every other day of the week, and eating on the remaining days.” 

Most people discover IF because they’re interested in losing or maintaining their weight, because it seems to promise dramatic and fast results. It’s definitely a simple way of restricting calories and avoiding less nutrient-rich foods.

“Breakfast in America is usually a high carbohydrate, high sugar, dense calorie meal,” says New York doctor James Stulman, a physician in my local practice. “And then after 7 pm, that is a really challenging time. A lot of my patients, including myself, are hungry at 9:30 pm. We’re snacking on cookies or something sweet. So if you’re disciplined enough not to eat after seven o’clock, you’re probably getting rid of all the nasty carbohydrates, which are the real problem.”

But unlike other diets, intermittent fasting seems to offer real possible health advantages, because it kicks in different processes that can make the body more efficient. A 2021 paper in the journal Nutrients explains, “As a result of periods of restricted food intake, the human body initiates a metabolic switch from glucose to stored lipids, which leads to a cascade of metabolic, cellular, and circadian changes that are associated with numerous health benefits in animal models and humans. Periods of IF have not only been associated with weight- and metabolism-related diseases, but also with reducing the risk/prevalence of neurological diseases.”

And a widely circulated 2019 New England Journal of Medicine review of the “Effects of Intermittent Fasting on Health, Aging, and Disease” reported that “The metabolic switch from glucose-based to ketone-based energy” may result in “increased stress resistance, increased longevity, and a decreased incidence of diseases, including cancer and obesity.” 

There is science that attests to why intermittent fasting can be healthy for your cells. Christine Kingsley, Health and Wellness Director of the Lung Institute and an advanced practice registered nurse, explains that “during intermittent fasting, the body attains lower levels of glucose more efficiently, catalyzing the activation of brain synapses and stress resistance. This allows the brain to function at its fullest capacity as humanly possible, which is why verbal memory is notably improved during and after the practice.”

There are other potential benefits as well.

“One of the main effects is a reduction in insulin levels,” says John Landry, a registered respiratory therapist and the founder and CEO of Respiratory Therapy Zone. “High levels of insulin have been linked to an increased risk of chronic diseases like diabetes and heart disease. By reducing insulin levels through intermittent fasting, individuals may be able to reduce their risk of these conditions. Intermittent fasting has also been linked to a decrease in inflammation, which is believed to be a contributing factor to several chronic diseases.” He adds, “There is currently limited research on the effects of intermittent fasting on lung health. However, some studies suggest that intermittent fasting may have potential benefits for respiratory function, such as reducing inflammation and improving oxidative stress.”

Intermittent fasting also typically means your body isn’t busy digesting during your resting hours. That can lead to better sleep, experts say.

“An IF schedule that has your last meal at least two to three hours before you go to bed (caveat: for the general population, not night shift workers) can support healthy sleep and optimal daily energy in many ways,” says Chester Wu, a board-certified MD in Psychiatry and Sleep Medicine with the sleep and energy app RISE. “It allows for better digestion, reducing the risk of heartburn and acid reflux keeping you up at night.” Furthermore, he says, “when we sleep, our brains clear out waste products. But if your body is busy digesting a meal, blood gets diverted to the digestive system, leaving the brain with fewer resources to do this job.” 

Regardless of the perceived benefits of IF, some people absolutely shouldn’t attempt it. As Elizabeth Ward explains, that includes “people under 18 and over 75; pregnant and breastfeeding women; those on medication that must be taken with food at certain times of the day; those with a chronic medical condition, such kidney disease; people with a history of disordered eating.” She adds, “IF can be triggering. Preoccupation with the timing of eating can encourage obsessive behaviors concerning food. In addition, exercise reduces glucose and insulin levels, and people relying on IF may need to change the intensity, and timing, of exercise to prevent fatigue.”

I may be intrigued by intermittent fasting, but my lifestyle right now isn’t realistically compatible with it. I could get by with just black coffee for breakfast, but I’m not yet ready for consistently early bird dinners. People who have families, who travel or socialize, or keep erratic hours would probably likewise struggle to stay on intermittent fasting. And any eating plan is only as good as your ability to stick with it. So for the time being, I’ll continue to pay more attention to what I eat than when. “Of first and foremost most importance,” says Dr. Stulman, “is your choices of food.”