Intermittent Fasting May Be Popular, but Experts Still Aren’t Sold

There’s no question that intermittent fasting is now firmly embedded in the cultural zeitgeist. Case in point: Last year, more than one in 10 Americans tried intermittent fasting, according to the International Food Information Council. People who promote the diet cite our prehistoric hunter-gatherer ancestors’ tendency to go long stretches of time without food as proof that this is a natural and healthy eating pattern. But . . . is it?

“Intermittent fasting has the potential to improve your health by helping with weight loss, improving blood sugar control, and possibly lowering the risk of diseases like diabetes and heart problems,” says Raj Dasgupta, MD, FACP, FCCP, FAASM, quadruple board-certified physician. Yet, these purported benefits aren’t quite as clear cut as they may seem. And even though social media is ripe with intermittent fasting proponents, the potential drawbacks aren’t always advertised.

So we spoke with experts and did a deep dive into the latest research for the full lowdown on the true health impacts of intermittent fasting — both good and bad.

Experts Featured in This Article

Raj Dasgupta, MD, FACP, FCCP, FAASM, is a quadruple board-certified physician specializing in internal medicine, pulmonology, critical care, and sleep medicine, who is currently the Internal Medicine associate program director at Huntington Health Hospital in California.

Bryan Quoc Le, PhD, is a food scientist and the author of “150 Food Science Questions Answered.”

Dalina Soto MA, RD, LDN, is a registered dietitian and founder of Your Latina Nutrition.

What Is Intermittent Fasting?

Intermittent fasting is a diet that requires you to fast during certain hours of the day and/or days of the week. The idea is that restricting your calorie intake forces your body to burn fat for fuel, rather than relying on the usual and readily-available glucose that comes from food. “Researchers speculate that daily consumption of food increases the levels of oxidative stress in our bodies, which in turn exacerbates the risk for chronic conditions,” says food scientist Bryan Quoc Le, PhD. The theory is that “intermittent fasting allows the body to recover from the intense strain of breaking down, distributing, and storing food over time,” he adds.

Types of Intermittent Fasting

There are different ways to course out your eating. One popular intermittent fasting regimen is the 5:2 approach — eating normally for five days of the week, but restricting your daily caloric intake to 500 to 600 calories on the other two.

Another option is the 16:8 approach — you fast for 16 hours per day, eating only during the remaining eight hours.

“The 16:8 plan, where you eat during an eight-hour window every day, might be easier if you prefer a consistent routine. The 5:2 plan, where you eat very little on two days a week, gives you more flexibility but can be harder on those fasting days,” says Dr. Dasgupta. Whichever option you choose ultimately boils down to a matter of personal preference. “The benefits and risks of intermittent fasting are pretty similar whether you do 5:2 or 16:8. Both can help with things like weight loss, better metabolism, and improving how your body uses insulin. The main difference is how these methods fit into your lifestyle,” he says.

Unlike many other diets, intermittent fasting doesn’t regulate what you eat, just when you eat; you can eat whatever you want during your allotted windows. That said, many people try to fill up on high-protein, low-calorie foods, particularly on days when their intake is limited to 500 to 600 calories, says Dalina Soto MA, RD, LDN.

Health Benefits of Intermittent Fasting

Your Heart Health Might Improve

Research, including that 2022 study in Nature Reviews Endocrinology we referenced above, shows that intermittent fasting might improve a few heart health markers including blood pressure, cholesterol, and triglycerides. “It lowers LDL cholesterol, the ‘bad’ cholesterol, and triglycerides, reducing the risk of heart disease,” Dr. Dasgupta says.

According to a 2020 study in the American Journal of Medicine, these heart health benefits may be related to reducing oxidative stress, optimizing the body’s circadian rhythm, and inducing ketogenesis, the production of ketone bodies in the liver when your body uses fat instead of glucose for fuel. This is because using fat for energy — rather than storing it — may help lower LDL and raise HDL.

However, other studies have found that intermittent fasting may harm heart health. For example, a more recent study with over 20,000 participants found that people who restricted eating to eight hours per day had a 91 percent higher risk of dying from cardiovascular disease than those who ate during a more standard 12 to 16 hour window.

You May Lose Weight. . . In The Short Term

First, it’s important to say that weight alone isn’t an accurate measure of health, and therefore weight loss isn’t always correlated with any health benefits. But many people who try IF do so with weight loss as a goal. And the eating plan could lead to weight loss, at first. But it also can come with some significant drawbacks. “Intermittent fasting can lead to weight loss by limiting how much you eat, but it’s not always quick or easy. While some people may see results faster, it can also cause fatigue, headaches, or nutrient deficiencies in others,” Dr. Dasgupta says. “Because you are cramming all your eating in fewer hours, you will probably eat less in that small window because you will not be as hungry,” Soto adds. It’s a means of reducing your food intake without having to count calories everyday.

According to a 2022 study in Nature Reviews Endocrinology, intermittent fasting can result in three to eight percent weight loss over eight to 12 weeks. It’s worth noting, however, that these results are similar to other diets rooted in calorie restriction. Research has long demonstrated that diets can help you lose weight in the short term, but keeping the weight off is a different story. Over time, the body begins to normalize the amount of calories consumed, explains Dr. Le. As a result, he says that, on average, individuals will regain the weight after one year of intermittent fasting. And calorie restriction diets come with many potential downsides it’s worth being aware of, including an increased risk of disordered eating — discussed in more detail below.

Your Blood Sugars Might Improve

Studies have found that intermittent fasting may help lower blood sugar and weight in people with type 2 diabetes. For example, a 2023 study in JAMA Network Open found that people with type 2 diabetes who restricted their eating to eight hours per day (for six months) lost 3.6 percent of their body weight on average and had a nearly 50 point decrease in average blood sugar.

“The current research is showing promising health data around [intermittent fasting and] blood sugar control, but I think it’s important to note that if you eat less and reduce caloric intake you are going to see a decrease in blood sugars,” Soto says. Since more research is needed before experts can say for sure that IF is linked to blood sugar benefits and intermittent fasting carries risks as well, if lowering your blood sugar is your goal, it’s best to discuss an appropriate eating plan with your doctor or healthcare provider before trying something new.

Health Drawbacks of Intermittent Fasting

Your Social Life May Suffer

While diet and physical health are inextricably linked, don’t underestimate the effects of dieting on your social life. “What happens if you’re invited to a later dinner or earlier breakfast? I think the mental Olympics people have to go through in order to stick to it is more of the problem,” Soto says.

It May Lead to Disordered Eating or an Eating Disorder

Dieting is one of the main risk factors for developing an eating disorder, particularly binge eating disorder, according to the National Eating Disorder Association. More specifically, an alarming 2022 study found that practicing intermittent fasting in the past 12 months and 30 days was significantly associated with eating disorder psychopathology among adolescents and young adults. Point being, if you have a history of an eating disorder or disordered eating, it’s probably best to avoid this diet, as restricting your eating could exacerbate or trigger disordered behaviors.

You May Not Meet Your Nutritional Needs

The restricted eating time frame may also mean you’re not meeting your nutritional needs. Soto says you’ll probably get less fiber, vitamins, and nutrients from intermittent fasting since your eating is limited to certain hours of the day.

It May Alter Hormone Levels

Research that examined data from both human and rodent studies raised concerns that intermittent fasting may alter production of hormones such as thyroid hormones, cortisol, growth hormone, estradiol, serotonin, and vaspin. This could in turn affect thyroid function, muscle mass, and menstrual health.

Although some studies have found that intermittent fasting benefits blood sugars, a 2023 study in Diabetes Care linked high levels of vaspin — a potential consequence of intermittent fasting — with an increased risk of type 2 diabetes. In other words, research is still mixed on intermittent fasting’s impact on blood sugar control and diabetes. And a 2022 study in the journal Nutrients found that intermittent fasting may decrease testosterone production, which could harm metabolic health and libido in men. However, while these two were conducted on people, human studies on the hormonal effects of intermittent fasting are still extremely limited.

So Is Intermittent Fasting Worth Trying?

In a word, maybe. “Intermittent fasting affects people differently because everyone has unique metabolic rates, lifestyles, and genetics,” Dr. Dasgupta says. As a result, he says it shouldn’t be touted as a universal solution. “People with insulin resistance or metabolic issues might experience greater benefits, like improved blood sugar control. However, athletes or those with high energy needs may struggle with fatigue on fasting days,” he says.

But before you hop on the intermittent fasting bandwagon, keep in mind that science-backed results are still mixed. Much of the research has been conducted on animals. Human studies are limited and the ones that do exist are often small in size, yield mixed results, and don’t examine on-going effects. “All the research on intermittent fasting does state that we need long-term data to make any conclusions,” Soto says.

At the end of the day, your best bet is to talk with a healthcare provider to ensure intermittent fasting is first and foremost a safe option — but also one that’s worth your time. Although intermittent fasting may sound sexy, the less-glamorous yet more tried-and-true route of eating a well-balanced, nutrient-rich diet may ultimately be the simplest and safest choice for staying healthy.

Isabel Vasquez, RD, LDN, is a registered dietitian and expert health and nutrition writer practicing from a weight-inclusive, non-diet, culturally humble framework. In addition to PS, her work has been published in EatingWell, Self, Health.com, Verywell Health, Outside, and more.