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What Are the Pros and Cons of Intermittent Fasting?

Contributor Mary Hyer, RD, LDN, CCRP
7 minute read
A clock on a dinner plate, with a woman's hands holding a fork and knife on either side.

Food is fuel for your body. But getting the right balance of calories and nutrients can be tricky. Intermittent fasting is one approach to eating that’s gained a lot of popularity. But is it a smart strategy, or a passing fad?

“It’s not a magic cure for losing weight,” cautions Mary Hyer, RD, LDN, CCRP, a Mass General Brigham cardiac rehabilitation dietitian. Hyer works with patients at the Elfers Cardiovascular Center at Newton-Wellesley Hospital. “But the research on intermittent fasting is promising.”

So how does it work—and should you try it? Hyer shares the intermittent fasting pros, cons, and facts you should know before you start.

What is intermittent fasting?

A fast is just a stretch of time when you go without eating. Intermittent fasting is an eating plan where you alternate between eating and not eating for a certain amount of time.

How much time? That depends. “There are lots of options to choose from,” Hyer says. “But in the scientific world, you have to go at least 12 hours for it to be considered a fast.”

Time-restricted eating plans are some of the more approachable models, Hyer explains. With these plans, you eat every day but only during certain windows. You might eat all your meals and snacks between 10 a.m. and 6 p.m., then fast the other 16 hours a day. Some people choose 6-hour windows for eating, others 10 or 12.

More intense intermittent fasting plans follow the so-called 5:2 approach. You eat normally for five days a week. On the other two days, you dramatically restrict your diet, cutting calories by at least 75%. (For example, if you normally aim for 2,000 calories a day, you’d eat no more than 500 calories on fast days.) But this more extreme approach is definitely not for everyone. Always talk to a health care provider before trying any low-calorie meal plans.

Intermittent fasting: Pros and cons

Intermittent fasting has been shown to help with weight loss and improve some markers of health. But it’s not for everyone. If you’re thinking of trying a fast, consider the pros and cons.

Pros: Intermittent fasting benefits

One big thing that intermittent fasting has going for it: “It’s very easy to do,” Hyer notes. “You don’t have to track your meals or count calories. It’s something you can fit easily into your life.”

Plus, evidence shows fasting can be good for you. Scientists have done a lot of research on fasting in animals, and some in people. Those studies show a range of potential benefits, both for heart health and overall health. The benefits include:

  • Weight loss
  • Less inflammation
  • Better brain health
  • Fewer age-related diseases
  • Longer life

 

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Cons: Intermittent fasting cautions and concerns

While intermittent fasting has promise, it’s not right for everyone. A lot of the research has been done in animals, so it’s not clear if people would get all of the same benefits. And early studies mostly looked at the more extreme 5:2 diet, Hyer points out. Researchers are still exploring how helpful it is to restrict eating to 8 or 12 hours a day.

For example, a recent analysis of over 20,000 U.S. adults found that people who limited their eating across less than 8 hours per day were more likely to die from cardiovascular disease compared to people who ate across 12 to 16 hours per day. This data was part of preliminary research presented at the 2024 American Heart Association’s Epidemiology and Prevention│Lifestyle and Cardiometabolic Scientific Sessions. Research is considered preliminary until published in a peer-reviewed scientific journal. You should always consult with your doctor prior to making changes to your lifestyle.

Michelle O’Donoghue, a Mass General Brigham cardiologist, spoke with Boston 25 News about research and what it means for patients.

“The key takeaway for people who are doing intermittent fasting is: don’t panic,” says Dr. O’Donoghue. “This study is not definitive and it doesn’t tell about cause and effect…Nonetheless it does raise some questions about the safety of intermittent fasting.”

Dr. O’Donoghue adds, “There have been some really intriguing studies suggesting that a pattern of intermittent fasting may improve your blood pressure, improve your cholesterol, and really lead to weight loss. But at the end of the day, it seems like it’s more about what you eat, rather than the pattern of eating.”

Fasting might not be a good idea for certain groups or people with some health problems. Hyer recommends not fasting (or talking to a doctor first) if you:

  • Are over 65
  • Are still growing (children, teens, and young adults who haven’t finished developing)
  • Are pregnant or breastfeeding
  • Have low blood pressure
  • Take medications such as blood thinners, diuretics, blood pressure medications, or medications that affect your blood sugar
If fasting sounds scary, try to avoid snacking between meals and especially late-night snacking. That can be an easy way to give yourself time for your blood sugar to recover between meals.

Mary Hyer, RD, LDN, CCRP
Cardiac Rehabilitation Dietitian
Mass General Brigham

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Tips for intermittent fasting

If you want to give intermittent fasting a try, these tips will get you started:

  • Think about your lifestyle: If family dinners are important to you, it doesn’t make sense to start fasting every day at 3 p.m. Think about which option best fits your life.

  • Read up: What are your goals for fasting: to lose weight, live longer, control your blood sugar? “Educate yourself so you can choose the approach that makes sense for you,” Hyer says.

  • Eat well: Intermittent fasting isn’t a substitute for healthy eating. In other words, you can’t just gorge on burgers and fries when your fasting period ends. Aim to eat plenty of fruits and vegetables, whole grains, and lean protein. “Eating during a very small window and not focusing on healthy eating can be nutritionally inadequate,” Hyer says.

  • Drink up: Make sure you drink plenty of water when fasting so you don’t get dehydrated.

  • Plan ahead: Hyer recommends planning and prepping meals ahead of time so you have healthy foods ready to grab when you’re hungry. That way you’ll be less likely to fill up on snacks once your eating window opens.

  • Don’t overdo it: Hyer says she’s seen some intermittent fasting diets that advise people to eat only one meal a day. That’s a plan you should probably skip, she cautions. “It’s hard to get enough vitamins, minerals, and other nutrients if you’re eating just once a day.” Even the more common 5:2 plan isn’t appropriate for everyone. Talk to your doctor before trying one of these more extreme approaches to intermittent fasting.

  • Ease in: If you’re interested but worried about going a long time without eating, start small. Eat during a 12-hour window, then fast for 12 hours. Or start even smaller by “fasting” between meals. “If fasting sounds scary, try to avoid snacking between meals and especially late-night snacking,” Hyer says. “That can be an easy way to give yourself time for your blood sugar to recover between meals.”

As with most eating plans, it’s always a good idea to speak to your doctor before overhauling your diet, Hyer says. That’s especially true if you take medications or have health problems or if you’re considering more extreme fasting plans. But for most people, she says, a fast of 12 to 14 hours is a pretty safe bet. “You still have to make sure you’re picking healthy foods,” Hyer adds. “But intermittent fasting is one tool that can kickstart a healthy lifestyle change.”

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Contributor

Mary Hyer, RD, LDN, CCRP
Cardiac Rehabilitation Dietitian