Intermittent Fasting Benefits and Risks
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, but the research on intermittent fasting is promising,” says Mary Hyer, RD, LDN, CCRP. Hyer is a cardiac rehabilitation dietitian with the Mass General Brigham Heart and Vascular Institute and works with patients at the Elfers Cardiovascular Center at Newton-Wellesley Hospital.
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.”
How does intermittent fasting work?
Intermittent fasting isn’t just about eating less, it’s about how your body responds when it’s in a fasting state. Your body usually burns glucose (sugar) for energy, but after about 10 to 12 hours without food your glucose stores run low.
This is the key part of intermittent fasting—the metabolic switch. Your body switches from using glucose for energy to using fatty acids. Using fatty acids for energy triggers a change in your metabolism, making your body burn fat more easily.
Incorporating intermittent fasting into your routine may help you hit the metabolic switch more often, encouraging your body to use its fat stores for energy.
Intermittent fasting can help induce autophagy, the body’s built-in cell cleanup process. Research—mainly from animal and cellular studies—suggests this housekeeping of damaged or injured cells may help reduce inflammation and prevent damage to healthy cells. Reduced inflammation in the body can sometimes improve energy and comfort levels, making it easier to be active and indirectly support weight loss.
In a review of multiple studies about intermittent fasting, it has been found to reduce insulin resistance and fasting insulin levels especially when weight loss happens. Insulin resistance can lead to a variety of conditions including type 2 diabetes, polycystic ovary syndrome (PCOS), and fatty liver. While intermittent fasting isn’t a prevention or cure for any condition, talk to your primary care provider about whether it may be beneficial for you.
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
- Lower cholesterol
- Lower blood pressure
- Improved blood sugar
- Reduced insulin resistance or improved insulin sensitivity
- Less inflammation
- Better brain health
- Cellular repair
- Fewer age-related diseases
- Longer life
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.
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)
- Have diabetes
- Have heart, kidney, or liver disease
- Have a history of an eating disorder or disordered eating
- 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
Intermittent fasting can also be a challenge to stick to or to do correctly. As noted, fasting isn’t an excuse to eat whatever you want in your non-fasting time. People may struggle with choosing nutritional foods or with hunger when they break their fast. Listen to your body and consult your health care provider to ensure intermittent fasting is safe and appropriate for you.
Learn about Mass General Brigham Heart and Vascular services
Common intermittent fasting schedules
You may want to experiment with different intermittent fasting schedules to find what works best for you, your body, and your lifestyle. Each method has a pattern of eating and fasting for specific timeframes, allowing you to customize your approach.
Time-restricted eating or the 16:8 method
A common approach to intermittent fasting is the 16:8 method, a form of time-restricted eating. Time-restricted eating (TRE) is when your daily calorie intake is limited to a consistent timeframe, usually 8 to 10 hours. In the 16:8 method specifically, you fast for 16 hours and eat within the 8-hour timeframe.
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.
Under the 16:8 method, you might decide to skip breakfast and start your day with lunch, leaving time to eat a later dinner. However, recent studies suggest that eating earlier in the day may be more beneficial — breaking your fast with breakfast and ending with an earlier dinner.
A time-restricted eating pattern can be tailored to your schedule often ranging from 12 to 14 hours of fasting, or even just limiting nighttime snacking after a specific time.
The 5:2 diet
Where the 16:8 method refers to hours in the day, the 5:2 method of intermittent fasting refers to days of the week.
In this plan, you eat normally for 5 days a week. On the other 2 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.)
This method offers more flexibility, so it may be easier to stick to. Scheduling your fasting days around life events like weddings, family gatherings, or birthday parties may make it easier to adhere to your intermittent fasting goals while not missing out on the joy of shared food and celebration.
But this more extreme approach is definitely not for everyone. It's generally recommended only for healthy individuals who aren't living with chronic diseases. Always talk to a health care provider before trying any low-calorie meal plans.
Alternate day fasting (ADF)
If keeping track of which 2 days per week you’re fasting on the 5:2 method seems challenging for your schedule, alternate day fasting might be easier for you. The concept is simple: fast every other day.
If a complete fast isn’t right for you, you could try alternate day fasting modified. In this version, on fast days you eat only 25% of your recommended daily calories; on regular eating days you eat 125% of your recommended daily calories. Overall, you’ll be eating less. Pairing this with healthy food choices could help you reach your health goals.
Tips for intermittent fasting
FAQs about intermittent fasting
What should I eat when I’m not fasting?
When you’re not fasting, follow a healthy diet filled with lean protein, whole grains, vegetables, fruits, and healthy fats. Consider adopting the Mediterranean diet or a plant-based diet, and update your grocery list to ensure you have plenty of nutritious foods on hand. Avoid fast foods, salty or sugary snacks, and fried or greasy foods.
What can I drink while fasting?
Any clear, zero-calorie liquids are good options to stay hydrated while fasting—water, black coffee, black or herbal tea, or sparkling water. Avoid sugary drinks like soda or fruit juice, drinks like smoothies, and any additives to your tea or coffee like creamer or sugar or artificial sweeteners.
Will I feel hungry all the time?
When you first start an intermittent fasting schedule, yes, you’re going to be hungry. Your body is used to eating at regular intervals and changing that schedule can impact your hunger cues. Some people report feeling less hungry over time as the body adapts, though this can vary from person to person.
How long does it take to see results from intermittent fasting?
Intermittent fasting isn’t intended to be a timed challenge, but a different way of approaching your diet overall. It can help with weight loss, decreased cholesterol, and reduced blood pressure, among other health benefits, but it isn’t a quick fix. Combined with healthy food choices, an active lifestyle, and regular doctor visits, intermittent fasting can help you improve your overall health over time.
Who should not attempt intermittent fasting?
Like any diet or exercise program, you should discuss your interest in intermittent fasting with your health care provider before starting. Anyone under 18, those who are pregnant or breastfeeding, and individuals who have had disordered eating should avoid intermittent fasting. If you have certain medical conditions or take certain medications, intermittent fasting may impact them, so it’s important to discuss it with your doctor.
Which intermittent fasting schedule should I follow?
The best schedule is the one you can stick to try out the different schedules and see which works best for your lifestyle.