Key takeaways

Intermittent fasting, which alternates between periods of eating and fasting, is considered safe for most people and offers benefits like weight loss and improved heart health, but it’s not suitable for everyone, including children, seniors, and those with certain medical conditions.
Extended fasts over 24 hours are not recommended due to increased risks and potential for negative side effects like reduced energy and dehydration, with professionals advising against fasting for more than two months without further research.
Intermittent fasting doesn’t restrict what foods you can eat during eating windows, but healthy, nutrient-dense meals are advised to avoid malnutrition and ensure complete nutrition.
Specific groups, including children, adolescents, seniors, those with kidney disorders, a history of eating disorders, diabetes, pregnant or breastfeeding women, should avoid intermittent fasting due to higher risks of adverse effects.

Intermittent fasting is all the rage these days, and it makes sense. It’s an eating plan that doesn’t restrict what you eat, just when you eat. Many people find it freeing to be on a diet where they are still able to eat their favorite foods and don’t have to constantly count calories. Intermittent fasting involves switching between periods of eating and not eating on a regular schedule. There are two different ways to practice intermittent fasting: alternate-day fasting and time-restricted feeding. During the periods that you eat, you can eat however much you want, though you should ensure that you are getting complete nutrition by eating a balanced, healthy diet and prioritizing proper hydration.

However, many wonder: Is intermittent fasting safe? After all, there may be long periods of time when you aren’t taking in calories, and it’s understandable to be uncertain if that’s dangerous—especially if you have underlying health conditions or other risk factors. Here, learn how to decide whether intermittent fasting is a good choice for you.

Is intermittent fasting safe?

The consensus among health professionals is that intermittent fasting is safe for most people. It’s proven that intermittent fasting works, at least in the short term. Numerous studies have shown that intermittent fasting is effective at helping people lose weight. The health benefits of intermittent fasting include reducing cholesterol, preventing obesity, lowering blood pressure, and even improving memory. The mechanisms by which intermittent fasting affect health are not completely understood, but may include improved insulin sensitivity and anti-inflammatory effects.

However, there are certain considerations—fasting duration, potential side effects, and what to eat when you’re not fasting. Like any new eating plan, you should check with your healthcare provider before getting started.

Duration

The most popular fasting plans are the daily approach (where you eat during an eight-hour window and fast for 16) and the 5:2 method, which involves eating normally for five days of the week, and fasting for the remaining two. Though some plans suggest fasting for longer periods such as 48 or 72 hours, most experts do not recommend it. Because of the risks of side effects of intermittent fasting like reduced energy and fluid intake, Kristin Gillespie, MS, RD, an advisor for exercisewithstyle.com, doesn’t recommend fasts lasting more than 24 hours. The risks of extended fasts (more than 24 hours) increase while the benefits do not. Extended fasts can actually put your body into starvation mode, which causes the body to store more fat instead of burning it.

Additionally, most of the research about intermittent fasting looks at short-term safety and benefits and doesn’t take into account what can happen if you practice fasting for months or years. “Until more research is available, I would recommend intermittent fasting for short-term periods of up to two months,” Gillespie says.

Side effects

When you begin intermittent fasting, it may result in some unwanted side effects. “Some people experience headaches, irritability, low energy, and low blood sugar,” says Melissa Mitri, MS, a registered dietician for Wellness Verge. Some are also emotionally daunted by the prospect of not eating.

Nutrition

In many ways, intermittent fasting is easier than fad diets, like keto, that restrict foods or programs like Weight Watchers that rely on tracking calorie intake and calorie restriction. With IF, there are no specific rules about what to eat or to avoid. However, you shouldn’t just “eat whatever you want” during the window of time that you are consuming food, explains Juliana Tamayo, MS, a licensed nutritionist and editor at fitnessclone.com. Rather, you should make sure that you eat healthy foods, like whole grains, lean proteins, and lots of produce.

“The important part of intermittent fasting is to have healthy, nutrient-dense meals during eating hours,” Tamayo advises. “Otherwise, malnutrition might occur, whether it is at the macro or micronutrient level.” In other words, just because you skip breakfast doesn’t mean you should have french fries for lunch.

Pros and cons of intermittent fasting

When you’re starting a new eating pattern, educate yourself about the benefits and risks, so that you are well informed.. Here are some of the pros and cons to consider when it comes to intermittent fasting.

Pros

Weight loss
Increased fat burning
Decreased cholesterol
Improved heart health
Lower blood pressure
Reduced insulin resistance
Improved physical performance
Better tissue health
Improved working memory

Cons

Can make social eating awkward or uncomfortable if an event is during your fasting window
May make you more moody and irritable
Can lead to inadequate nutrition and dehydration if you are not thoughtful of your food choices
May trigger or exacerbate eating disorders
Can lead to low energy and fatigue
May be difficult to stick to long-term

Who should not do intermittent fasting?

For most people, intermittent fasting is a safe choice. Certain groups should avoid it altogether, including:

Children and adolescents under age 18
Seniors and anyone who is underweight (with a BMI less than 20)
People with kidney disorders
Anyone with a history of eating disorders
People with diabetes or blood glucose problems
Pregnant or breastfeeding women

People who don’t fall into any of these groups can generally safely try intermittent fasting.

Children and adolescents

Children and teens should not practice intermittent fasting. “While it is safe for most healthy adults, intermittent fasting is not good for children because their dietary needs are so different,” Tamayo says.

Seniors and anyone who is underweight

Intermittent fasting may also be risky for elderly people, people who are underweight, as well as anyone who is taking daily medication. You may also need to alter your intermittent fasting routine if you become ill, need a medical procedure performed, or need lab work done, Tamayo advises.

People with kidney disorders

There are also concerns for people who live with kidney disease. “Longer fasts can potentially increase the risk of kidney damage in those that already have kidney disease,” says Mitri. “This is most likely because prolonged fasting can lead to volume depletion, which can affect kidney function.” If you live with heart disease and take medication for it, you should consult with your doctor before trying intermittent fasting, or any type of diet.

Anyone with a history of eating disorders

Intermittent fasting is generally not recommended for people who have experienced eating disorders—or those at risk for developing one. “Fasting is not appropriate for someone with a history of an eating disorder, as it can trigger unhealthy thoughts and disordered eating behaviors,” Mitri says. Intermittent fasting also has the potential to make it more difficult to recognize hunger cues and may cause people to binge once they are in their eating window.

People with diabetes

At first glance, intermittent fasting may seem beneficial for people with Type 1 and Type 2 diabetes. After all, it leads to weight loss and stabilizes insulin levels. Unfortunately, there is not enough research available to definitively prove its safety.

However, fasting can cause your blood sugars to crash, says Mitri. Not eating for extended periods of time can lead to hypoglycemia (blood sugar less than 60). Additionally, if you lose weight or your insulin sensitivity changes, your diabetes medication may need to be decreased. Be aware that the potential benefits may not outweigh the risks.

Pregnancy and breastfeeding

While there is some evidence that fasting can reduce your chances of gestational diabetes and unwanted weight gain during pregnancy, your caloric needs during pregnancy are increased. Fasting might be potentially harmful to you and your baby. “During pregnancy, you are more at risk of low blood sugar, and fasting can exacerbate this,” Mitri explains. “Fasting also makes it more difficult to meet your and baby’s nutrition needs during pregnancy, and can increase the risk of pregnancy complications.”

Breastfeeding mothers need additional calories while nursing, and intermittent fasting may decrease milk supply and deplete a breastfeeding mom’s energy levels, especially if she does not remain well hydrated. That being said, some nursing mothers practice intermittent fasting successfully by keeping their fasting periods short, drinking adequately, and eating nutrient-dense foods, explains Gillespie. “It is also a good idea for these individuals to work with a registered dietitian to ensure that mom and baby are getting all of the nutrients they need,” Gillespie recommends.

How to fast safely

If you are intermittent fasting, keep these points in mind to make the experience as healthy and positive as possible:

Keep fasting windows short. Fasting for one or two days of the week may be more difficult than a plan where you simply eat between the hours of 9 a.m. to 5 p.m.
Eat a small amount on fasting days. Some plans allow for a small number of calories, and in some interpretations a fasting diet can include some very low calorie food or drinks.
Stay hydrated even while you are refraining from consuming food. Drink water or black coffee.
Make sure to eat foods that provide you with balanced nutrition during your “feasting” windows. In other words, don’t go from fasting to eating only simple carbohydrates like pasta and sweets. Prioritize protein, fruit, vegetables, and other whole foods.
Note how you are feeling as you are fasting. If you start to experience mood changes, or anxiety, consider stopping intermittent fasting.
Stop fasting if you feel ill. Seek medical help if you feel so weak or tired that you’re unable to complete daily tasks.
Complement fasting with healthy lifestyle changes. Successfully, managing body weight requires more than intermittent fasting and healthy eating. Complement your new diet with smoking cessation, limiting alcohol consumption, exercise, and positive mental health practices.

As with everything health-related, you should consult your healthcare provider for professional medical advice before beginning a new diet or eating plan.