Is Intermittent Fasting Safe? Benefits, Risks and Who Should Avoid It

Intermittent fasting has gone from a niche idea to one of the most popular ways to try to lose weight. Instead of changing what you eat, it changes when you eat, by setting windows for eating and fasting across the day or week.

For many healthy adults it is a reasonable option, but it is not right for everyone, and some of the bigger claims run ahead of the evidence. This piece can help you decide whether it is worth trying and whether you should check with your doctor first; it cannot assess your personal health risks.

What is intermittent fasting?

Intermittent fasting is an eating pattern that cycles between periods of eating and periods of eating little or nothing. It sets rules about timing rather than specific foods.

The three most common approaches are:

• Time-restricted eating: eating only within a set window each day, such as 8 hours (often called 16:8, for 16 hours of fasting and 8 hours of eating)
• The 5:2 diet: eating normally five days a week and eating about 500 to 600 calories on two non-consecutive days
• Alternate-day fasting: alternating normal eating days with very low calorie or no-calorie days

Fasting is only one of many ways to approach eating, and it works best when it sits on top of the basics covered in the guide to nutrition and wellness. What you eat during your eating window still matters.

How do the main fasting methods compare?

The methods differ mainly in how long the fasts last and how hard they are to keep up. Shorter daily windows are usually the easiest place to start.

Method	How it works	Difficulty	Main drawback
12:12	Eat within a 12-hour window, such as 7 a.m. to 7 p.m.	Low	Modest effect if your usual pattern is already similar
16:8	Eat within an 8-hour window, such as noon to 8 p.m.	Moderate	Hunger in the morning; easy to overeat in the window
5:2	Two days a week at about 500 to 600 calories	Moderate to high	Low-calorie days can bring headaches, irritability, and fatigue
Alternate-day fasting	Very low calorie days every other day	High	Hard to sustain; higher dropout in studies

If you are drawn to the 5:2 approach, planning low-calorie days ahead makes them easier, and these 500-calorie day meal ideas show what that can look like.

Does intermittent fasting help you lose weight?

Intermittent fasting can help with weight loss, but in most studies it works about as well as ordinary daily calorie reduction, not better. The main reason it works is that a shorter eating window or low-calorie days tend to reduce total calories.

The NIH's National Institute on Aging notes that much of the early excitement came from animal studies, and that human trials so far show results similar to other calorie-cutting approaches. Some people find fasting easier to stick to than counting every meal; others find it harder.

Because the underlying math is the same, it helps to understand how calorie targets are calculated before you choose a method. If you overeat during your eating window, the fasting hours will not make up for it.

What are the other possible benefits?

Some studies suggest intermittent fasting may improve insulin sensitivity, blood pressure, and cholesterol in some people, but the evidence is still mixed and often tied to the weight lost. It is not yet clear whether fasting has benefits beyond those of losing the same amount of weight another way.

Possible benefits researchers are studying include:

• Better blood sugar control in some people with prediabetes
• Modest improvements in blood pressure and blood fats, mostly alongside weight loss
• A simpler routine for people who dislike tracking food
• Less late-night eating when the window ends earlier in the evening

Claims that fasting "detoxes" the body, reverses aging, or prevents disease go well beyond what human research has shown. Treat them with caution.

What are the side effects of intermittent fasting?

The most common side effects are hunger, headaches, tiredness, irritability, and trouble concentrating, especially in the first couple of weeks. Most ease as your body adjusts, but they are a sign to go slower if they persist.

Side effect	Why it happens	What may help
Hunger	Your body is used to regular meals	Start with a 12-hour fast and lengthen slowly
Headaches	Low fluid intake, lower blood sugar, or less caffeine than usual	Drink water; keep caffeine timing consistent
Fatigue and low focus	Fewer calories and changing routines	Avoid hard workouts during fasts at first
Irritability	Hunger affects mood	Schedule fasts on calmer days
Constipation	Less food and fiber overall	Include vegetables, beans, and whole grains in your window
Poor sleep	Hunger at bedtime	End your eating window a few hours before bed rather than in the afternoon

Fewer meals also means fewer chances to get enough protein, which matters for keeping muscle while you lose weight. Knowing how much protein you actually need makes it easier to plan meals inside a short window.

Who should avoid intermittent fasting?

Some groups should avoid intermittent fasting or try it only with guidance from their doctor, because skipping meals can be risky for them. The main groups are listed below.

• People who are pregnant or breastfeeding: energy and nutrient needs are higher, and fasting is not recommended
• Children and teens: they need regular meals for growth and development
• People with a current or past eating disorder: strict eating rules can trigger restriction, bingeing, or relapse
• People taking insulin or sulfonylureas: these diabetes medicines can cause dangerously low blood sugar when meals are skipped
• People who are underweight or frail, and many older adults: fasting can speed loss of muscle and strength
• People with certain medical conditions: including a history of low blood sugar, kidney disease, or conditions that require medicine taken with food

Why is diabetes a special concern?

If you take insulin or other medicines that lower blood sugar, fasting can cause hypoglycemia, meaning blood sugar that drops too low. MedlinePlus lists shakiness, sweating, a fast heartbeat, confusion, and in severe cases seizures or passing out as warning signs. Never start fasting with these medicines without a plan from your doctor, who may need to adjust your doses.

What about eating disorders?

Rigid food rules can feed an unhealthy relationship with eating. If you notice you are fasting longer and longer, feeling guilty when you eat, or bingeing after fasts, stop and talk to your doctor. The National Eating Disorders Association (NEDA) offers a free online screening tool and support resources.

How can you try intermittent fasting more safely?

If you are a healthy adult and want to try fasting, the safest approach is to start gently, stay hydrated, and focus on nutritious food during your eating window. Here are practical steps:

1. Check with your doctor first if you have any medical condition or take regular medicine.
2. Start with a 12-hour overnight fast before trying 14 or 16 hours.
3. Drink water, plain tea, or black coffee during fasting hours.
4. Build meals around vegetables, fruits, whole grains, lean protein, beans, and healthy fats, in line with MyPlate.
5. Do not compensate with large, ultra-processed meals when the window opens.
6. Stop if you feel faint, unwell, or preoccupied with food.

Black coffee can make fasting hours easier, but it is easy to drink more than you realize when you are hungry. The FDA cites 400 milligrams a day as an amount not generally linked with negative effects in healthy adults, and the signs of too much caffeine are worth knowing.

A registered dietitian (RD or RDN) can help you design an eating pattern, with or without fasting, that fits your health, schedule, and goals.

Can you exercise while intermittent fasting?

Yes, most healthy adults can keep exercising while intermittent fasting, but it helps to time harder workouts carefully. Light activity such as walking usually feels fine during a fast, while intense or long sessions may feel harder without food beforehand.

The Physical Activity Guidelines for Americans recommend 150 to 300 minutes of moderate activity a week plus muscle-strengthening activity on at least two days, and that advice does not change when you fast. Regular strength training is especially useful while losing weight, because it helps you hold on to muscle.

Some practical tips:

• Schedule hard workouts during or just before your eating window, so you can refuel afterward
• Drink water before, during, and after exercise, especially in hot weather
• Stop and eat something if you feel dizzy, shaky, or unusually weak
• Pay attention to how you recover; persistent soreness or poor performance can mean you are not eating enough overall

If you take diabetes medicine, exercise during a fast raises the risk of low blood sugar further, so ask your doctor how to plan it.

When should you see a doctor or call 911?

Talk to your doctor before starting intermittent fasting if you have diabetes, heart disease, kidney disease, a history of eating disorders, or take any regular medicine. See your doctor if you develop dizziness, ongoing fatigue, missed periods, or rapid weight loss while fasting.

Call 911 or go to the emergency room if you or someone else has:

• Confusion, slurred speech, or unusual behavior, especially in someone who takes diabetes medicine
• Fainting or a seizure
• Chest pain or a racing, irregular heartbeat
• Severe weakness or inability to keep fluids down

If fasting has become tied up with intense distress about food or your body, or you are having thoughts of harming yourself, call or text 988 to reach the 988 Suicide and Crisis Lifeline.