Find Your Perfect Intermittent Fasting Plan: Schedules, Benefits and Safety

Intermittent fasting means cycling between periods of eating and periods of not eating. There is no single right way to do it: some people eat within an 8-hour window every day, others cut way back on two days a week, and a few try longer fasts. The best plan is the one that fits your life, supports your training and that you can keep up.

This guide covers the most popular intermittent fasting schedules, how to choose one, what the research actually shows, how to train and keep your muscle while fasting, longer fasts such as water fasting and the “monk fast”, side effects, and who should not fast.

The short version

• Popular schedules include 14:10 and 16:8 daily eating windows, the 5:2 diet, a 24-hour fast once or twice a week, and alternate-day fasting.
• Beginners usually do best starting with a 12-hour overnight fast and working up to 14:10 or 16:8.
• In most studies, intermittent fasting leads to weight loss similar to cutting calories every day. It works mainly by helping you eat less.
• Eat enough protein and keep strength training to protect your muscle.
• Fasts longer than a day carry more risks. Talk to your doctor before trying one.
• Do not fast if you are pregnant or breastfeeding, have a history of an eating disorder, are under 18, are underweight, or have type 1 diabetes and take insulin. Check with your doctor first if you have type 2 diabetes or take daily medicines.

Popular intermittent fasting schedules

How to choose the right plan

• Start small. Stop eating after dinner and wait 12 hours before breakfast. If that feels easy after a week or two, move to 14:10, then 16:8.
• Fit it to your day. If you are rarely hungry in the morning, a later eating window may suit you. If you have family dinners, end your window after dinner instead of before it.
• Fit it to your training. Plan hard workouts inside or close to your eating window so you can fuel and recover.
• Consider fasting days instead of windows. If daily windows feel restrictive, the 5:2 approach lets you eat normally most days.
• Judge it by how you feel and perform. If your energy, sleep, mood or training suffer for more than a couple of weeks, adjust the schedule or stop.

What the research shows

Intermittent fasting has been widely studied, and the results are more modest than many headlines suggest.

• Weight loss: in most trials, intermittent fasting produces weight loss similar to cutting calories every day. It mostly works because a shorter eating window or fasting days help people eat less overall. If you eat the same amount, the timing alone makes little difference.
• Blood sugar and insulin: some studies show improvements in insulin sensitivity and blood sugar, especially with eating windows earlier in the day, but results vary.
• Heart health: blood pressure and cholesterol often improve along with weight loss.
• Autophagy: fasting triggers autophagy, the body’s process of breaking down and recycling old cell parts, in animal and cell studies. In people, there is no reliable way to measure it, and claims that a specific number of fasting hours “switches it on” are not established.
• Long-term effects: most studies last months, not years, so the long-term effects of intermittent fasting are still being studied.

Fasting, fat loss and keeping your muscle

Some intermittent fasting studies have found that part of the weight people lose comes from muscle, not just fat. A few habits help you lose fat while keeping the muscle you have:

• Get enough protein. Spread it across your meals, and make sure each meal in your eating window includes a good source of protein.
• Keep strength training at least 2 days a week.
• Time your hardest workouts inside or close to your eating window, and eat a meal with protein and carbohydrates afterward. Light cardio, such as walking, is usually fine while fasted.
• Do not cut too hard. A very short eating window can make it hard to eat enough to fuel training.

For more on fueling workouts, see our guide to whether to have a protein shake before or after a workout.

What to eat and drink

• During a fast: water, plain sparkling water, black coffee and plain tea are fine. Avoid anything with calories, sugar or cream. See our guide to coffee before a workout if you train in the morning.
• Breaking a fast: start with a balanced meal that includes protein, fiber and some healthy fat, rather than a huge or very sugary meal.
• During your eating window: build meals around lean protein, vegetables, fruit, whole grains, beans, nuts and healthy fats. Fasting does not cancel out a poor diet.

Some people combine intermittent fasting with a low-carb diet. If that interests you, see our complete keto guide.

Longer fasts: water fasting, the monk fast and fasting-mimicking diets

Some people try fasts that last longer than a day. They carry more risk than daily eating windows and are not needed to get the benefits of intermittent fasting.

• Water fasting means only water for a day or more. Longer water fasts can cause dizziness, low blood pressure, low blood sugar and electrolyte problems, and in some people they can trigger gout flares. Going back to normal eating too fast after a long fast can also cause refeeding syndrome, a dangerous shift in body fluids and minerals. Water fasts longer than 3 days should only be done with medical supervision.
• The monk fast is a 36-hour, water-only fast, often done once a week, such as from dinner one evening to breakfast two days later. It carries the same risks as other water fasts and is not a good starting point.
• Fasting-mimicking diets are a few days of very low-calorie, mostly plant-based meals designed to mimic some effects of fasting while still allowing food. Most of the research has been small, and they are best done with medical guidance.

If you try a longer fast, talk to your doctor first, stop if you feel faint, confused or unwell, and ease back into eating with smaller, simple meals.

Side effects

Most side effects are strongest in the first few weeks and ease as your body adjusts. They include:

• Hunger and food cravings
• Headaches, especially if you are dehydrated or cutting back on caffeine
• Low energy, dizziness, irritability or trouble concentrating
• Constipation or other changes in digestion
• Trouble sleeping
• Overeating when the eating window opens

Drinking enough water, eating enough protein and fiber, and easing in slowly help. If side effects don’t settle after a few weeks, intermittent fasting may not be right for you.

Who should not fast

Intermittent fasting is not right for everyone. Do not fast if you:

• Are pregnant or breastfeeding
• Have a history of an eating disorder
• Are under 18
• Are underweight
• Have type 1 diabetes and take insulin

Talk to your doctor before trying intermittent fasting if you have type 2 diabetes (especially if you take insulin or other medicines that lower blood sugar, because your doses may need to change to prevent low blood sugar), take medicines that need to be taken with food, have a history of low blood sugar, low blood pressure, gout or kidney stones, are at high risk of bone loss or falls, or have any other chronic health condition.

A note for women: some women find that longer daily fasts affect their energy, sleep or menstrual cycle. The research here is limited, so if you try intermittent fasting, a gentler schedule such as 12:12 or 14:10 is a sensible place to start.

Working with a trainer

Intermittent fasting works best alongside regular training and good food choices. A Type A Training trainer comes to your home, your building gym or a park in Manhattan and builds a program around your goals and your schedule. For help with what and when you eat, see our nutrition services.

Frequently asked questions

Which intermittent fasting schedule is best for beginners?

Start with a 12-hour overnight fast, then move to 14:10 and 16:8 if it feels comfortable. Daily eating windows are usually easier to stick with than full fasting days.

Does intermittent fasting work better than regular dieting?

In most studies, weight loss is similar. Intermittent fasting is one way to eat less, and it suits some people better than counting calories every day.

Can I drink coffee while fasting?

Yes, black coffee and plain tea are fine. Adding sugar, cream or milk adds calories and breaks the fast.

Will I lose muscle with intermittent fasting?

Some studies have found that part of the weight lost is muscle. To help protect it, keep protein high during your eating window and lift weights at least 2 days a week.

Can I work out while fasting?

Light exercise such as walking is usually fine. For hard or long workouts, most people perform and recover better if they train inside or close to their eating window.

Is a water fast or monk fast safe?

Fasts longer than a day carry real risks, including dizziness, low blood pressure and electrolyte problems. Talk to your doctor before trying one, and do not attempt water fasts longer than 3 days without medical supervision.

Is intermittent fasting safe for women?

Many women do it without problems, but some notice changes in energy, sleep or their cycle with longer fasts. Start with a gentle schedule, and do not fast while pregnant or breastfeeding.

Does fasting trigger autophagy?

Fasting triggers autophagy in animal and cell studies. In people, there is no reliable way to measure it, so claims about exactly when it starts or how much fasting you need are not established.