48-Hour Fast: How to Do It, What Happens and Who Should Avoid It

A 48-hour fast means going two full days without food. The usual way is to finish dinner on day 1 and eat again at dinner on day 3. During the fast you drink water, and usually black coffee and plain tea; a 48-hour water fast is the strict version, with water only.

It is a big step up from daily fasting like 16:8. Most healthy adults can do it safely if they build up to it, but it is hard, the weight you see come off is mostly water, and some people should not try it. Here is what happens, how to prepare, and how to end it well.

48-Hour Fast Schedule

To see the timeline for your own start time, including when you are likely to reach ketosis, use our extended fasting calculator.

What Happens in Your Body During a 48-Hour Fast

• 0 to about 12 hours: your body uses the energy from your last meal and the sugar stored in your liver (glycogen).
• About 12 to 36 hours: as liver glycogen runs low, your body burns more fat and starts making ketones. Researchers call this the “metabolic switch”. When it happens depends on your last meal and how active you are. (1)
• 24 to 48 hours: insulin is low, ketone levels keep rising, and most of your energy comes from fat. Hunger usually comes in waves rather than building steadily.

Your metabolism doesn’t slow down in this time frame. In one study, resting metabolic rate was slightly higher (by 3.6%) after 48 hours without food. (2)

You will often read that a 48-hour fast “switches on autophagy”, the process cells use to recycle worn-out parts. That is well shown in animals, but there is no reliable way yet to measure it in people, so treat any specific hour as a guess. See our autophagy calculator for what the research does and doesn’t show.

48-Hour Fast vs 48-Hour Water Fast

In body terms the two are almost the same. Choose water only if that is the fast you set out to do, and cut down on caffeine a few days before so withdrawal headaches don’t hit during the fast. Not sure if a drink counts? Check our what breaks a fast tool. For longer water fasts, see our water fasting guide.

How to Prepare for a 48-Hour Fast

1. Build up to it. Get comfortable with daily fasting (12:12, then 16:8), then a 24-hour fast and a 36-hour fast, before you try 48 hours.
2. Check with your doctor if you have any medical condition or take regular medication (see who should not fast below).
3. Eat well the days before. Normal, balanced meals with protein, vegetables and whole foods. A huge “last meal” doesn’t help.
4. Pick quiet days. Avoid days with hard training, long drives, big social meals or demanding work.
5. Plan your fluids and electrolytes. Drink regularly through the day. Our fasting electrolyte calculator gives daily sodium, potassium and magnesium reference amounts.

Does a 48-Hour Fast Help You Lose Weight?

Two days without food creates a calorie gap of about twice your daily calorie burn, often 4,000 to 5,000 calories. That is roughly 1 to 1.5 pounds of body fat.

The scale will usually show more, often 2 to 4 pounds, because your body also loses stored glycogen and the water that goes with it. Most of that comes back within a few days of eating normally. That is normal and doesn’t mean the fast failed.

A 48-hour fast now and then can be part of a weight loss plan, but only if you don’t make up for it by eating more on the other days. For steady weight loss, a daily routine such as 16:8, or alternate day fasting, is easier for most people to keep up.

Side Effects of a 48-Hour Fast

Common, and usually mild:

• Hunger, often worst on the evening of day 1 and the morning of day 2
• Tiredness, low energy and poor focus
• Headaches, often from dehydration or caffeine withdrawal
• Dizziness when you stand up quickly
• Poor sleep and irritability
• Muscle cramps, a sign you may need more electrolytes

Stop the fast and eat if you feel faint, confused, have a racing or irregular heartbeat, severe weakness, or vomiting. Seek medical help if those symptoms don’t settle after eating.

How to Make It Easier

• Sip water steadily rather than drinking large amounts at once.
• Add electrolytes, especially sodium, if you get headaches, cramps or dizziness.
• Black coffee, green tea or sparkling water can blunt hunger (skip them on a water-only fast).
• Keep busy, and keep exercise light: walking and stretching are fine; skip hard workouts.

How to Break a 48-Hour Fast

1. Start small. A light, easy meal: soup or broth, eggs, Greek yogurt, a small portion of fish or chicken with cooked vegetables.
2. Go slowly. Eat over 20 to 30 minutes and stop before you are full. A big, greasy or very sugary meal right away can cause bloating, cramps and diarrhea.
3. Have your next meal a couple of hours later, then go back to normal meals the next day.
4. Don’t “make up” the calories. Return to your normal portions rather than eating extra.

Who Should Not Do a 48-Hour Fast

Do not try a 48-hour fast without your doctor’s supervision if you:

• Have type 1 diabetes, or type 2 diabetes treated with insulin or other glucose-lowering drugs
• Take blood pressure medication, blood thinners or other regular medication
• Are pregnant, breastfeeding or trying to conceive
• Are under 18, underweight, or have a history of an eating disorder
• Have kidney, liver or heart disease, gout, or often have low blood pressure

Frequently Asked Questions

What can I drink during a 48-hour fast?

Water, sparkling water, black coffee and plain tea. On a 48-hour water fast, water only, with electrolytes if you need them. Anything with calories, such as milk, juice or bone broth, breaks the fast.

Can I do a 48-hour fast to lose weight?

It can help as an occasional tool: it removes about two days of calories, roughly 1 to 1.5 pounds of fat. Most of the bigger drop on the scale is water and comes back. Daily fasting is easier to sustain.

What happens to glycogen during a 48-hour fast?

Your liver’s glycogen stores run low within roughly the first day, which is when your body shifts to burning mostly fat and making ketones. You refill them within a day or two of eating normally.

How often can I do a 48-hour fast?

There is no research-backed schedule. Many people who use it do it once a month or less. Doing it every week is hard to sustain and leaves less room to eat enough protein and nutrients.

Can I exercise during a 48-hour fast?

Light activity such as walking or gentle stretching is fine. Skip hard or long workouts until you have eaten again.

Are there any side effects of a 48-hour fast?

Hunger, tiredness, headaches, dizziness, poor sleep and cramps are common and usually mild. Stop and eat if you feel faint or unwell.

The Bottom Line

A 48-hour fast is two days without food, usually from dinner to dinner. Healthy adults who build up to it can do it safely, but it is hard, the scale drop is mostly water, and it isn’t more effective for long-term weight loss than daily fasting you can stick to. Prepare, drink plenty, use electrolytes, break the fast gently, and check with your doctor first if you have a medical condition or take medication.

References

Fitness Volt is committed to providing our readers with science-based information. We use only credible and peer-reviewed sources to support the information we share in our articles.

1. Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity. 2018;26(2):254-268.
2. Mansell PI, Fellows IW, Macdonald IA. Enhanced thermogenic response to epinephrine after 48-h starvation in humans. Am J Physiol. 1990;258(1 Pt 2):R87-R93.