What you’ll learn:
- A 48-hour fast can temporarily reduce weight and lower blood sugar, but these effects are usually short-lived once normal eating resumes.
- Much of the weight lost during a 48-hour fast comes from water and glycogen depletion rather than true fat loss.
- Prolonged fasting places stress on the body and isn’t safe for everyone; it should be approached cautiously and ideally discussed with a healthcare provider.
Fasting has become a popular health strategy. Many people are experimenting with eating less in hopes of improving their weight and overall health. Intermittent fasting and calorie restriction are common. But some people cut out food entirely for days in the hope of detoxing their bodies and potentially even living longer.
Some believe that extending calorie reduction for multiple days may influence insulin sensitivity, calm inflammation, initiate cellular repair processes like autophagy, and even slow aging. That curiosity has led some people to try longer approaches like a 48-hour fast or two full days without food.
People who try a more extreme fast, like a 48-hour fast, are typically looking for accelerated weight loss, improved insulin levels, and reduced inflammation.
While there are some studies that show extended fasting can have health benefits, it can also be dangerous, and it’s not a strategy that’s right for everyone.
If you’re considering fasting for longevity or metabolic health, it’s important to understand how a 48-hour fast actually works—and what it can really do.
We’ll break down what happens in your body during a 48-hour fast, how it compares to other fasting methods, and what the research says about potential benefits and risks. Dr. Karen Mann, MD, Noom Medical Director, will also share her insights into what truly makes a difference for long-term health and whether a 48-hour fast is worth it.
What is a 48-hour fast?
A 48-hour fast means eating nothing for two consecutive days. Most people drink water, black coffee, or unsweetened tea during the fast to remain hydrated.
Why do people try 48-hour fasts?
People try a 48-hour fast for various reasons. Many think it will help with weight loss, help them live longer, or reduce inflammation.
What are the benefits of a 48-hour fast?
Fasting can have some benefits. A study in the journal Obesity found that fasting for more than 12 hours:
- Triggers a metabolic shift: As your body runs out of stored glycogen, it will gradually use stored body fat for energy, a shift called ketosis. Your liver creates “ketones” from fat. These act as a fuel for your brain and body that can also help protect your muscles from being broken down as energy.
- Lowers insulin levels: When you stop eating, your insulin levels will drop. Low insulin is a signal to your body to stop storing fat and start burning it.
- Initiates cellular repair: Your cells start a “deep-cleaning” process that makes them stronger, more resistant to stress, and potentially slower to age, called autophagy.
But another study points out a flaw: most fasting studies are small, involve very few participants, or are conducted in animals rather than humans.
Even in human trials, studies aren’t focused on the long-term benefits and aren’t generalizable to the average person. That makes it difficult to draw firm conclusions about long-term safety, sustainability, or real-world effectiveness.
So while temporary changes in ketosis, glucose, or insulin are measurable in some studies, the evidence is much less clear on whether those shifts lead to lasting changes for your body.
Does a 48-hour fast improve your health?
People try 48-hour fasts for many reasons. Some believe that skipping food for two full days can accelerate weight loss, lower blood sugar, reduce inflammation, calm stress, and even support long-term longevity. On the surface, it sounds logical: eat less, give your body a break, and let it “reset.”
Some health markers do change during a 48-hour fast. The bigger questions are whether those shifts are helpful or harmful, and whether they are temporary or a long-term fix. Here’s what the research says about what does and doesn’t happen.
Claim 1: A 48-hour fast can help you lose weight
Verdict: Temporarily.
The Easy Way
to lose weight and get healthy.
See if you qualify*Initial 3-week subscription and 4 weeks of medication from $99 plus tax and $199 per month plus tax for 12-week subscription thereafter.
Another main reason people try fasting is to lose weight.
Since fasting reduces calorie intake, it can be easy to assume that several days of no calories will result in weight loss. But it’s not just about the number on the scale.
Research suggests that during prolonged fasting, roughly two-thirds of the weight lost is categorized as “lean mass.” That doesn’t mean you’re losing mostly muscle instead of fat. Lean mass includes everything that isn’t fat—including glycogen (stored carbohydrate), the water attached to it, organs, and muscle tissue.
While fat may account for about one-third of the weight lost during the fast, much of the remaining loss reflects glycogen and water depletion, with a smaller portion coming from actual muscle tissue.
“You will likely weigh less after a 48-hour fast. But most of that is large water and glycogen, not fat loss. It’s also temporary unless you work to keep it off,” says Dr. Mann.
So while the weight loss as measured by the scale is real, it’s likely temporary and not purely fat loss.
Claim 2: A 48-hour fast helps with blood sugar and insulin regulation
Verdict: Maybe, but not as a long-term strategy.
A common reason for fasting is to lower blood glucose (blood sugar) and regulate insulin.
But does that really work as a long-term strategy?
It’s true that when you stop eating, glucose levels fall and insulin decreases.
In a study comparing a 2-day fast with a 6-day fast, both approaches lowered blood sugar during the fasting period. The difference showed up after people started eating normally again. Those who completed the 6-day fast appeared to maintain better insulin release and glucose control once food was reintroduced. The 2-day fast didn’t show the same lasting improvement—but it also didn’t appear to worsen glucose tolerance. It just didn’t have the same sustained metabolic effect as the longer fast.
This suggests that even when fasting improves markers like fasting glucose, insulin, and insulin resistance, the effects may not last. Another review of prolonged fasting studies found that improvements in glucose regulation often disappeared within 3 to 4 months after returning to a normal eating pattern.
So while blood sugar does shift during a 48-hour fast, it might not be a long-term metabolic transformation.
Claim 3: A 48-hour fast helps reduce inflammation
Verdict: Temporarily.
Reducing inflammation is often linked to longevity, and there’s a popular belief that giving your digestive system a “break” helps calm inflammation or even “cleanse” the body. But the research in humans is more nuanced than the headlines suggest.
Some studies show that certain inflammatory signals may decrease during short-term fasting. But other research has found that during longer fasts—particularly those lasting 48 hours or more—markers such as CRP, IL-6, and TNF-α can actually rise. In other words, fasting doesn’t consistently push inflammation in one direction.
Overall, there isn’t strong human evidence showing a clear, lasting anti-inflammatory effect from fasting alone. In some cases, the temporary rise in inflammatory markers may reflect a short-term stress response as the body adapts to a lack of energy intake. That doesn’t necessarily mean fasting is harmful—but it does challenge the common assumption that longer fasts are automatically anti-inflammatory.
And that stress response doesn’t only show up in lab values. It can also show up in how you feel—including fatigue, irritability, headaches, or disrupted sleep during extended fasts.
Claim 4: A 48-hour fast is a good mental challenge
Verdict: Stress and anger can worsen with fasting.
You might be attracted to fasting because you think it will make you stronger physically and mentally. It’s not only a challenge to your body not to eat, but also to your mind.
But, just like inflammation, your stress and mood can be negatively affected by fasting. Think about it: fasting puts stress on the body because it senses a lack of incoming energy. Because inflammation and stress signaling are closely connected, it’s not surprising that when inflammation doesn’t consistently decrease, stress may not either.
In a study of healthy women, researchers observed increased stress markers during a 48-hour fast. The researchers measured cortisol, a stress hormone, over the course of the fast. They found that fasting altered the daily rhythm of cortisol so that levels stayed elevated longer. Cortisol stayed higher for longer during the day instead of tapering down normally.
Another study of nine male weightlifters found that fasting increased anger.
These studies were small and studied very specific populations, so it doesn’t automatically mean that fasting is unsafe. But we do know that the body interprets prolonged calorie deprivation as a stress event, and that might be something people want to avoid.
How does a 48-hour fast compare to a 72-hour fast?
Both a 48-hour fast and a 72-hour fast are forms of prolonged fasting—meaning you go multiple days without calories, typically drinking only water and other non-caloric beverages. Both can cause metabolic shifts, but the main difference is how long your body stays in that fasted state.
The science isn’t 100% clear on whether one is better than the other. Many studies are small, short-term, or use animal models. While we might assume that a 72-hour fast may extend certain effects because of the extra day, it’s not guaranteed to produce dramatically different outcomes.
Here’s how they compare:
- Ketosis: A 72-hour fast might keep you in ketosis longer. In theory, that could extend fat loss, but it’s unclear how much additional benefit an extra 24 hours provides beyond 48 hours. And, as we already learned, people lose mostly water, not fat.
- Autophagy: Autophagy is the body’s process of clearing out damaged cells. A study in mice suggests it may begin after 24 to 48 hours of fasting. A 72-hour fast gives more time for autophagy to occur, but human data are limited, so more studies are needed.
- Glucose and insulin response: In one small study, both a 2-day fast and a 6-day fast lowered blood sugar levels and increased ketones while people weren’t eating. The difference showed up after food was reintroduced. After the 2-day fast, the body didn’t handle sugar quite as smoothly when participants started eating again. After the 6-day fast, insulin release improved, and blood sugar control was better maintained. In simple terms, the longer fast seemed to help the body respond more effectively to carbohydrates once eating resumed—at least in the short term.
Is a 72-hour fast better than a 48-hour fast?
Maybe, but it’s not a simple “longer is better” equation. Short fasts may temporarily make it harder for your body to handle sugar when you start eating again, while longer fasts may lead to different metabolic adjustments. Scientists are still studying these effects, and longer fasts aren’t right for everyone. Whether fasting makes sense for you depends on your overall health and should ideally be discussed with a healthcare provider.
Is a 48-hour fast safe?
A 48-hour fast can be safe for some otherwise healthy adults, but it places real stress on the body. Going without food for two full days affects blood sugar, hydration, electrolytes, and energy levels. Some people may tolerate it without major issues, while others may experience dizziness, headaches, irritability, or fatigue.
Always be sure to check in with your doctor, who knows your personal medical history, to see if there are any concerns.
Who shouldn’t try a 48-hour fast?
Some people face higher risks during prolonged fasting and should speak with a healthcare provider before attempting one.
According to the National Institutes of Health, you shouldn’t fast without medical supervision if you:
- Are under age 25
- Are pregnant or breastfeeding
- Take insulin or other diabetes medications
- Take medications that must be taken with food
- Have a seizure disorder
Newer research also raises questions about fasting in people with underlying cardiovascular risk. One recent study found that extended fasting was associated with changes in inflammatory and clotting-related markers that could, in theory, increase cardiovascular risk in some people. While this doesn’t prove that fasting directly causes heart problems, it does suggest that prolonged fasting should be cleared with your doctor first.
For people with heart disease, clotting disorders, metabolic conditions, or those taking medications that affect clotting—including some oral contraceptives—fasting without medical supervision isn’t recommended. Individual risk factors matter, and what may be tolerated by one person could pose concerns for another.
Common side effects of a 48-hour fast
Among people who fasted for at least 48-hours the most frequently reported side effects included:
- Nausea
- Headache
- Insomnia
- Fatigue
- High blood pressure
Even though these symptoms aren’t unusual, it doesn’t mean you need to keep pushing through if you’re not feeling well.
Dr. Mann warns that you should stop a fast immediately and seek medical guidance if you experience:
- Dizziness or fainting
- Confusion or difficulty concentrating
- Severe weakness
- Heart palpitations
- Persistent vomiting
- Signs of dehydration
Your body sends signals to let you know when you need to stop. It’s important to listen to them and seek medical attention when needed.
Frequently asked questions about 48-hour fasts
What does a 48-hour fast do?
During a 48-hour fast, your body uses up its stored carbohydrates (glycogen) and begins increasing ketone production, shifting toward greater reliance on fat for fuel. Blood sugar and insulin levels drop temporarily, and stress-response hormones such as cortisol and adrenaline rise to help maintain energy and alertness. Many people also notice short-term weight loss during this time.
Much of that early weight loss reflects water, glycogen depletion, and some muscle, rather than pure fat loss. Longer-term benefits are less clear and depend heavily on overall health, eating patterns, and what happens once normal eating resumes.
Does lemon water break a 48-hour fast?
Plain water with a small squeeze of fresh lemon (minimal juice) likely contains very few calories and may not meaningfully disrupt fasting metabolism.
If your goal is strict metabolic fasting, you shouldn’t consume anything with calories.
Does black coffee break a 48-hour fast?
Plain black coffee (no cream, milk, sugar, or sweeteners) doesn’t contain enough calories and generally doesn’t break a fast.
Some people use black coffee to help manage hunger during fasting or to ensure they don’t get a headache from caffeine withdrawal.
What can you drink during a 48-hour fast?
During a strict 48-hour fast, most people stick to non-caloric beverages such as still or sparkling water, black coffee, unsweetened tea, and plain water. These options help keep you hydrated without adding any calories.
Does chewing gum break a fast?
Most gum contains small amounts of calories and artificial sweeteners. While the calorie amount is low, sweeteners may stimulate insulin in some people.
If you’re aiming for a strict fast, gum technically breaks it. If you’re fasting more loosely for calorie reduction, a piece of sugar-free gum may not meaningfully affect results.
The bottom line: 48-hour fast weight loss and metabolic changes are temporary
A 48-hour fast can trigger measurable changes in your body, like shifts in ketones, insulin, stress hormones, and even inflammation markers. But most of those changes are temporary, and the long-term health impact is far less clear. While the scale may drop and certain lab values may improve during the fast itself, much of the weight loss reflects water and glycogen depletion, and many metabolic effects fade once normal eating resumes.
Extended fasting is not a guaranteed shortcut to better health, and it isn’t inherently superior to more sustainable approaches like consistent calorie moderation, balanced nutrition, and regular movement.
If your goal is longevity, metabolic health, or weight loss, the bigger picture matters more than a two-day experiment. Long-term improvements tend to come from habits you can maintain—adequate protein, strength training, cardiovascular movement, good sleep, and stress management. For some healthy adults, a short fast may be fine. But for many people, steady, evidence-based lifestyle strategies are more predictable, safer, and ultimately more effective than pushing the body into repeated cycles of extreme restriction.
For those looking for structured support in managing long-term health, Noom’s Proactive Health Microdose GLP-1Rx Program is an evidence-based way to make small changes that matter. The program combines microdose GLP-1 therapy, behavior-change support, and regular biomarker monitoring so individuals can see how their daily choices influence their health over time.
Editorial standards
At Noom, we’re committed to providing health information that’s grounded in reliable science and expert review. Our content is created with the support of qualified professionals and based on well-established research from trusted medical and scientific organizations. Learn more about the experts behind our content on our Health Expert Team page.
