Can’t stomach a daily intermittent fast? An occasional 48-hour fast might be all you need for benefits.
The field of longevity is constantly evolving. The one thing experts can agree on: eating less—through methods like calorie restriction or intermittent fasting—can do more than help you lose weight. Eating less may also boost your health and lifespan. For example, cutting back on your daily nosh may reduce your risk of chronic disease by triggering autophagy—a process that recycles damaged cell parts and is linked to a longer, healthier life (1).
But what if you’re simply not down with eating less daily or doing popular intermittent fasting methods like 18:6? That’s where 48-hour fasting comes in.
Fasting for 48 hours may increase your basal metabolic rate (BMR), or how many calories you burn at rest (2, 3), and boost weight loss and cognitive function (4, 5). Better yet, you really only need to do it once or twice a month to see benefits, says Imashi Fernando, M.S., R.D.
While it might be effective, 48-hour fasting isn’t for the faint of heart. Everything you need to know about 48-hour fasting, including the benefits, side effects, and exactly how to do it for best results, below.
Imashi Fernando, M.S., R.D., C.D.C.E.S. is a registered dietitian who works in a large hospital system as a clinical dietitian.
Melissa Mitri, M.S., R.D., is a registered dietitian who specializes in weight loss, nutrition writer, and owner of Melissa Mitri Nutrition.
A 48-hour fast is simple: give yourself a full, two-day break from eating, then resume your diet as normal after the 48 hours are up.
Fasting can help reduce hypertension, obesity, rheumatoid arthritis, and asthma (6). It also allows your body to take a break from the energy-consuming process of digestion—giving it time to focus its energy on other tasks, like repairing itself.
According to human and animal research, this shift may (7):
There’s no right time of day to start a 48-hour fast, Fernando notes. The most important factor in successfully fasting for 48 hours is finding a schedule that’s sustainable for you. “The simplest method is to stop eating after dinner on the first day and begin eating again at dinnertime on the third day,” says Fernando. “This way, you’ll go to bed full and wake up with eight to ten hours of your fast already complete.”
When your cells are deprived of external food sources for extended periods of time they’re forced to tap into stored forms of energy for fuel. First, your body turns to glycogen (the stored form of glucose). But as glycogen stores dwindle, your body shifts into ketosis, a process that involves breaking down fat into ketones for energy.
Ketosis can boost mitochondrial function—poor mitochondrial function is believed to be associated with heart disease (8) and dementia (9). It’s also been linked to type 2 diabetes (10), metabolic syndrome (11), cancer (12), and insulin resistance (13). Plus, ketosis can reduce inflammation, which plays a role in everything from high blood pressure and arthritis to low testosterone (14, 15).
Experts note more research is needed to confirm exactly how and why ketosis induces these positive changes. Though limited to animal research, some experts believe the effects may be due to increased levels of adenosine monophosphate-activated protein kinase (AMPK) (16), an enzyme that blocks DNA from premature aging (17).
Without food in your system, your body also triggers autophagy; the eating of your cells’ junk parts for energy. This process activates (and suppresses) specific proteins, enzymes, and pathways that appear to play critical roles in aging so that your cells stay healthier for longer (18, 19, 20, 21).
But when exactly do these things happen? Registered dietitian Melissa Mitri, M.S., R.D. explains.
Complete glycogen depletion typically starts after at least 24 hours of fasting (22). But factors like how many carbs you eat, your glycogen stores, age, weight, and BMR can impact how quickly your body burns through its glycogen stores and enters into ketosis (23).
Those with a higher BMR, are of a younger age, are highly active, or have less glycogen stores to begin with may achieve glycogen depletion faster, says Mitri. “Following a very low-carb keto-style diet before a fast, for example, may lead to more rapid glycogen depletion because there are fewer carbs being stored as glycogen to begin with.”
Improved insulin sensitivity has been seen in studies from as little as a 10-hour fast to more prolonged fasts of 36 hours or more (24, 25). However, fasting for longer isn’t necessarily better. “More studies are needed to determine the best fasting method and schedule for improving insulin sensitivity, which may vary depending on the person,” Mitri explains.
Since it takes most people about 24 hours to deplete glycogen levels, ketosis typically kickstarts somewhere between the 24 and 48 hour mark into a fast. However, diet and exercise can help speed things along. “You may also be able to speed up this process by taking a medium-chain-triglyceride (MCT) oil supplement, a type of fat shown to speed the initiation of ketosis (26),” says Mitri.
Animal studies show autophagy begins within 24 to 48 hours of fasting (27), “but more research is needed on the ideal timing in humans,” says Mitri.
The transition into ketosis kickstarts fat loss (28). “Fat loss can occur once glycogen stores are depleted and the body makes the metabolic switch to use fat stores for energy (29),” Mitri explains.
While many good things happen when you fast, when you push beyond the 48 hour mark, less than ideal shifts can begin to occur. “There’s a short window of time before the body starts to enter a starvation state (after about 48 hours), when it then starts to use protein stores in muscle for energy (30),” says Mitri. So, fasting longer than 48 hours is best approached with caution.
“We don’t yet have evidence pointing towards the ideal fasting protocol for benefits. What we do know is that when done too often, 48-hour fasting can become dangerous to your health,” says Fernando.
With 48-hour fasting, you’ll need to find a schedule that gives your body an appropriate amount of recovery in between. “Many people aim to do a 48-hour fast once or twice a month as tolerated, and this appears to be safe and achievable for most people,” Fernando adds.
An obvious downside of 48-hour fasting is hunger. Although the feeling is temporary, that doesn’t make the visions of Taco Bell’s Mexican pizza (or any other strange but completely reasonable cravings) any easier.
On top of hunger, one study found that 72 percent of people who fasted for at least 48 hours also experienced a range of side effects including fatigue, insomnia, and dizziness (31). One small study also suggests you can expect a temporary dip in blood oxygen levels—which might reduce your workout performance (5).
Fasting for a full 48 hours isn’t easy, which is why Fernando recommends starting with shorter fasts. “Try fasting for 16 hours, then 24 hours, and so forth—slowly working your way up to the full 48 hours as tolerated,” she says. This will help you tune into your body before diving in headfirst.
During a fast, drink plenty of fluids to prevent dehydration (32). “Stick to drinks with zero calories like plain tea, black coffee, water, sparkling water, or seltzer (look for PFA-free options) to avoid breaking your fast,” recommends Fernando.
While electrolyte imbalances aren’t typically a concern for fasts lasting 48 hours or less, if you’re fasting for an extended period, you’ll also want to be conscious of your intake of essential electrolytes and minerals.
You’ll also want to load up on essential electrolytes and minerals. “Supplementing with electrolytes during a fast can prevent refeeding syndrome, a phenomenon where your electrolytes become dangerously low when you reintroduce large amounts of food after extended fasting (33),” says Fernando. Look for an electrolyte powder with sodium, potassium, chloride, calcium, and magnesium (34, 35).
Have your nutrient levels checked by your doctor before engaging in an extended fasting or supplement regimen.
After your fast, it might be tempting to indulge in a cheat meal. Resist the urge. The quality and size of a fast-breaking meal can make a difference. Resuming normal eating—especially foods high in fat or sugar—after a period of fasting can lead to faster bowel movements and sometimes diarrhea.
“Break your fast with a small but energy-dense snack that pairs a carb with a protein like nuts and dried fruit, greek yogurt with granola, an apple or banana with nut butter, or an egg on toast,” says Fernando. “Have your next meal one to three hours later, depending on how your gut is feeling.”
48-hour fasts are generally safe for healthy adults, according to Fernando. “If you do have a pre-existing medical condition or are on any medications, talk with your healthcare provider before trying a fast,” she says. “Kids and pregnant women should avoid fasting.”
If you feel unwell during a 48-hour fast, it’s okay (and probably wise) to stop. “In particular, if you’re experiencing any prolonged symptoms like dizziness with standing, blurred vision, or vomiting, stop and talk to your healthcare provider immediately,” says Fernando.
“A 48-hour fast is an extreme diet change that might be difficult for some people to maintain,” says Fernando. “If you just can’t swing a full 48-hour fast, consider shorter fasting periods, which offer a similar range of benefits and might be easier to stick to in the long run.”
It might not be. “While autophagy and fat-burning may not fully kick in at the 24-hour fasting mark, it may not be worth the potential risks of pushing it to 48 hours,” warns Mitri.
Once you get past 24 hours, there is a short window before the risk of nutrient depletion, muscle loss, and fatigue sets in (30). “Getting enough nutrients is essential to health, especially for those in perimenopause or menopause, to preserve bone health and muscle mass,” Mitri adds.
There is also limited research to show that prolonged 48-hour fasts provide additional benefits beyond more common (and shorter duration) fasting approaches like 16:8 and 24-hour fasting (36). Most people will get the most benefits from a more sustainable approach, says Mitri.
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