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 can 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 can increase how many calories you burn at rest (2, 3), and boost weight loss and cognitive function (4). 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.
What Is a 48-Hour Fast?
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.
How to Do a 48-Hour Fast
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.”
What Happens to Your Body During a 48-Hour Fast?
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. At 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 linked to heart disease (5), dementia (6), type 2 diabetes (7), metabolic syndrome (8), cancer (9), and insulin resistance (10). Plus, ketosis can reduce inflammation, which plays a role in everything from high blood pressure and arthritis to low testosterone and erectile dysfunction (10, 11).
Experts note more research is needed to confirm exactly how or why ketosis induces these positive changes. However, thanks to emerging research, some experts believe the effects might be due to increased levels of adenosine (12), a chemical that blocks DNA from premature aging.
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 genes that appear to play a critical role in aging so that your cells stay healthier for longer (13, 14, 15, 16).
Benefits of a 48-Hour Fast
Fasting can reduce hypertension, obesity, rheumatoid arthritis, and asthma (17). 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.
This shift can (18):
- Boost weight loss
- Enhance brain function
- Reduce inflammation
- Reduce glucose levels
- Improve digestion
- Reduce blood pressure
- Reduce blood sugar levels
- Improve insulin sensitivity
- Increase human growth hormone
- Slow the aging process
- Boost immunity
- Trigger autophagy
How Often Should You Do a 48-Hour Fast for Benefits?
“We don’t yet have evidence pointing towards the ideal fasting protocol for benefits. What we do know is 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, this appears to be safe and achievable for most people,” Fernando adds.
Side Effects of 48-Hour Fasting
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 (19). One small study also suggests you can expect a temporary dip in blood oxygen levels—which might reduce your workout performance (4).
How to Minimize Side Effects
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 (20). “Stick to drinks with zero calories like plain teas, black coffee, water, sparkling water, or seltzers (look for PFA-free options) to avoid breaking your fast,” recommends Fernando.
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,” says Fernando. Look for an electrolyte powder with sodium, potassium, chloride, calcium, and magnesium (21).
How to Safely Break a 48-Hour Fast
After your fast, it might be tempting to indulge in a cheat meal. Resist the urge. Eating too much following a fast can overstimulate your gut and cause bloating, nausea, and diarrhea (22). “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.”
Is a 48-Hour Fast Safe?
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 for 48 hours.”
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.”