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 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 and provides one-on-one nutrition counseling through her virtual private practice, Brown Sugar Nutrition PLLC. She holds a master’s degree in Nutritional Sciences from the University of Washington.
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.
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. 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 believed to be associated with heart disease (6) and dementia (7). It’s also been linked to type 2 diabetes (8), metabolic syndrome (9), cancer (10), and insulin resistance (11). Plus, ketosis can reduce inflammation, which plays a role in everything from high blood pressure and arthritis to low testosterone (12, 13).
Experts note more research is needed to confirm exactly how or why ketosis induces these positive changes. Though limited to animal research, some experts believe the effects might be due to increased levels of adenosine monophosphate-activated protein kinase (AMPK) (14), an enzyme that blocks DNA from premature aging (15).
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 (16, 17, 18, 19).
Fasting can help reduce hypertension, obesity, rheumatoid arthritis, and asthma (20). 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 (21):
“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.
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 (22). 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 (23). “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.
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 (24),” says Fernando. Look for an electrolyte powder with sodium, potassium, chloride, calcium, and magnesium (25, 26).
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.”
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