Everything You Need to Know About Intermittent Fasting
Can you really consume anything you want and still lose weight? Almost. When it comes to intermittent fasting—dieting’s hottest trend since giving up sliced bread—it’s all in the timing.
Intermittent Fasting (aka IF) is having a moment right now as one of the most-buzzed-about ways to fit into your skinny jeans again. This seemingly super-easy diet is based on the premise that if you stop eating long enough, your body begins burning fat instead of carbs—and it has you watching the clock instead of counting points, calories or grams of fat. Because you’re eating less often, the diet requires less prep, less mess and less stress. Plus, if you can ignore your growling stomach for a week or two as your body adjusts, research suggests it may have surprising health benefits.
How to Pull it Off
There are several ways to go about IF, but some are easier to sustain than others—and not all approaches have study results to back them up. From hard-core to basically a breeze, here are three ways to do it.
Option 1: Whole-Day Fasting
Fast for 24 hours once or twice each week. For example, you might finish your last meal on Monday at 6 p.m. and not eat again until Tuesday at 6 p.m. Here's what you're allowed to consume for 24 hours:
- Plain tea
- Black coffee
- Butter coffee
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Option 2: 5:2 Plan
Eat normally five days out of the week, then drop down to 500 to 600 calories the other two (nonconsecutive) days. For example, you might choose to fast on Wednesdays and Sundays and follow a regular diet on the remaining days. The photo below shows a 500-calorie menu plan for a 5:2 reduced-calorie day:
- 1 small apple 80 calories
- 2 tbsp hummus + 1 medium cucumber 75 calories
- 1/2 cup blueberries 40 calories
- 2 ounces tuna, canned in water 45 calories
- 2% cheddar-mozzarella twist cheese stick 50 calories
- 2 hard-boiled eggs 160 calories
- 2 cups spinach with 7 cherry tomatoes + 1 tbsp balsamic vinegar 50 calories
Option 3: Time-Restricted Feeding (TRF)
The popular 16:8 regimen falls under this umbrella. Basically, you eat only during an eight-hour window each day (say, between 10 a.m. and 6 p.m.). For a less stringent approach, you could try a 12-hour eating window (7 a.m. to 7 p.m.). Some people eat this way naturally—and food doesn’t even enter their consciousness before 10 a.m. If that’s you, TRF isn’t a far leap.
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Your Most Pressing Fasting Questions Answered
Is there real research backing fasting?
Yes! You’ve already read what it does for weight loss, but there’s also research linking fasting with health benefits ranging from disease prevention to longer life. The not-insignificant catch is that most of the research has been done on overweight rodents. They lose weight, their cardiovascular markers improve and they may even reverse their diabetes. But they’re rodents, not humans. Researchers hypothesize that IF improves health by giving your body a break from digesting food to focus on other things, like repairing damaged cells and lowering LDL, but there’s a definite need for more human-based research.
How much weight could I potentially lose?
Weight loss varies from person to person, but some studies show you could lose as much as 2 pounds each week—if you don’t cheat. According to a recent study in the Journal of Nutrition and Healthy Aging, people who fasted from 10 a.m. to 6 p.m. for 12 weeks lost about 2% to 3% more weight compared to those who didn’t restrict their eating times. Similar weight loss results were found for alternate-day fasting as well.
Who should skip IF?
• Pregnant and nursing women, children, the elderly and anyone who has (or has ever had) an eating disorder.
• Anyone with a chronic disease that requires medication, particularly if they need to take their meds with food. Although IF has shown promise for people with diabetes, “it’s harder to rebound from low blood sugar, which can cause weakness, dizziness, brain fog and more,” says Fatima Cody Stanford, MD, MPH, an obesity specialist at Massachusetts General Hospital.
Everyone always says you shouldn’t let yourself get hungry or your body will stockpile fat. Are you sure fasting won’t confuse my body into making me gain weight?
Yes and no. An IF regimen can make you pack on pounds if you’re prone to binge eating. “When you think of feeding hours as feasting hours and overeat, you can easily take in excessive calories and gain weight,” says Monique Tello, MD, MPH, internal medicine physician at Massachusetts General Hospital in Boston. “The people who get results eat a healthy, plant-based diet low in sugar and refined carbohydrates during non-fasting periods.”
Will I lose muscle from IF?
No. Unless you have an eating disorder, the only key players in muscle loss are aging, hormone changes and lack of exercise. IF may actually help you build fat-burning muscle. Research shows that muscle cells in fasters have an improved ability to make use of energy and proteins—and protein is the building block of muscle.
I’ve heard intermittent fasting will make me smarter. Is that true?
Back to the rodents again—this time in a maze. There are plenty of animal studies that link IF with enhanced brain function and a longer life-span. Studies suggest it may protect against the brain’s top saboteurs—Alzheimer’s disease, dementia and Parkinson’s—if you can treat it as a lifestyle instead of a temporary diet.
“From an evolutionary perspective, fasting makes sense,” says Mark Mattson, PhD, chief of the Laboratory of Neurosciences, National Institute on Aging. “Stressors like starvation and malnutrition strengthen brain areas tied to learning and memory. You’ll only survive if your memory and cognition kick in and help you figure out where you’ve found food before and how you can get it again.”
Unfortunately, IF may also make you less focused in the beginning. During the first few weeks, studies show that women on a 5:2 intermittent-fasting program are hungry and irritable and can’t concentrate; many suffer from headaches. To minimize the hit, Tello suggests adopting a healthy diet first (mostly plant-based foods without added sugars or refined flour) and then adding the IF component after your body adjusts to fewer carbohydrates and refined foods.
Should my teen fast with me?
Probably not. Granted, their natural clocks have them going to bed and waking up later, so a lot of them already skip breakfast and even lunch. But anything beyond that could have drawbacks, such as the risk of developing a calcium or vitamin D deficiency.
How to stay fit when doing IF
- Give your body time to adjust. “A lot of people go into IF trying to do their normal kickboxing workout and they can’t get through it,” says Stanford. Since you’ve been giving your body 24/7 access to food, going without can make workouts a challenge—at least until your body adapts to the new schedule. “It’s important to modify activity to accommodate the fact that the body is not able to pull on immediate fuel sources,” says Stanford.
- Wait until the end of your fasting period to exercise. When you start your workout in a fasted state, your body can't use the breakfast you just had for energy. Instead, it runs on a build-up of fat-derived molecules in the blood called ketones that could help you work out longer. “Interestingly, the main ketone that’s produced during fasting, called beta hydroxy butyrate, has been shown to enhance endurance,” says Mattson.
Meals to help you stay full while fasting
Each method of fasting is far more sustainable if you make smart choices when you do eat. Avoid hunger pangs with our breakfast, lunch or dinner.
—Recipes by Sarah Wharton
She Tried IF!
When 38-year-old Jenna Zawadowski of Lafayette, CO, got divorced in 2014, she packed up her four kids, moved in with her parents and turned to food for comfort. “I gained 30 pounds in three months,” she says.
Even though Jenna did CrossFit three times a week, she continued gaining weight, and by September 2017, she was up to 160 pounds on her 5'3" frame. “I started researching ways to take the weight off and learned about intermittent fasting on YouTube,” Jenna says.
She started slowly, fasting for 10 hours at first and creeping up to a total of 18 hours without eating. “The first two weeks were the roughest because my body was so used to waking up and eating two bowls of cereal,” she says.
To get over the hump and help her body adjust, Jenna drank lemon water around the clock, chewed sugar-free gum and sipped on black coffee with butter at the start of her non-fasting period for an energy boost—and to keep her away from food.
Within six months, Jenna lost 30 pounds and her dress size went down too, from an 8 to a 4.
She subsists on 1,400 calories concentrated between 2 p.m. and 8 p.m. daily. Her mainstay meals still include granola or Rice Chex, Crock Pot favorites like beef stroganoff and broccoli cheese soup, and casseroles like lasagna and mac and cheese. Jenna likes to bank some of her calories for an indulgent dessert—usually ice cream or cookies.