One of the well-known rules of weight loss asserts that when you eat is as important as what you eat. Which is why many of us have long held to certain food-timing conventions we believe will help keep our metabolisms humming from the moment we wake up: We kick-start our mornings with a solid breakfast; we eat several small meals throughout the day. We try not to go more than a few hours — heaven forbid a whole day — without food because we fear it might slow our metabolic rate to a crawl.
Now, though, a small-but-growing group of nutrition and fitness experts is beginning to question that conventional wisdom. These experts argue that constant grazing can actually disrupt metabolic pathways and that the best way to kick our metabolisms into high gear is to occasionally eat less often — in short, to observe an eating pattern known as intermittent fasting.
When most of us hear the word “fasting,” we’re likely to think of politically motivated hunger strikes or endurance stunts like illusionist David Blaine’s infamous 44-day stint, sans food, in a transparent, Plexiglas box. But the occasional fasting these experts are recommending is something different. They point out that humans evolved through multiple short-term periods of caloric restriction, and that our metabolisms operate more efficiently when freed from the burdens of 24/7 digestion and nutrition assimilation.
The time frames recommended for intermittent fasting vary. Popular options (all of which can incorporate sleep time) include daily 14- to 16-hour fasts, once- or twice-weekly 24-hour fasts, and alternate-day fasts of assorted time durations.
Ultimately, the goals and results of these programs are similar. Studies indicate that short-term fasting can increase longevity, help regulate glucose levels, and help treat everything from asthma and autoimmune diseases to cardiac arrhythmias.
In many ways, short-term fasting has more in common with detox regimens than with extended fasts of multiple days or weeks. But detox programs follow specific dietary rules, while short-term fasts rely on only the period of not eating to achieve health benefits.
It should be noted, however, that some health experts, including detox experts, are not big fans of intermittent fasting. (For more on that, see “Fasting vs. Detoxing,” below.) And even advocates acknowledge it’s not for everyone (see “Note,” below).
Integrative nutritionist Kathie Swift, MS, RD, LDN, has mixed feelings. On the one hand, she notes, observing what happens to the body during a short fast can help people become more observant of their habitual pattens and learn to identify real hunger.
“I’m fascinated by the idea of alternate-day fasting,” she says. “Some people do that almost intuitively — eat heavily one day, and then really, really cut down the next day.” But she also notes that because the body relies on key nutrients to eliminate toxins, build healthy tissue and more, intermittent fasting can cause problems for people who are not generally well nourished.
The biggest skepticism about fasting stems from the concern that it will disrupt metabolism and lead to weight gain. Yet a new study published this year in the journal Cell Metabolism suggests that limiting periods of food intake to eight hours a day might reduce the risk of obesity and obesity-related diseases.
In the study, researchers noted that mice who continually grazed on fatty food for 100 days gained weight and developed high cholesterol, high blood glucose and liver damage. The mice that fasted for 16 hours a day — but ate the same total amount of food during their nonfasting periods — weighed less, stayed healthy and performed better when they exercised.
Why? Researchers have found that after a few hours of fasting, the body starts to burn fat and break down cholesterol into beneficial bile acids — as if it were flipping a fuel-selector switch. The liver, meanwhile, shuts down glucose production for several hours, lowering blood glucose levels. Instead of ending up in the bloodstream, extra glucose is used to repair damaged cells and make new DNA, which can help prevent chronic inflammation. Meanwhile, liver enzymes are activated and help in the creation of brown fat (the good kind, which converts extra calories to heat).
“You’re enhancing your body’s ability to use fat as an energy source,” says strength and conditioning specialist Mike T. Nelson, a PhD candidate at the University of Minnesota in kinesiology and an intermittent faster. “Being more metabolically flexible allows you to shift back to burning fat faster after a meal.”
Frequent eating, on the other hand, means the body keeps making and storing fat, enlarging both fat and liver cells. Take that too far, and liver damage can occur. Plus, the liver keeps right on making glucose and raising blood-sugar levels.
Beyond Weight Loss
Many intermittent fasters report experiencing something not often associated with calorie restriction: relief. Those who have struggled with trying to eat five to six meals a day (or to count calories) report feeling eased of those burdens. Intermittent fasters also say they have more patience around food, and a stronger preference for eating high-quality fare on nonfasting days.
“Eating six meals a day can train you to get hungry more often,” says strength coach John Romaniello, founder of Final Phase Fat Loss. “Once you start practicing some sort of fasting, you usually eat less on nonfasting days, too.”
Fitness enthusiasts have discovered that fasting makes exercising for fat loss more efficient, and, in turn, exercising makes fasting easier. “Athletes performing long endurance activities while fasting actually burn more body fat than athletes who are fed (because the fed athletes are burning through food energy before they get to the stored energy in their body fat),” writes fitness guru Brad Pilon, MS, in his e-book, Eat Stop Eat (Strength Works, 2012). “The very act of burning fat also releases something called ‘glycerol’ from your body-fat stores. When the fatty acids are released, so is the glycerol. Glycerol is a valuable precursor for gluconeogenesis [a metabolic pathway] in the liver that helps keep blood glucose stable.”
If you tend to get hungry before working out, Pilon notes that the sensation might be more psychological than physical. “Most likely, what we call hunger is really a learned reaction to a combination of metabolic, social and environmental cues to eat,” he says. “Consider that most people get noticeably hungry or irritated if they have gone more than two to three hours without eating. But during this time, metabolically speaking, they are still in the fed state.”
While the case for fasting as a general-health tool is compelling, the case for fasting as a longevity booster may be even more compelling. Studies have shown that severely restricting calorie intake can increase the lifespan of rats and mice. (Data published this year in the journal Science Translational Medicine showed that mice with cancer improved their survival rates when they fasted during chemotherapy.)
The National Institute on Aging announced earlier this year that fasting for one or two days a week may also help stave off Alzheimer’s, Parkinson’s and other degenerative brain conditions. When fasting, the researchers said, brain cells are mildly stressed, benefiting the brain in a similar way to how the mild stress of exercise benefits muscles.
How to Get Started
Several well-regarded experts recommend fasts of a day or less, but you can start with whatever seems doable and appealing to you. Pilon, for example, advocates a 24-hour fasting period, one to two times per week. If you spread the fast over a two-day period, he points out, you can still eat every day — and sleep through the hardest part.
“Depending on your schedule, and what you’re most comfortable with, you could do a lunch-to-lunch or dinner-to-dinner fast,” Pilon says. “Going longer than that is very intrusive on life. Really, what I’m asking people to do is not fast, but take a break from eating. You learn patience, and that you don’t have to eat all the time.
Once you know your muscles aren’t going to melt, your metabolism is not going to crash, you regain the ability to be patient when it comes to eating.”
Pilon also likes this method because it allows him to eat food he loves at socially appropriate times: “I’d rather have dinner with my wife and kids than eat by myself all day at work.”
If a 24-hour fast is not your style or seems too extreme, Swedish nutritional consultant and personal trainer Martin Berkhan advises condensing your nutritional intake into an eight-hour period to maximize potential hormonal benefits. The idea behind his 16-hour fast works on the theory that you produce growth hormone when you sleep. Once you start eating, the spike in insulin shuts it down. By avoiding food in the evenings, and pushing the first meal of the day back, you may be able to maximize growth-hormone production.
Romaniello has even developed a system that allows for cheat days: You eat whatever you want throughout the day (he usually splurges on wings and burgers), then fast for 36 hours, and then eat a balanced diet during the week. Similar to Berkhan’s idea, Romaniello’s belief is that leptin (the hunger hormone’s counterpart, which makes you feel full), gets bumped up after the cheat day.
Whatever the method, many people who thrive on short-term fasts like them for their simplicity: You’re either fasting, or you’re not. No counting calories; no restricting certain foods. But, as in all things, practice in moderation until you’re able to gauge exactly how intermittent fasting affects you physically and emotionally.
“You have to keep it in check,” Pilon says. If you’re missing out on a lot of life events or stressing over foods because of your diet, then it’s time to detach, relax and reassess.
Sheila Mulrooney Eldred is a Minneapolis-based health-and-fitness writer.