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 occasionally to 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. And even advocates acknowledge it’s not for everyone (see “Note,” page 4).
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.