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Does Intermittent Fasting Really Work?

by | Jul 10, 2021 | Nutrition | 331 comments

A photo of an alarm clock and a cup of coffee

Imagine eating all you want—and not only losing weight, but racking up a handful of health benefits to boot. That’s actually the hook behind one of the latest diet trends. Of course, there are caveats. The main one being that when you’re not pigging out, you’re actually starving yourself (or almost starving yourself). So it’s not a diet that’s too good to be true since you’re still miserably hungry some of the time. Feasting is balanced by fasting.

The technical term is intermittent fasting, and a green-light-on-cheat-day approach is one reason why diet books on intermittent fasting are on bestseller lists—and why you probably already know one or two people who are doing it.

Intermittent fasting, the basics:

Intermittent fasting stems from a structured practice known as caloric restriction, or CR, which has been shown to increase longevity and reduce disease in mice and monkeys. Believers think it does in humans, too—although there is no real evidence yet to prove that.

More than a few steps beyond your regular calorie-cutting diet, caloric restriction is a deliberate lifestyle where, essentially, you are semi-dieting forever. Health and longevity, not weight loss, are the goals, and, on a long-term basis, you eat 25 percent and up to 45 percent fewer calories than the body requires each day. This consistent state of energy deprivation at levels that still allow you to obtain essential nutrients triggers hormesis, a biological response to stress that makes the body stronger and more resilient.

Intermittent fasting may be a more palatable alternative to both caloric restriction and conventional dieting. Instead of eating less every day for a set number of days or weeks or months (or indefinitely), you alternate days of eating less (or nothing) with days of eating normally—or even ad libitum, aka “all you can eat.”

Regimens vary: Fasting days might entail eating nothing at all (but drinking plenty of water) on one day a week and eating as much as you want on the other six days. Or you might alternate days, with fast days consisting of eating anywhere from, say, 500 to 800 calories per day, and eating all you want on the other 3 to 4 days per week. Some popular examples are The Fast Diet, The 8-Hour Diet, and Fasting and Eating for Health.

Does it work for weight loss? Yeah, it does.

You would think that after cutting calories to drastic lows on fast days, people would naturally compensate for the loss on all-you-can-eat days. On the contrary, even though you can theoretically “make up for it” on feast days, most people don’t.

Research shows that there’s a ceiling to overeating: One small study found that people only ate around 10 to 22 percent extra on their feast days and incurred an overall caloric reduction of around 28 percent over the week. You might eat a little more, but not enough to stay in energy balance over a whole week. So, overall, you eat fewer calories and you lose weight.

But it doesn’t seem to work any better than regular dieting.

A photo of a healthy breakfast bowl

Studies on intermittent fasting show that an average of 7 to 11 pounds was lost from 10 weeks of intermittent fasting—similar to regular dieting. Fat loss and the loss of lean body mass were also no better than conventional dieting. And there appeared to be no difference in how the body responded—activity levels were lower and overall energy expenditure tended to be lower just like with regular dieting.

Most diets, including Atkins, consist of around 1,200 to 1,500 calories per day. For the average adult, that can be a 55 to 70 percent drop. That’s pretty extreme, so no wonder most dieters get cravings and can’t stick to a drastic energy deficit for very long.

Also according to studies, the hunger pangs one gets during IF do not seem to be lower than with conventional dieting. And though you’re only hungry one a couple days a week instead of every day, dropout rates between both patterns of dieting are similar, suggesting that intermittent fasting is not necessarily easier to stick to than a regular diet.

Still, because weight management is not just about losing weight, but finding lifestyle strategies for keeping it off, people who have longer-term maintenance goals in mind might find the intermittent fasting approach easier to follow over the long haul.

There’s one theory as to why IF might have an edge on other diets, on a physiological level. With conventional dieting, as weight loss occurs, physiological systems kick in that make the body plateau, making it harder to lose additional weight and easier to gain the lost weight back. It’s been suggested that intermittent fasting might trick the body into not plateauing, but that has yet to be demonstrated scientifically.

However, IF doesn’t seem to stop the body from going into “starvation mode.” The thought is that when you cut out a large number of calories, your body senses a dearth of resources (i.e. calories), and responds by storing every extra calorie as fat to prepare to weather the “famine.” Keeping in normal or greater-than-normal eating days doesn’t seem to avoid these periods of physiological slowdown. But, as long as you’re eating fewer calories than you burn over the week, you will still lose weight.

Are there other positive effects aside from weight loss?

A photo of a woman eating a snack

According to research, cells of a person on IF seem to act “younger” from the physiological transformation that takes place in tissues and organs. Plus, “inflammation and oxidative stress are decreased, as are blood glucose levels and the way that mitochondria in cells produce energy is improved. Positive effects are also seen in cholesterol and hypertension levels over time.

Want to give it a try?

A photo of a person consulting a doctor

The first thing to do is to consult your doctor if you are pregnant, have diabetes or other difficulties regulating blood sugar, or have other health concerns that might make fasting a questionable idea, such as a history of eating disorders. Second, pick a day or even part of the day each week where you eat nothing or next to nothing. That’s probably the simplest way to test out the method.

You may also opt to try alternate-day fasting. In this method, you eat without worrying about calories every other day and eat about 25 to 50 percent fewer calories on their fasting days. The popular Fast, aka 5:2 diet, is one version of this: You eat normally five days a week and quasi-fast on 500 calories per day for the other two days (600 for men). You can choose any days of the week to fast.

The bottom line is that weight loss through intermittent fasting appears to be similar to conventional dieting. However, the jury is still out on whether intermittent fasting is more effective for health than normal dieting. Since the hardest part of dieting is sticking to one and maintaining the weight loss long term, some personalities might find this sometimes-flexible eating pattern easier to swallow.

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