What Is Intermittent Fasting?

Intermittent fasting (IF) is a calorie-restricting approach that asks you to go without food for long periods of time. Some IF programs propose fasting for a few hours or for a specific part of the day, while others require fasting for a few days per week or for several days per month.

Intermittent fasting has gained popularity as a result of the widespread assumption that it can help you enhance your fitness, increase your longevity, and achieve faster and longer-lasting weight loss outcomes.

There has been substantial research conducted on different variations of intermittent fasting, but much of it has been conducted on animals. Long-term studies are needed to determine if there is enough scientific evidence to recommend this eating style.

What Experts Say

“Intermittent fasting, restricting food intake for certain periods of time, has been studied for potential effects on longevity and other health outcomes but is often used for weight loss. Many experts agree food restriction is not sustainable and that frequent fasting could lead to social isolation or binge eating.”—Willow Jarosh, MS, RD

Background

“Eat, Fast, and Live Longer,” a BBC documentary from 2012, is generally credited with popularising intermittent fasting, but it’s been studied for much longer for its possible benefits in preventing breast cancer, heart disease, diabetes, and cognitive decline.

Calorie restriction has also been studied for its potential impact on longevity, however, most studies have been conducted on rodents, with mixed findings.

Intermittent fasting has also been extensively researched for its weight-loss potential. Many of the early experiments were conducted on mice, but as public interest in the regime has grown, more human investigations are being conducted.

Long-term research is still needed to determine whether IF is safe and effective in the treatment of obesity and other diseases.

Lastly, it is helpful to understand that almost all of us follow an intermittent fasting program every day to some extent, except we don’t call it “intermittent fasting.” We call it to sleep. We eat dinner in the evening, then fast during sleep, and finally eat again at breakfast.

Most of us practice intermittent fasting every night when we sleep. The 10–14 hours when our body is at rest is a fasting phase that aligns with our circadian rhythm. Researchers believe that this alignment is key for optimal health.

How It Works

Unlike many healthy eating and weight loss programs, food choice is not a part of an intermittent fasting plan. There are no macronutrient recommendations and there is no list of foods to limit or avoid. Instead, intermittent fasting simply regulates the timing of your food intake.

Intermittent fasting can be done in a variety of ways, but they all contain a “feast” and a “fast” phase. Most programs propose that you consume an “ad libitum” diet during the feast phase, which means that you don’t limit or restrict your food consumption in any manner.

However, eating past the point of fullness or overeating unhealthy foods is not encouraged. During this time, it’s still a good idea to eat well. During the fasting period, you either severely restrict or completely forego eating.

The 5:2 diet is one of the most common approaches. You eat less-restricted, healthful food five days a week and then fast two days a week on this plan. Fasting, however, does not imply complete abstinence from food under this program.

It means severely restricting your food intake. For women, that represents about 500 calories, for men, about 600 calories. On the other days, you consume a typical healthy diet, although a calorie recommendation is provided.

Other variations of IF include alternate-day fasting (ADF) plans that require you to completely abstain from food or severely restrict food every other day, or time-restricted plans where food is eliminated during certain hours of the day.

Severely restricting your food intake may mean that you consume roughly 25% of your daily calorie needs per day.

Religious fasting has also been studied, including intermittent fasting programs that take place during the holy months of Ramadan, and fasting programs followed by Seventh Day Adventists and Latter-Day Saints.

Pros and Cons

Intermittent fasting is a popular dietary change method because it allows participants to keep eating the things they enjoy. Other regimens can be difficult to stick to because the individual following the program has to give up things that they’ve grown accustomed to eating.

While this is occasionally manageable in the short term, many people find it difficult to give up their favorite foods in the long run.

Furthermore, while there is still a lack of information on the diet’s long-term effectiveness or safety, some studies have revealed that intermittent fasting is equally successful as continuous calorie restriction in terms of weight reduction.

Some experts believe that these diets could be a more effective way to manage obesity and obesity-related diseases.

The feast or famine attitude to eating, on the other hand, is a major source of concern among researchers and nutrition professionals. Short-term fasting might lead to binge eating or overeating at other times. Surprisingly, research to date has not substantiated this issue.

In one study, people who ate 20-30% of their average calorie requirements on fasting days ate only 10% more on non-dieting days. Furthermore, many participants stated that their hunger on low-calorie days decreased considerably with time.

Finally, experts are concerned that there is little guidance on how to make healthy eating choices. Although an intermittent fasting strategy may meet a person’s nutritional needs, it does not encourage them to follow healthy eating standards.

Common Myths and Questions

Because there are different types of intermittent fasting and no specific authority or dedicated source for information, there are quite a few myths about the eating style.

Myth: Intermittent fasting is more effective for weight loss than traditional diets.

Current evidence suggests that those who follow traditional calorie-restriction diets lose about the same amount of weight as those who follow intermittent fasting programs.

Several studies have found that while there is a slight advantage for those doing IF, the advantage isn’t significant. Additionally, experts still don’t know if IF programs are sustainable.

Myth: Intermittent fasting causes muscle loss.

Starvation can cause a loss of lean muscle tissue. So, it would seem reasonable to assume that intermittent fasting would also cause some degree of muscle wasting.

However, the evidence so far has shown that intermittent fasting may spare muscle when compared to conventional dieting.

In a 2011 review, 90 percent of the weight lost through intermittent fasting was fat (rather than muscle), compared with only 75 percent with daily dieting. This would suggest that conventional dieting causes greater muscle loss than IF programs.

Maintaining lean muscle mass while dieting offers a metabolic advantage for trying to maintain weight loss because muscle burns more energy than fat even at rest.

Myth: Intermittent fasting works better for losing belly fat

Belly fat, also known as visceral fat, is the spare tire that surrounds your internal organs, leading to a greater risk of diabetes and heart disease.

A 2011 review found that both traditional dieting and intermittent fasting reduce similar amounts of belly fat.

Myth: Intermittent fasting will improve your level of fitness.

Some people believe that the human body maximizes fat loss and cardio efficiency in a fasted state during aerobic exercise first thing in the morning.

The practice, called “fasted cardio,” has caught on in certain fitness communities. However, there isn’t a lot of scientific evidence to support the practice.

One study reported by the National Institutes of Health found that mice who were subjected to an alternate-day fasting program developed a more efficient energy metabolism and improved their running endurance.

However, researchers acknowledged that further study is needed to see if these results hold true in humans.

Myth: You’ll live longer if you practice intermittent fasting.

This is one of the most widely held beliefs by many people who adhere to an intermittent fasting protocol. But there hasn’t been enough research conducted on humans to know if it is a fact, according to the National Institutes of Health Institute on Aging.

Rodent studies have suggested that intermittent fasting boosts longevity. But humans have very different lifestyles than mice and those differences have substantial implications. The bottom line is that we don’t know how intermittent fasting affects human longevity.

Myth: Intermittent fasting is safe for everyone.

Intermittent fasting is not appropriate for:

  • Children or teenagers
  • Women who are pregnant or breastfeeding
  • Those with a history of eating disorders

The NIH also recommends that adults with diabetes or heart disease should consult their healthcare provider before trying any new diet.

How It Compares

Because there are no meal suggestions or restrictions, comparing intermittent fasting programs to other commercial and traditional diet plans can be difficult.

Almost every other eating plan requires you to consume more of some items and less of others (for example, high-protein diets and low-carb diets).

On the other hand, on an IF diet, you can eat as much or as little of any item or food group as you want as long as you consume it during the “feasting” window.

USDA Guidelines

The USDA publishes daily recommendations for specific foods (such as fruits and vegetables) and nutrients (such as fiber, protein, and fat). On certain days of the week or month, certain varieties of intermittent fasting urge you to eat nothing but water and clear liquids.

As a result, sticking to your dietary guidelines on those days would be impossible. Other IF modifications could help you fulfill your nutritional requirements, but only if you were very careful about what you ate.

Most IF programs make it impossible (or nearly impossible) to meet USDA nutritional guidelines on days when you are fasting.

Juice Cleanse

Some people follow juice cleanses or detox diets that are variations of intermittent fasting. For example, on a typical juice cleanse, you might consume a range of fruit or vegetable juices and avoid solid foods for a number of days to lose weight. Detox diets typically last three days and also severely limit food intake.

However, juice cleanses and detox diets are generally not permanent eating styles. That is, they are usually one-time programs to gain a specific benefit such as reduced bloating or weight loss.

Body Reset Diet

This eating program, developed by celebrity fitness trainer Harley Pasternak, requires that you go through an introductory phase in which food is restricted. During the first five-day phase you drink only liquid smoothies and forgo solid food. This part of the diet has a fasting feeling to it.

However, after five days you begin to incorporate solid food back into your plan and the fasting phase is complete. There is no return to a phase where you fast unless you repeat the diet at some point.

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