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Intermittent Fasting

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What Is Intermittent Fasting?

Intermittent fasting (IF) is a method of calorie restriction that requires you to go for extended periods of time without eating. There are different types of IF programs—some recommend fasting for a few hours or for a certain part of each day, some require fasting for a few days each week, and some involve fasting for several days each month. Intermittent fasting has gained momentum because of popular beliefs that the regimes can help you improve your level of fitness, boost longevity, and get faster and longer-lasting weight loss results.

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 frequent fasting could lead to social isolation or binge eating."

—Willow Jarosh, MS, RD


A 2012 BBC documentary, "Eat, Fast and Live Longer," is often credited with bringing intermittent fasting into the mainstream, but it's been researched longer than that for its potential benefit in reducing breast cancer, heart disease, diabetes, and cognitive decline. Calorie restriction has also been researched for its potential impact on longevity, but studies have been primarily performed on rodents and results have been inconsistent. 

Intermittent fasting has also been studied extensively for its potential weight loss benefits. Many of the earlier studies were performed on rodents, but as interest in the regime has increased more studies are now being conducted on humans. Long-term studies are still needed to see if IF is safe or effective in the treatment of obesity and other conditions.

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 sleep. We eat dinner in the evening, then fast during sleep, then 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.

Different methods of intermittent fasting exist, but all of them include a "feast" phase and a "fast" phase. During the feast phase, most programs recommend that you eat an "ad libitum" diet, meaning that you don't limit or restrict your food intake in any way. But it is not recommended that you eat past the point of fullness or overeat unhealthy foods. Healthy eating is still recommended during this time.  On the fasting phase, you either severely restrict or avoid food altogether.

One popular approach is called the 5:2 diet. On this plan, you eat a less-restricted, healthy diet five days of the week and then spend two days per week fasting. However, in this program, fasting does not mean total abstinence from food. 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.

 What to Expect When Intermittent Fasting

Pros and Cons

Intermittent fasting is a popular approach to dietary change because it allows followers to continue to eat the foods that they love. Adherence to other programs is sometimes difficult because the person following the program has to deny themselves beloved foods that they've become accustomed to eating. While that is sometimes tolerable in the short-term, many people have a hard time giving up familiar foods over the long-term.

Additionally, while there is still little evidence on the long-term effectiveness or safety of the diet, several studies have suggested that intermittent fasting works as well as continuous calorie restriction for weight loss. Some researchers have suggested that these diets may be a smarter approach in the treatment of obesity and obesity-related conditions.

However, a primary concern among researchers and nutrition experts is the feast or famine approach to eating. Starving yourself for limited periods may lead to overeating or binge-eating at other times. Although surprisingly, studies haven't supported this concern, so far.

In one study, subjects who ate 20-30 percent of their normal calorie requirements on fasting days generally ate just 10 percent more than usual on their non-dieting days .In addition, many people reported that their feelings of hunger on low-calorie days dramatically diminished over time.

Lastly, experts worry that there is no guidance provided about making healthier food choices. Someone following an intermittent fasting plan might meet their nutritional needs, but these plans do not provide encouragement to meet healthy eating guidelines.

 Pros and Cons of Intermittent Fasting

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 in 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.

 Will Working Out on an Empty Stomach Help You Lose Weight?

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

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

Comparing intermittent fasting programs to other commercial and traditional diet programs is tricky because there are no food recommendations or limitations on the plan. Almost every other eating plan requires you to increase your intake of certain foods (such as high protein diets) or limit your intake of others (low-carb diets). But on an IF program, you can eat as much or as little of any food or food group as you like, as long as it is consumed in the "feasting" window.

USDA Guidelines

The USDA provides guidelines for the daily intake of certain foods (such as fruits and vegetables) and important nutrients (such as fibre, protein, and fat). Certain types of intermittent fasting require that you avoid food except for water and clear liquids on certain days of the week or month. It would, therefore, be impossible to meet your nutritional guidelines on those days. Other variations of IF would allow you to meet your nutritional needs, but only if you were very careful in your food choices.

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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