Intermittent fasting has shown some promise when it comes to weight loss. But before you dive in to days of serious calorie restriction mixed in with more normal eating days, read on to see if it's worth a shot.

Allison Knott, MS, RDN, LDN and Abbey Sharp, RD, AbbeysKitchen.com
Updated January 19, 2020

Throughout history, the human body has learned to cope with both feast and famine (thanks to necessity). In today's world, with more than half of the American population attempting to lose or maintain weight, some people are turning once again to this feast-or-famine approach for weight loss. Intermittent fasting, as it's known today, has shown some promise when it comes to weight loss, but like any other diet, there are questions about its long-term efficacy.

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What Is Intermittent Fasting?

Intermittent fasting is defined as an eating pattern that cycles between periods of minimal calorie intake (as low as 500-600 calories) and times when you can eat as much as you want (no calorie restrictions). While the premise is simple, the "rules" of intermittent fasting are less so; there's no official schedule to follow to know when to fast and when to eat, and there are no clear-cut caloric designations to help guide your intake. Some adherents consider a 24-hour period with minimal calorie intake followed by a 24-hour period of adequate intake to be the definition of intermittent fasting; others limit the days of fasting to just two days per week and follow an unrestricted diet the other five days of the week.

Does Intermittent Fasting Work?

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Most people will end up losing weight on this type of plan, but that doesn't mean it's for everyone. "This lack of a clear definition has the potential to create confusion," says Rachele Pojednic, Ph.D., assistant professor of nutrition at Simmons College in Boston. Yet Pojednic says the overall concept of intermittent fasting generally translates successfully to losing weight. "No matter the pattern of restriction, the result is reduced calorie intake-which leads to weight loss," she notes.

What Does Science Say About Intermittent Fasting?

Intermittent fasting sounds promising, until you take a closer look at the research. Karen Collins, M.S., R.D.N., nutrition advisor to the American Institute for Cancer Research, notes one significant problem with recommending intermittent fasting for weight loss: most of the research to date has been conducted on animals. "Previous [animal] studies have shown success using intermittent fasting to decrease insulin and the visceral fat that is connected to health risks, but the problem is translating those results to humans," Collins says.

Yet, research continues to be conducted. A recent study published in JAMA Internal Medicine looked at the effects of intermittent fasting on 100 overweight and obese people. The study participants were randomly assigned to one of three groups: alternate-day fasting, where participants consumed 25 percent of total calorie needs on fasting days and 125 percent of total calorie needs on "feast" days; a calorie-restriction group, where individuals consumed 75 percent of total calorie needs on all days; and a control group with no intervention. The study lasted for one year, and was broken into two phases-a six-month weight-loss phase and a six-month weight-maintenance phase. The primary measured outcomes were change in body weight, followed by metabolic changes such as blood pressure, heart rate, insulin resistance and fasting glucose, among others.

The results of the study showed no difference between intermittent fasting and standard calorie restriction in terms of changes in body weight or metabolic factors. Pojednic says the results are predictable: "No matter what diet you're on, there will be metabolic changes," she says. In other words, most diets are successful at first because of the calorie restriction. The difficulty lies in long-term maintenance. "There may be some for whom [intermittent fasting] works, but they need to be conscious of how they're going to learn to eat in a sustainable way that maintains a healthy weight," Collins says.

Collins points to the reality of our food environment as a limiting factor in sustaining weight loss. "Most of us are living in a world where there are very high-calorie foods in very large portions available to us 24/7," she says. "Somehow we have to figure out how to enjoy foods so that we aren't deprived. That becomes the challenge for long-term weight loss."

Intermittent fasting developed partially as a response to this challenge. "It's clear that people have a tough time restricting calories," Collins notes. "Some researchers thought that perhaps alternating restriction days could make it more sustainable."

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Intermittent Fasting Pros and Cons

Intermittent Fasting Benefits:

1. Regain fewer pounds

In one study, the IF regime was compared to a heart-healthy diet, and researchers found that while both groups lost comparable amounts of weight, the IF-regime group regained less of the weight lost. Since many dieters who lose weight gain back most of it, regaining fewer pounds is a benefit to long-term weight loss. Another study found that, compared with daily calorie restriction, participants who used a IF weight-loss regime were able to maintain more lean muscle mass while losing weight.

2. Burn more calories

Although research in this area is in its infancy, some early research suggests that fasting may slightly increase your metabolic rate, meaning you may burn slightly more calories, even at rest.

3. Potentially boost your brain

Emerging research suggests that fasting may have some benefits for your brain. One early study on rats found that IF helped improve memory, while another animal study found it helped protect the brain from damage due to aging. While we can't necessarily generalize these results to humans quite yet, it's worth keeping an ear out as evidence evolves.

4. Improve blood sugars

One preliminary study found that a fasting regime helped improved insulin secretion while producing new pancreatic cells in mice with type 1 and type 2 diabetes. In human trials, fasting helped improve participants' insulin sensitivity, while a new review of the literature found that IF was just as effective as daily calorie restriction at reducing insulin resistance.

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5. Keep your cholesterol in check

A short preliminary human study found that fasting reduced cholesterol and blood pressure levels, while another study found it to increase good cholesterol and decrease the bad cholesterol levels in men. Unfortunately, a recent expert report on IF suggested that due to some inconsistencies in the data and preliminary study methodology, we still need a lot more research to confirm IF's heart health potential.

6. Cut cancer risk

In an early animal study, IF seemed to reduce the risk of lymphoma in rat subjects, while another study found that fasting increased the longevity of rats who were inoculated with cancer cells. Unfortunately, while these studies may be an interesting jumping-off point for future research, it's far too early to suggest that there's any real measurable benefit of fasting on cancer in human patients.

Cons of Intermittent Fasting:

So even if a lot of the research is young, and animal-based, it seems like there may be some serious perks from IF. But what about the potential risks involved? Let's take a look.

1. Fertility issues

Severe dietary restriction is generally not recommended for women trying to conceive, so it makes sense that an early animal study found that fasting impaired the fertility capacity of rats. Again, while we shouldn't put too much weight on this early research, this type of calorie restriction (via IF or another restrictive diet) isn't recommended for pregnant or nursing women or those trying to conceive.

2. High dropout rates

Diets with the most restriction usually have the highest dropout rates, and IF is no walk in the park. One study found that the dropout rate was significantly higher among IF dieters compared to dieters restricting daily calories, so if long-term change is your goal, it might not be the best diet for the job.

3. Binge-like tendencies

While there are many iterations of IF, the underlying concept remains constant-dieters have a fasting phase and a feasting phase, the latter of which can lead to some dangerous tendencies. When you've been restricting most of the day, just trying to manage the hangry beast inside each time you get a whiff of someone's lunch, it's quite natural to go overboard when the clock strikes "feast." For some, these feasting episodes have the potential to overshadow the caloric deficit you make during the fast, thwarting any weight-loss efforts and risking dangerous disordered eating behaviors.

Should You Try Intermittent Fasting?

Before giving intermittent fasting a try, it's important to consider a few points.

1. First, fasting days are not zero-calorie days, but instead days where 25 percent of required calories are consumed. This may mean a person requiring 2,000 calories per day would eat 500 calories on fasting days.

2. Second, "feast" days are not unlimited-calorie days, so don't get excited about "cheat" days filled with doughnuts, double cheeseburgers and french fries. In the study, the "feast" days consisted of 125 percent of usual calorie intake, so a person following this diet pattern would eat around 2,500 calories on those days (if they typically eat a 2,000-calorie diet).

3. And finally, the focus should be on nutrient density on all days to ensure adequate micronutrient intake, as there's some evidence that intermittent fasting may not provide adequate nutrition-which raises questions for nutrition experts like Pojednic.

"The idea of eating foods is to first and foremost nourish your body, so if you're on a diet that requires you to take a supplement, then that is a serious red flag," Pojednic says.

Bottom Line

As is the case with many weight-loss-related questions, the answer to whether fasting leads to weight loss is complicated. Any diet that restricts calories can be an effective strategy for short-term weight loss. But data on the long-term effectiveness of intermittent fasting is limited, and there's no research to suggest that intermittent fasting is better than consistent caloric restriction for weight loss. For some, intermittent fasting may be a way to jump-start the weight-loss process. Others may be turned off by the thought of days with severe calorie limitation.

No matter the pattern of eating, Collins stresses the importance of consistent habits. "The more important thing is to find ways to make tweaks in eating habits that can reduce calorie consumption comfortably in a way that can be continued long-term," she says.

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