Intermittent Fasting: Everything You Need to Know (A Comprehensive Research Report)

by Melissa Gallanter, RD

An overview of intermittent fasting. Part of an ongoing series highlighting various diets and fitness trends

Key Points:

  • What it is: an umbrella term for a few different fasting diets, including time-restricted fasting done each day and alternate-day fasting with minimal calories consumed every-other-day.  
  • What we know: There is no single consensus regarding intermittent fasting, but research continues to explore its benefits, including weight loss at least comparable to standard calorie restriction and improved metabolic markers like decreased LDL cholesterol and triglyceride levels. Much more research needs to be done to establish intermittent fasting as a medically sound long-term intervention for weight loss and overall improved health.
  • Recommendations: Time-restricted fasting seems to be the most doable, most closely resembling a standard non-diet eating pattern. Sticking to an eight-hour eating period during daylight hours (or somewhat close to daylight hours) does create a regular rhythm and has suggested benefits for the metabolism. If considering intermittent fasting, consult with your physician and a registered dietitian.

How It Works

Intermittent fasting is an umbrella term for a few different types of fasting diets, in which a person does not eat at all or severely limits intake during certain times within a day, a week, or a month. Intermittent fasting has recently gained traction as a method of calorie restriction, a pattern widely studied as a way to maintain health and a nutritious lifestyle.

There are a few different ways to go about intermittent fasting, but the overall idea is to reduce calories by eating unrestricted during some hours in the day or on some days of the week while fasting outside of these eating periods. 

The Hype 

Fasting has its place in ancient traditions, for health, and for religious practices. But in recent years intermittent fasting has gained a lot of attention in research and in the media, with famous fasters like Twitter CEO Jon Dorsey, Jennifer Aniston, and Reese Witherspoon

The Claims

Some research shows that intermittent fasting works as a method of weight loss – at least on a short-term basis. Intermittent fasting is also promoted to improve metabolic markers that are associated with chronic disease, for example, LDL cholesterol levels and triglycerides. However, there is still a lot of research to be done about its long-term effects on developing a feasible healthy lifestyle. 

Studies suggest that methods of intermittent fasting can result in similar weight loss results to those of standard daily, continuous calorie restriction regimens. Intermittent fasting was introduced as an alternative and a more accessible diet since the standard calorie restriction requires daily adherence. With a more flexible approach fasting in short term periods of time, there is more freedom to meet energy requirements during eating periods. 

How You Actually Do It

Intermittent fasting is often done in a few different ways:

Time-Restricted Fasting: Meals are consumed within a limited period of time each day (usually six to eight hours), with nothing consumed during the other hours. Some people choose to eat in a six-hour window, but most commonly, fasts last 16 hours with an eight-hour eating window. The eating window can be decided upon based on individual schedule and lifestyle, for example, a window can be from 7 a.m. to 3 p.m., 9 a.m. to 5 p.m., or 1 1 a.m. to 7 p.m.

Alternate-Day Fasting: Eating is unrestricted every other day, and no or minimal calories can be consumed on fasting days, consisting of at most one meal (usually at lunchtime) that provides about 25 percent of daily calorie needs.

5:2 Eating Pattern: Eating is unrestricted for five straight days each week, followed by two days of restricted calorie intake of up to 25 percent daily calorie needs. These two days could consist of one bigger meal, usually at lunchtime. 

Periodic Fasting: Caloric intake is restricted for multiple days in a row (such as five days in a row within a month), and unrestricted on all other days. 

So, What Can You Eat?

In theory, non-fasting periods have no restrictions on what you can eat. But if intermittent fasting is being used as an intervention for weight loss and reduced risk of chronic disease (such as cardiovascular disease and type II diabetes), it is best to consume a balanced, healthy diet during the non-fasting periods. 

The importance of food quality during eating windows takes part in the effects and benefits reaped by the fasting periods. It is important to include fruits, vegetables, and whole grains as well as protein sources and unsaturated fats to create a wholesome and balanced diet to satiate and provide adequate calories and micronutrients. 

Because there are fewer opportunities to eat, making sure to consume a nutrient-dense diet is highly recommended, especially to avoid long term issues such as malnutrition. Incorporating nutrient-dense foods provides “more nutrition bang for the calorie buck,” as explained in a Harvard Health Letter. In other words, nutrient-dense foods are those that have high nutrition content (vitamins, minerals, fiber, lean protein) but are not comparably high in calories. 

Diversity of Foods?

As with any healthy diet, it is recommended that you eat a variety of foods from as many food groups as possible. Intermittent fasting allows for a diversity of foods because during eating windows nothing there are no restrictions. The recommendations are in fact to include a diversity of foods, meaning a wide variety of nutrient-rich fruits and vegetables, whole grains, nuts, beans, lean proteins, fish, and dairy.

Hard to Maintain?

It is not easy, nor a natural eating pattern, to skip meals intentionally and to rely on water, coffee, and tea to keep full during fasts. It is also not easy to maintain a balanced meal plan during eating periods or non-fasting days. Non-fasting days are meant to include healthy meals and eating in moderation.

Only having to grocery shop, plan meals, and prepare food some of the time makes intermittent fasting makes it a little less overwhelming for some people to make healthy choices. For those who find preparing healthy meals each day a chore, not having to prepare each day may make it easier during the eating periods to find the motivation to make a variety of healthy meals. It creates fewer instances to make decisions and provides more structure to abide by. For some people, this aspect makes intermittent fasting much easier to maintain. For others, this makes it more challenging. 

Exercising while fasting can also be a challenge. In complement with physical activity, the diet is meant to fuel the body and provide energy for the activity and recovery, both of which can be difficult with intermittent fasting. If incorporating physical activity into one’s lifestyle is compromised during fasting periods, the benefits of intermittent fasting overall might be in question. Finding the balance and being able to maintain consistent and well-fueled physical activity may be difficult. 

The social aspect of eating makes intermittent fasting difficult to maintain as well. Say a family gathering, work event, or a friend’s birthday celebration is called for 7:30 p.m. but one’s eating window ends at 7 p.m.; depending on the social environment, it can be hard to come along for the event and refrain from sharing the table’s appetizer or tasting a spoonful of Grandma’s famous soup. 

But, whether intermittent fasting is easy to maintain is very dependent on the individual. Regardless of the method, there are many who attest that it is a straight-forward and easy-to-maintain means of calorie restriction. There is research that suggests that intermittent fasting can be an option for those who have a hard time adhering to daily calorie restriction. Supporters of alternate-day and 5:2 fasting endorse that after getting used to it, it becomes easier to only worry about what to eat part of the time. Others who practice time-restricted fasting find their eating windows as discipline barriers against midnight snacking.

Even so, there is still little data on weight loss maintenance and long-term maintenance of intermittent fasting. Others have found it difficult to maintain their weight loss after stopping an intermittent fast regimen. A notable aspect of a 2017 study published in JAMA was a very high dropout rate (38 percent) among those assigned to the fasting group, which can be telling. In contrast, it’s difficult to confirm claims regarding long-term maintenance because of the lack of studies and data on diet sustainability. 

Cost of Diet?

Because there are no restrictions during the eating period, there is a wide variability and flexibility in the cost of intermittent fasting. The cost of the diet will depend on what type of intermittent fasting method is being used and the make-up of meals during the eating window. Theoretically, intermittent fasting results in eating less food (in comparison to the standard American diet), so less money needs to be spent on food. Fewer meals eaten per day or per week means less money spent on lunch breaks and groceries. 

What the Research Says

Most largely studied research done on intermittent fasting has focused on the weight-loss aspect of the lifestyle, primarily in people with obesity. Only a few small clinical trials have been conducted. More research is needed to determine which, if any, types of intermittent fasting have long-term benefits. 

Hypothesized benefits of intermittent fasting are more extensively studied in animal models than in humans but suggest improved insulin sensitivity and reduced inflammation and oxidative stress. Higher insulin sensitivity allows the cells in the body to use glucose more effectively, reduce blood sugar levels, and therefore prevent type II diabetes. New research continues to find associations between intermittent fasting and improved metabolic markers including BMI and fasting blood glucose levels. Research continues to emerge and recognize new and potential associations, but also continues to recognize the need for further randomized control trials to deepen the understanding. 

A 2017 review of 12 animal studies examined the effects of alternate-day fasting on chronic disease risk and found that fasting glucose concentrations generally decrease in response to alternate-day fasting in animal models within a short-term study conditions. However, in one study included in the review, the glucose and insulin concentrations decreased to a similar extent in the alternate-day fasting group as in the standard calorie restriction group.

To assess cardiovascular response to alternate-day fasting, trials have examined heart rate, blood pressure, circulating lipids, and ischemic injury. The 2017 review notes that recent studies suggest that reductions in heart rate may be associated with alternate-day fasting. 

In a more recent 2019 review, alternate-day fasting was found to lower LDL cholesterol levels. Studies found that getting 100 percent of needs during the eating window can help individuals avoid night eating and follow a better circadian rhythm, but assuring a well-balanced diet during the eating window in a real-world setting (outside of a study environment) becomes the challenge in maintaining the benefits. 

In a 2017 systematic review and meta-analysis, which looked at the data from five different studies, weekly intermittent fasting interventions were found to be just as effective as the more standard weight-loss intervention of continuous calorie restriction. The standard calorie restriction used as a comparison involves eating fewer calories each day in order to lose weight, for example eating 2000 calories when estimated individual needs are 2500, or consciously choosing a lower-calorie snack and meal options to lower total calorie intake. The study concluded that instead of this more well-known calorie restriction method for weight loss, intermittent fasting provides a more flexible alternative that can yield the same weight loss results. 

A clinical trial published in 2018, with 100 participants found that participants in the fasting group ate more than prescribed on fasting days and less than prescribed on non-fasting days, while those in the daily calorie restriction group typically met their prescribed energy goals on a daily basis. Mean weight-loss was similar for participants in the alternate-day fasting group and the daily calorie restriction group after six months. This clinical trial concluded that alternate-day fasting did not result in greater adherence, weight loss, weight maintenance, or cardioprotection compared to daily calorie restriction. 

Rodent studies show that intermittent fasting — especially time-restricted fasting — might directly influence the gut microbiota, which has an important role in digestion, immunity, and much more. Our circadian rhythms have evolved to be in sync with the day/night cycle, and so gastric emptying and blood flow are greater during the day. A chronically disturbed circadian rhythm can affect gastrointestinal function long-term. Time-restricted fasting mimics a steadily regimented day/night eating pattern to match that of our natural sleep schedule, and creating a structured fasting rhythm may contribute to a beneficially diverse gut microbiome as well as a decrease in inflammation, which is typically elevated in obesity. 

There is a lot of research on intermittent fasting, as it has become a popular approach to weight loss and of interest to many in the general public. There are studies that suggest inconsistent or inconclusive results, but some with a potential of favorable outcomes. One limitation of current research is that much of the research being done on animals rather than humans, and many of the human studies are limited by small sample sizes. Because of short-term follow-ups for much of the research as well, information about long-term maintenance and benefits of intermittent fasting are still not understood.

Within current research are some studies with inconsistent or inconclusive results and some with the potential of a promising long-term outcome. Limitations from the majority of the research include a small portion of studies on humans rather than animals, small sample sizes, short-term follow-up, and inconsistency among the methods of intermittent fasting being studied.

Nutritionally Sound?

The Academy of Nutrition and Dietetics, the world’s largest organization of food and nutrition professionals, does not have an official statement regarding the effects of intermittent fasting. Among nutritionists and dietitians, intermittent fasting is still up for debate. Some dietitians endorse intermittent fasting as one method to calorie reduction if maintained thoughtfully. Supporters in the medical field include Dr. Jason Fung, a Toronto-based nephrologist, co-founder of the Intensive Dietary Management Program, and has written three best-selling books, The Obesity Code, The Complete Guide to Fasting, and The Longevity Solution

Others aren’t as supportive that intermittent fasting is nutritionally sound. There is a concern for malnutrition when fasting, which is the deficiencies or imbalances in a person’s intake of energy and nutrients. With malnutrition often comes the concern for something called refeeding syndrome, which is the potentially harmful shifts in fluids and electrolytes after reintroducing food too quickly after a period of not eating. Like Barbara Gordon, RDN, LD, many dietitians feel that “there needs to be more research, especially regarding the negative side effects of fasting,” before intermittent fasting can be a strongly recommended treatment for weight loss. 

Medically Sound?

While there is no single consensus amongst physicians, medical research continues to explore the benefits of intermittent fasting, especially as the data continues to emerge with more studies. Many physicians are involved in the research that continues to investigate intermittent fasting as a weight loss intervention. Some, like Dr. Shilpa Ravella, a gastroenterologist at NewYork-Presbyterian, emphasize that anyone interested in trying intermittent fasting should adopt a healthy, plant-based diet and lean closer towards a time-restriction fasting method in that  meals are consumed within a limited period of time each day (usually six to eight hours), with nothing consumed during the other hours. One of Time magazine’s 50 Most Influential People in Healthcare and USC Leonard Davis School of Gerontology Professor Valter Longo has been at the forefront of the intermittent fasting conversation and is a strong voice supporting intermittent fasting’s potential role in disease prevention, promotion of healthy aging, and expedite cellular repair. 

According to Clinical Director of the Yale Metabolic Health and Weight Loss Program Artur Vargas Viana, the proposed benefits of intermittent fasting do not stem from the method of weight loss specifically but rather because of the general principle that someone trying fasting eats less in one week than they normally would. 

The current research suggests that there are tangible short-term benefits of intermittent fasting, including weight loss at least comparable to standard calorie restriction, increased insulin sensitivity, and improved metabolic markers like decreased LDL cholesterol and triglyceride levels. However, medical experts have not yet come to a consensus about intermittent fasting. Much more research needs to be done to establish intermittent fasting as a medically sound long-term intervention for weight loss and improved overall health. 

Scientific Literature 

Final Verdict

With planning and incorporation of a healthful, balanced diet during eating periods, intermittent fasting can be an effective tool for weight loss with potential metabolic benefits as well, including decreased inflammation and oxidative stress, increased insulin resistance, and lower LDL cholesterol levels. Through social media attention and continued research emerging, intermittent fasting has hit the headlines, and with potential for good reason. Celebrities and public figures constantly mention their intermittent fasting habits, and research continues to suggest the short-term benefits of the diet. 

Intermittent fasting has emerged as a popular trend, and research aside, it is essential to remember that it is not the right lifestyle for everyone. Feeling hungry during fasting can be a distraction during work, a detriment to a regular exercise routine, and sticking to a strict eating schedule can make it hard to maintain the diet in the midst of a social life. 

The time-restricted fasting seems to be the most doable for a lot of people, allowing for eating periods during each day, most closely resembling a standard non-diet eating pattern. This method of intermittent fasting also is recognized for deterring late-night snacking, which can often be an obstacle for weight loss and a difficult behavior change to make. Sticking to an eight-hour eating period during daylight hours (or somewhat close to daylight hours) does create a regular rhythm and has suggested benefits for the metabolism.

All of that being said, the final verdict is still up in the air, with gaps in knowledge and lack of strong evidence that intermittent fasting has long-term adherence and weight loss maintenance results. 

If considering intermittent fasting, consult with your physician and a registered dietitian. It is not recommended for pregnant or breastfeeding women, children, elderly, individuals with eating disorders, or people that perform heavy physical work, as well as individuals who are required to eat meals at regular intervals such as type 1 diabetics. 

Diet quality is paramount for the promotion of health and there is no one way to define what works best. On an individual basis, there are many ways to make intermittent fasting work into one’s lifestyle in order to maintain or lose weight; however, the long-term metabolic benefits, as well as the potential to maintain weight loss, are still uncertain. 

Fact Sheet and Resources

For “Intermittent Fasting”:

Google Search: About 63,200,000 results

PubMed Search: About 200,819 results

Google Scholar Search: About 124,000 results

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