Intermittent Fasting

What the Science Says About This Trendy Way of Eating

Intermittent Fasting

Fasting, for cultural or religious reasons—or because the hunt was unsuccessful—is as old as the hills. As a healing agent, fasting dates at least as far back as Hippocrates, who is said to have declared that “To eat when you are sick is to feed your illness.” Lately, people wishing to lose weight and optimize their health have embraced various forms of intermittent fasting, eating patterns structured around short-term fasts and time-restricted dining.

“Intermittent fasting is being touted to fix problems that might have developed over decades of unhealthy eating habits, to get you on the road you wish you had followed,” says Ed Blonz, Ph.D., assistant clinical professor in the Department of Clinical Pharmacy at the University of California, San Francisco, and a member of our editorial board. Among the claims made are that intermittent fasting will help with weight loss, heart health, insulin resistance, cancer prevention—you name it. “And though it shows some promise as a strategy to achieve certain short-term goals with appropriate guidance, it must be stressed that its long-term effects are not yet known,” Dr. Blonz adds.

Some studies have found intermittent fasting feasible and practical, and some people may find this eating pattern easier to adhere to than more restrictive diets, such as daily calorie restriction, ketogenic diets, or veganism. But if you’re considering adopting some form of intermittent fasting for weight loss or other medical reasons, it’s important to consider your lifestyle, your overall health status and comorbidities (other medical conditions you may have), and whether you are on any medications that need to be taken with food, among other factors.

Also, bear in mind that intermittent fasting is not without potential discomfort and risk. People on such diets have reported headaches, dizziness, weakness, and low blood sugar (hypoglycemia), especially initially. And fasting is not recommended for people who have immune deficiencies, dementia, or an eating disorder (or history of one), or for anyone who is underweight, pregnant or breastfeeding, or frail. Fasting is also not a good idea for children or people who do work that has heavy physical demands. Adolescents should fast only under medical supervision.

Intermittent fasting comes in different forms

The term intermittent fasting refers to eating patterns that involve fasting for varying lengths of time, typically 12 to 40 hours, either on certain days of the week or for a specified window of hours each day. During the fasting portion, calorie intake is sharply reduced (to 25 percent of usual calories, for instance) or entirely avoided. 

Some popular types of intermittent fasting include 5:2, where you eat normally (“feast”) five days a week and consume only about 500 calories on two non-consecutive “fast” days a week; alternate-day fasting (ADF), with little or no food intake every other day; and time-restricted eating (TRE), in which you abstain from food for a set number of hours a day and eat your meals (two or more) within the remaining hours. One popular TRE method is 16/8, which involves a 16-hour daily fast and an 8-hour eating window. In other types of TRE, you may fast for 12 hours a day, for instance.

Fasting for weight loss

It makes sense that intermittent fasting, which restricts calories on fasting days, would result in weight loss, as long as dieters don’t overcompensate on non-fast days. After all, people who consume fewer calories over an extended time generally lose weight.

And when you consider the way many Americans eat, with between-meal and post-dinner snacks and drinks, restricting the time in which you allow yourself to eat could work as a reasonable weight-loss strategy. A study in Cell Metabolism in 2015, for instance, found that participants consumed about 20 percent fewer daily calories when they narrowed their eating window.

And a review of studies published online in February in Nature Reviews: Endocrinology found more support for intermittent fasting at least for short-term weight loss. It identified 22 clinical trials that included anywhere from 10 to 150 participants who followed 5:2, alternate-day fasting, or time-restricted eating patterns for 5 to 12 weeks or so. The authors concluded that these three approaches led to weight loss of 3 to 8 percent over 8 to 12 weeks, similar to results from traditional calorie-restricted diets.*

But achieving weight loss with any form of intermittent fasting will depend on what you eat when you’re not fasting, and how compliant you are with the diet. If you fast one day but fill the next day with hamburgers, macaroni and cheese, fries, and ice cream—or even an excess of healthy, high-calorie foods like avocados and olive oil—you may more than compensate for the calories you didn’t consume the day before.

Some studies have found slightly greater weight loss with traditional calorie restriction (reducing average daily calories) but note that long-term calorie restriction is often difficult and frustrating because it requires vigilant calorie counting on a daily basis, while intermittent fasting may be easier to stick with, at least for some people, because it allows for free eating during non-fasting periods.

Most recently, a study from China, published in the April 21 issue of the New England Journal of Medicine, followed 139 people with obesity for 12 months. About half of them engaged in time-restricted eating (eating only between 8:00 a.m. and 4:00 p.m.), while also restricting calories. The other half restricted their daily calories but without prolonged fasting periods. The time-restricted-eating group lost about four more pounds than the calorie-restriction-only group, but the difference was not statistically significant. The researchers concluded that the two weight-loss regimens “had similar success in patients with obesity, regardless of whether they reduced their calorie consumption through time-restricted eating or through calorie restriction alone.”

Fasting for cardiometabolic benefits

Intermittent fasting may help protect against type 2 diabetes as it has been shown to lower characteristic features of the disease, including blood sugar levels and insulin resistance. A review of studies, published in Translational Research in 2014, reported decreasing fasting insulin levels of 20 to 31 percent in people with prediabetes after 8 to 12 weeks of intermittent fasting.

This could have implications for heart health, too, as insulin resistance is associated with several risk factors for poor cardiac health. Intermittent fasting may also have a positive effect on other risk factors for heart disease, such as cholesterol, triglycerides, inflammation, and blood pressure, though some studies have not shown benefits.

However, to date there is insufficient evidence to give intermittent fasting all the credit since improvements in heart health seem to correlate most strongly with the amount of weight lost rather than how it was lost, whether through some form of intermittent fasting or through more traditional calorie restriction.

Fasting to prevent cancer?

Excess body fat is associated with an increased risk for various cancers, including breast, colon, kidney, pancreatic, and endometrial cancer, and intermittent fasting has been shown effective in preventing cancer in some animal studies. It’s not clear that intermittent fasting can play a role in cancer prevention in humans, aside from the protection that weight loss might afford, but it is an area that warrants research, says Dr. Blonz. 

To fast or not to fast?

If you’re considering joining the ranks of those who engage in regular short-term calorie restriction or time-restricted eating, consider what you hope to accomplish and how feasible that might be. At this point, most studies on intermittent fasting are small and have assessed only short-term effects (up to a few months, with just a few studies lasting a year).  Moreover, though preliminary findings suggest that intermittent fasting may be helpful for weight loss, studies on whether it can treat or prevent diabetes, heart disease, or other metabolic disorders have had variable results or have not shown benefits. It’s also unknown whether any specific fasting protocol is more effective than others. In short, there is no consensus among experts about what the best diet is for weight loss, health, and longevity.

If you have diabetes or other medical conditions, check with your doctor before considering any type of fasting regimen. Intermittent fasting is not appropriate for children and should be supervised in adolescents and other at-risk people. According to the 2022 Nature Reviews: Endocrinology review, preliminary studies have found that intermittent fasting produces few adverse gastrointestinal, neurological, hormonal, or metabolic effects, but the authors also note that such outcomes are not often assessed in such trials, so “definitive conclusions regarding the safety of these diets are difficult to draw at present.”

Intermittent fasting may take getting used to, and it doesn’t suit everyone. It can affect your social life—dinner can get awkward when one diner at the table is having only water. And the possible ill effects of restrictive eating (hunger, lightheadedness, irritability) could make following an intermittent fasting way of life unsustainable, though such side effects often subside after an adjustment period of one to two weeks.

But even if it’s not something that you do forever, experimenting with intermittent fasting might help some people concerned about their eating habits hit a dietary reset button and make better food choices as they pay closer attention to when and what they eat. As with any diet, the key to staying healthy on it is to choose high-quality, nutrient-dense foods during the eating windows (avoid junk food), with an emphasis on vegetables, fruits, whole grains, and legumes. In that way, this dietary approach may serve as a bridge to long-term healthier eating.

Not So Fast: Some Practical Considerations

If you try intermittent fasting, beware of self-defined “experts” with a personal story and possibly something to sell (which could be themselves) but no academic training in health sciences. There is an abundance of intermittent fasting “gurus” on social media. Here’s more practical advice, mostly from the Nature Review: Endocrinology review:

  • To control hunger and minimize loss of lean mass (muscle), consume at least 50 grams of lean protein on fast days.
  • Be sure to eat plenty of fruits, vegetables, whole grains, and legumes to get enough fiber and essential micronutrients.
  • Stay adequately hydrated. If you don’t drink enough water, you may experience more headaches during your fasts. Alcohol is not recommended on fast days. Limit diet sodas to two servings a day on fast days because they may increase sugar cravings; caffeine (from coffee and tea) is okay.
  • It’s a good idea to be assessed by your doctor in the first few months to make sure you’re not becoming deficient in electrolytes or in nutrients such as B12 and vitamin D. If you lose weight, your doctor should assess your medications since some, such as those for high blood pressure, cholesterol, and blood sugar, might need adjusting.
  • For a better chance of long-term weight management, structured behavioral change programs are recommended. This could be any individual, group, or online program that sets diet and physical activity goals and teaches behavioral skills such as problem solving and stimulus control related to eating.

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