What Is Intermittent Fasting? A Detailed Beginner’s Guide
By now, you’ve no doubt heard of intermittent fasting (IF). Maybe your brother skipped out on brunch the last time you got together because it was too early for him to eat. Or maybe your friend couldn’t do a late dinner last time you saw her.
Although IF has become part of the popular diet lexicon in recent years, fasting overall is nothing new. Hippocrates was reportedly the first person to use fasting in the fifth century B.C. to treat illness, and it is an essential part of many religious traditions, including in Islam.right up arrow There are many reasons why you might try fasting, or specifically IF, from weight loss to wellness. Use this scientific guide to get the lowdown on IF specifically. You’ll also find tips for how to set yourself up for success if you decide to start.
Unlike some other diets, intermittent fasting doesn’t have a long list of rules. Instead, the approach is all about “entirely or partially restraining or abstaining from eating during a specific period of time,” says Heather Bauer, RDN, founder of Heather Bauer Nutrition in New York City.
In other words, IF involves pauses from eating. While some people find that they enjoy IF, this is not the right diet for everyone, she says.
How Intermittent Fasting Works You choose how you want to do IF by deciding which days of the week you will fast. On fasting days, you’ll likely follow a severe calorie-restricted diet or you may not eat at all. You can also fast for a certain time every day. Ultimately, this results in consuming fewer calories over the course of the week, and some experts, including Caroline Susie, RDN, a spokesperson for the Academy of Nutrition and Dietetics in Dallas, say that this calorie reduction is what sometimes leads to weight loss and then potentially additional metabolic benefits.
Types of Intermittent Fasting There is no one standard way to practice IF. “Intermittent fasting is an umbrella term for three different types of diets,” says Krista Varady, PhD, a professor of nutrition at the University of Illinois in Chicago and a researcher on intermittent fasting. Here’s what you’re most likely to see, she says:
Alternate-Day Fasting For the most common type of alternate-day fasting, you eat 500 calories every other day. On off days, you can eat what you want.
5:2 Diet Popular in the United Kingdom, you consume 500 calories on two nonconsecutive days per week. On the other days, you eat whatever you like.
Time-Restricted Eating You choose a window of time during which you can eat (feast); the rest of the day you don’t eat (fast). One popular setup is 16:8, which means you fast for 16 hours and you can eat during the other eight hours. For instance, you might set your eating window from 12 noon to 8 p.m. daily. (This could also be called skipping breakfast.)
Potential Health Benefits of Intermittent Fasting Overall, rigorous, long-term research on intermittent fasting is still limited, and many of the conclusions you’ll read online are based on animal studies, says Susie. It’s unclear whether any type of IF is truly safe or effective in the long run.right up arrow So proceed with caution. With that in mind, here’s how IF may benefit you:
Heart Disease Prevention Though more research is needed, one review concluded that IF is promising for improving cardiovascular health because it decreases cardiovascular risk factors, such as obesity, high blood pressure and cholesterol, and diabetes.right up arrow Right now, say researchers, it’s unclear which type of IF is best for heart health. Treat Type 2 Diabetes IF may be a promising treatment for type 2 diabetes.right up arrow Fasting aids in weight loss, decreases insulin resistance, and favorably affects hormones released by fat cells that impact appetite and inflammation levels. That said, if you have diabetes, you should not attempt IF on your own without talking to your doctor first. Fend Off Alzheimer’s Disease and Stroke While research is still ongoing, some studies find that intermittent fasting may help decrease the risk of stroke and Alzheimer’s disease.right up arrow That said, much of this understanding comes from animal research, and it’s not clear when one should start IF during the course of their life to decrease the risk of neurological conditions. What’s more, despite what proponents say about IF improving cognitive abilities, such as focus, IF does not appear to be a short-term brain-booster. Improve Nonalcoholic Fatty Liver Disease In a systematic review and meta-analysis of six studies, people with nonalcoholic fatty liver disease (liver disease seen in people with obesity, diabetes, and metabolic syndrome) who practiced IF saw an improvement in liver function tests, compared with individuals who didn’t fast. The authors concluded this was because IF leads to weight loss.right up arrow Nonetheless, the researchers point out that more large-scale, randomized studies are needed before making specific recommendations. Weight Loss Effects of Intermittent Fasting When on IF, you’re simply eating during fewer time periods, whether that be fewer hours in the day or fewer days of eating. “In our research, we’ve found that time-restricted eating naturally cuts out several hundred calories per day,” says Dr. Varady. That roughly results in losing about one to two pounds per week, she has observed in her research. It’s similar to doing a calorie-restricted diet every day, but a touted benefit is that you don’t have to count calories.
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One of her studies found that people who reduced their eating window to eight hours per day consumed about 300 fewer calories and lost about 3 percent of their body weight over 12 weeks. On the other hand, alternate-day fasting may help someone cut 25 to 35 percent of their daily calories (over the course of the week) and leads to weight loss of 4 to 6 percent of body weight over 12 weeks. right up arrow Another review of 11 meta-analyses concluded that IF, particularly alternate-day fasting, was beneficial for helping overweight or obese adults decrease their body mass index (BMI), a measure of body weight, better than a regular diet.right up arrow “With alternate-day fasting, the weight comes off twice as fast,” says Varady. The thing is, compared with time-restricted eating, many people find alternate-day fasting more difficult to sustain and fit into real-life scenarios, she says. In Varady’s research, 30 to 40 percent of people dropped out of alternate-day fasting studies, she says. Conversely, time-restricted eating only has a dropout rate of 5 percent in Varady’s research. That said, more long-term data (based on following people for one to two years) is needed. The majority of Varady’s published research has lasted a maximum of six months.
Intermittent Fasting Side Effects Before jumping into an entirely new way of eating, be aware that there are some side effects you may experience, says Susie:
Headaches Fatigue Low energy Irritability Hunger These can all happen because you’re not eating, says Susie. While they’re normal, they are uncomfortable, and it can affect your day-to-day sense of well-being. Some people will find that these are not good trade-offs and will choose to stop IF. This is completely okay. It’s not for everyone.
One note: Expect hunger to peak during the first 10 days, then decrease as your body adjusts to a new pattern of eating, says Varady.
Health Risks of Intermittent Fasting Do not try intermittent fasting if you are pregnant or breastfeeding, says Bauer. Similarly, if you have a low (underweight) BMI or a history of an eating disorder, IF is not right for you. Varady also cautions adults older than 70 against trying fasting, as it can lead to muscle loss, which is critical to preserve in older age.
Also, if you have any type of diabetes, speak to your doctor first, as this may not be safe for you. Skipping a meal can lead to low blood sugar, which can cause dizziness, fainting, and falls.right up arrow If you are on any medication that needs to be taken at a certain time and consumed with food, you’ll also need to connect with your doctor.