Intermittent fasting has become one of the most buzzed-about diets over the last several years. (The other? Keto, we're looking at you.)
In its simplest terms, intermittent fasting—or IF, for short—doesn't sound revolutionary: You designate a certain number of hours per day where you don't eat.
But what really happens to your body when you do IF? Will you lose weight? Feel hungry and irritable? Here's a look at what you might expect.
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As Harvard T.H. Chan School of Public Health explains, "intermittent fasting is a diet regimen that cycles between brief periods of fasting, with either no food or significant calorie reduction, and periods of unrestricted eating."
IF restricts the time you're eating, but it has no say in the type of food you eat or how much—that's up to you.
What's tough about IF is that there are numerous ways to do it, and if you're interested in IF, you'll want to land on the version that works best for your preferences and lifestyle.
Kaitlin Poillon, M.S., RDN, the Nutrition Counseling Services practice manager and outpatient registered dietitian at Drexel University in Philadelphia, notes that there are several popular versions, including time-restricted feeding (e.g., fast for 16 hours per day and eat for 8 hours per day; some people do longer or shorter fasting windows), whole day fasting (fast completely for 1-2 days or consume severely restricted calories), and alternate day fasting (similar as whole day, but you fast every other day).
Your body is used to eating at a certain time. And if you've decided that you're not eating until noon and you've always had breakfast at 8 a.m., then your body is going to know something is missing. "A delay in eating will likely lead to low blood sugar and feelings of increased hunger and irritability," says Poillon. Luckily, she notes, your body will adjust to your new eating schedule as time goes on. Another way you can make the transition easier to a time-restricted style of IF is to delay breakfast (or, conversely, move up your dinnertime) gradually, so the change is not as abrupt.
While the beginning may be tough, over time you may notice that you can handle the fasts better and/or are satisfied with less food when you do eat. "Some studies show that time-restricted eating may lower ghrelin levels, which is the hormone that signifies feelings of hunger in the body," says Poillon. In addition, she says, leptin levels may also increase, which is a hormone that tells the body when it is full. In the end, with lower ghrelin and higher leptin, you may naturally eat less rather than struggle with hunger.
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Insulin is a hormone made by the pancreas and is released after you eat food in response to rising blood sugar levels, per the CDC. Insulin's job is to "unlock" cells in your muscles, liver and fat and push glucose from your blood into those cells, thereby giving you energy now or storing it for later. There's a danger, though, in having insulin levels that are too high, as it can increase your risk of obesity, type 2 diabetes and cardiovascular disease, 2020 research published in BMC Medicine shows.
The benefit of IF is that it also acts on insulin. "Periods of fasting will decrease the amount of insulin circulating in the bloodstream because it will not need to be released as often," says Poillon. "This does not only help burn fat but also lowers the risk of developing certain diseases such as prediabetes and diabetes."
If you're aiming to lose weight, IF may help you get to your goal. A 2020 review of 27 IF trials published on the journal Canadian Family Physician found that people lost anywhere from less than 1% of their body weight to 13%. On average, people lost 4.3% of their body weight when they followed IF from 2 to 12 weeks. Some of these studies also compared IF to traditional calorie restriction and found that they were equal in terms of weight loss.
Overall, IF may work as a form of calorie restriction. With fewer hours in the day to eat, you may, in turn, consume fewer calories. (Though this all depends on what you eat exactly.) That said, Pollion also notes that a decrease in insulin and a change in appetite hormones may also help support weight loss.
Ultimately, intermittent fasting may be good as a weight loss jumpstart because it can help you lose weight faster than other diets in the beginning, according to 2020 research in the journal Nutrition. However, there is no one "perfect" weight loss strategy for everyone; the authors point out.
Among adults have overweight or obesity, certain types of IF helped adults decrease their body weight by 5% and lose fat, as well as lower "bad" LDL and total cholesterol, triglycerides, fasting glucose and insulin and blood pressure found a large umbrella 2021 review in JAMA Network Open. Only certain types of IF lead to these benefits, including "5:2" (eat for five days, and consume a very low-calorie diet for two) and modified alternate-day fasting (an every-other-day fast where you are eating 0 to 600 calories on fasting days).
In the end, IF may be a bona fide treatment for diabetes and fatty liver disease, say the authors. The only catch was that people were most successful with IF in the first one to six months. After that, people didn't lose any more weight, in part because they may have trouble following the diet long-term.
One important concern many health care experts have regarding popular nutritional trends is mental health and disordered eating. Restricting food during periods of time can increase the risk of overeating or binge-eating. Additionally, another concern is the lack of guidance regarding food choices. And while some people may meet their nutritional needs, others could be missing out of very important ones.
Even though IF doesn't guide you with food choices, what to eat still matters, says Poiillon. "A focus on adequate lean protein intake, whole foods, fiber and water is imperative," she says. Eating protein, fiber and healthy fats at meals will slow digestion to help prevent large insulin spikes at meals when you are eating. And hydrating with at least 64 ounces of fluid every day will help keep your body running as it should whether you're fasting or eating, Poillon adds.
Intermittent fasting may lead to feelings of hunger that makes it challenging to stick with at first. However, research shows that the diet may be able to help you lose weight and improve your metabolic health. Make sure you're focusing on eating a balanced, healthy diet during eating periods, which will help you feel your best.
It's highly important to note that fasting is not for everyone, especially in pregnancy and people with eating disorders. Speak to your health care provider before trying out IF.
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