Let’s Talk About Intermittent Fasting Benefits
Intermittent fasting might help with weight loss and improve certain chronic health conditions, but it comes with some risks as well. Learn more about the pros and cons, here.
Trendy diets and exercise fads that promise weight loss and better health come and go. But intermittent fasting, or the practice of restricting eating to specific times of the day or week rather than cutting calories, has been around for ages. What gives it such staying power? And is it worth a try? Let’s take a closer look at intermittent fasting’s benefits—and a few of its disadvantages, too.
Our Pro Panel
We went to some of the nation’s top intermittent fasting experts to bring you the most up-to-date information possible.
Vanessa Rissetto, R.D.
Registered dietitian and co-founder
Culina Health nutrition and health clinic
New York City
Christine Bishara, M.D.
Internal medicine doctor, certified in integrative medical weight loss
New York City
Steven Gundry, M.D.
Cardiothoracic surgeon; medical director
International Heart and Lung Institute Center for Restorative Medicine
Palm Springs, CA
It’s definitely normal to go through an adjustment period: Headaches and hunger pangs can occur, but usually even out after a few days. One way to help temper them? Water! Don’t forget to drink just because you aren’t eating as frequently. And stock up on fiber: When you focus on consuming nutritious, healthy, fiber-filled foods during your eating windows, you’ll feel fuller and less famished during your fast.
We’re not going to lie: It can be hard to be around people who are eating when you’re fasting. But that doesn’t mean it has to be torture. If you have a breakfast meeting, opt for an unsweetened/milk-free tea or coffee, which are allowed while you’re fasting. Another option: Replace food-related events (like brunch dates) with activities like a walk in the park.
No, coffee won’t break your fast if you take it without milk or sugar. (It’s the “extras” with coffee that have the calories—on its own, black coffee has two calories per cup.) Herbal tea is another option if you're craving some taste without any calories.
No, children shouldn’t practice intermittent fasting. Experts caution against kids fasting because it’s important for them to learn to listen to their hunger cues while growing. If you’re worried about your child’s weight, talk to their pediatrician to explore other options for improving their habits and increasing physical activity.
Remind Me, What Is Intermittent Fasting?
With everyone from Hollywood celebrities to Instagram influencers talking about intermittent fasting, it can feel like you’re the only one who’s not doing it. But what is it exactly?
Intermittent fasting is a way of eating that alternates between periods of eating and periods of fasting. It doesn’t prescribe which foods to eat or give up, but rather when to eat and when to let your body’s digestive system rest. This way of eating is actually nothing new. Our prehistoric ancestors did it naturally when hunting and gathering food. IF has also long been part of religious traditions and medical treatments. The ancient Greeks and Romans believed it allowed the body time to heal itself, and today, surveys suggest intermittent fasting is more popular than gluten-free regimens or traditional calorie-restricting weight loss plans.
There are several different types of intermittent fasting, but these are the three most popular types:
Time-Restricted or 16:8
This plan calls for structuring your day around an 8-hour eating window and a 16-hour fasting period. This means, for instance, that you stop eating at, say, 7pm and start again at 11am the next day. You can also start with a 12 or 14-hour fasting period and stretch it as you adjust.
Modified-Calorie or 5:2
This plan calls for following your normal eating routine (breakfast, lunch, and dinner, or two meals and a few snacks) for five days out of the week and then fasting for two days, which can be consecutive or spaced out. On fasting days, women can eat up to 500 calories (for men, 600).
Alternate-Day Fasting (ADF)
Here, you alternate one day of your regular caloric intake followed by a day in which calories are restricted to less than 500 calories for women and 600 for men. Keep in mind that all fasting plans allow for water, unsweetened/milk-free tea, and coffee to be consumed during the fasting windows.
The Benefits of Intermittent Fasting
Given the buzz about the health benefits of intermittent fasting, it’s no wonder everyone wants to know what it can do for them. While experts say there is no one-size-fits-all guarantee for how changes to diet and lifestyle might affect you versus your neighbor, in general, these are some of the intermittent fasting benefits you might experience:
Easier Weight Loss
One of the main appeals of trendy eating plans is the promise of dropping pounds, and intermittent fasting is no different. But unlike traditional diets, with IF there are no forbidden foods or strict calorie counting. Instead, intermittent fasting helps you lose weight simply by compressing the times of the day when you consume food and allowing for a substantial resting period (or “fast”) in between.
When you eat all day long, your body has a continuous stream of new glucose in the blood that it can use for energy (and any surplus gets stored for later use). When you stop eating for an extended period of time (during your fast), your body still needs energy so it finally gets a chance to burn those stored carbs and, when that gets used up, it turns to burning stored fat. The result: Increased fat loss.
Longer Life and Healthy Aging
It’s a natural fact: cell turnover slows down as we age. It can take longer for wounds to heal and unwanted cellular proteins can build up, which can play a role in neurodegenerative diseases such as Alzheimer’s and Parkinson’s.
However, studies show that fasting kickstarts the biological process called autophagy, or the breakdown and purging of damaged cells and cellular material in your body. The length of fasting necessary to trigger autophagy varies person to person because of differences in metabolism and lifestyle. For some people, autophagy can start by about eight hours into a fast; for others, it can take closer to 16 or 18 hours. Either way, the results—which may include better muscle performance, an increase in energy, and healthier skin and hair—have made many people believers in intermittent fasting.
More Efficient Metabolism
Every time you eat, the hormone insulin is released by the pancreas, the organ that produces digestive enzymes and hormones. It matches up with the sugar in the foods you just ate to help deliver glucose to your cells for fuel. But when you eat 12 or more hours a day (i.e. from breakfast at 7 am until dinner at 7 pm, or even later), your body maintains a high level of insulin in the bloodstream, which can lead to insulin resistance. And you don’t want that because when your body stops responding to insulin, your blood sugar level rises, which can increase your risk of developing diabetes. Fasting, however, can help improve your insulin sensitivity by decreasing the amount of insulin circulating in your blood, which helps your metabolism function more effectively.
Ask around and you will hear people who practice intermittent fasting rave about how much they get done in the morning. That may be because some research has found a correlation between fasting and improved cognitive function, learning, memory, and alertness. How it works: Fasting stimulates the production of a protein in nerve cells called brain-derived neurotrophic factor, or BDNF, which increases the resistance of neurons in your brain to degeneration and dysfunction. (Translation: Fasting protects health brain cells by reducing oxidative stress and improving their resistance to stress.)
Risks of Intermittent Fasting
While intermittent fasting can help you feel great and improve your health, the approach is a bit extreme, depending on which version of the plan you are following. Most people learn to tolerate it and even eventually thrive on it, but it’s not for everyone. Here are some of the ways intermittent fasting may be a disadvantage.
Disordered Eating Habits
Restricting when you can eat instead of simply eating when you’re hungry can set up a prickly relationship with food, especially if you have struggled with an eating disorder. If you’ve struggled with restricting food in the past, it may be best to skip intermittent fasting and focus on listening to your body cues regarding hunger and satiety. As with most dietary changes, it’s always good to consult a medical professional first to get the greenlight.
Food Cravings and Overeating
Some people can handle time-controlled eating and are able to alternate between enjoying food and restricting food with no problem. Their bodies become accustomed to filling up during compressed eating windows, and they may actually experience a decline in hunger. For others, however, the floodgates open the moment they take that first bite during their window—especially if it’s later in the day—and they have a hard time governing their calorie intake. If you find you’re overeating or bingeing during your eating periods, IF may not be the best fit for you.
The truth is, work commitments and real life don’t always cooperate with intermittent fasting—and that can make it hard to sustain. If you have a family, prepping meals for others during your fasting window can be torture. Plus it’s not always possible to say no to breakfast or dinner meetings that fall outside of your self-imposed eating window. Traveling for business and vacations can also upend your eating plan by disrupting your natural circadian rhythms, leaving you famished at unexpected times.
Could fasting actually cause your metabolism to slow down? Studies show that restricting too long or not consuming sufficient calories might actually coax your body into defending its energy stores. The result? More intense hunger pangs and fewer calories burned, along with a feeling of sluggishness from not enough fuel being added to your internal furnace.
Some fasters report digestive challenges such as constipation from taking in less food. Dehydration is another risk because a reduced level of insulin in your body may trigger the kidneys to release extra water. Plus, it can be easy to forget to hydrate while you’re steering clear of food.
Bottom line? Intermittent fasting can be an effective tool for improving your health and losing weight. But it’s essential to consider all the benefits and disadvantages of intermittent fasting before giving it a go. If you’re not sure it’s for you or don’t know where to start, chat with your healthcare provider about which eating plan might suit you best and what red flags to watch out for as you get started.
Intermittent Fasting Popularity: International Food Information Council. (2021.) “2020 Food and Health Survey.” https://foodinsight.org/wp-content/uploads/2020/06/IFIC-Food-and-Health-Survey-2020.pdf
Intermittent Fasting and Neurodegenerative Diseases: Experimental Biology and Medicine. (February 2018). “Intermittent Fasting Protects Against the Deterioration of Cognitive Function, Energy Metabolism and Dyslipidemia in Alzheimer's Disease-Induced Estrogen Deficient Rats.” https://journals.sagepub.com/doi/10.1177/1535370217751610
Health Advantages of Intermittent Fasting: New England Journal of Medicine. (2019.) “Effects of Intermittent Fasting on Health, Aging, and Disease.” https://www.nejm.org/doi/full/10.1056/NEJMra1905136
Health Benefits of Autophagy (1): Autophagy. (2010.) “Short-Term Fasting Induces Profound Neuronal Autophagy.” https://doi.org/10.4161/auto.6.6.12376
Health Benefits of Autophagy (2): Nature. (2006.) “Good Riddance to Bad Rubbish.” https://www.nature.com/articles/441819a
Intermittent Fasting and Metabolism (1): Annual Review of Nutrition. (2017.) “Metabolic Effects of Intermittent Fasting.” https://www.annualreviews.org/doi/full/10.1146/annurev-nutr-071816-064634
Intermittent Fasting and Metabolism (2): Molecular and Cellular Endocrinology. (2015.) “Do Intermittent Diets Provide Physiological Benefits Over Continuous Diets for Weight Loss? A Systematic Review of Clinical Trials.” https://www.sciencedirect.com/science/article/abs/pii/S0303720715300800
Intermittent Fasting and Improved Brain Functioning: Annual Review of Nutrition. (2005.) “Energy, Meal Frequency and Health: A Neurobiological Perspective.” https://www.annualreviews.org/doi/full/10.1146/annurev.nutr.25.050304.092526#_i16
Intermittent Fasting and Disordered Eating: Journal of Abnormal Psychology. (2008.) “Fasting Increases Risk for Onset of Binge Eating and Bulimic Pathology.” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2850570/
High Blood Sugar and Diabetes: Diabetes Care. (2011.) “Is Hyperglycemia a Cardiovascular Risk Factor?” https://care.diabetesjournals.org/content/34/Supplement_2/S128