Intermittent Fasting
Intermittent fasting has been shown to be a very practical and effective way to lose weight and burn fat. But, as with anything that works, it is important to consider what might go wrong (side effects, pitfalls, etc.) and make changes accordingly.
Intermittent fasting, or IF, has been around for a while now. From the days of the caveman, to the Stone Age, right up until right now. No wonder then that intermittent fasting is increasingly being used as a weight loss method by people all over the world. Whether you’re looking to jump start your weight loss journey, or continue it with an alternative dieting style — you’ve come to the right place.
Pretty much any health instructor, nutritionist or fitness coach will be quick to tell you the benefits of intermittent fasting for weight loss. They will also emphasize that it is not a diet, but rather a way of life. So, what is intermittent fasting? In simple terms, this is a diet aid and eating strategy which requires participants to cycle between periods of fasting and feeding, in order to promote weight loss and improve health. But, is there any scientific evidence to support the claims that intermittent fasting can, in fact, really help people lose weight?
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Can you lose weight with intermittent fasting?
Research has found a link between intermittent fasting and weight loss, but there isn’t much research to prove that intermittent fasting is a better weight loss method than other diets. For example, a meta-analysis published in the JBI Database of Systematic Reviews and Implementation Reports in 2018 found that intermittent fasting had similar weight loss results to a traditional calorie-restricted diet.
In some cases, even when intermittent fasting did lead to weight loss, it wasn’t the most sustainable diet. One randomized clinical trial of 100 metabolically healthy, obese adults (published in JAMA Internal Medicine in 2017) found that those who fasted every other day for a year only lost slightly more weight than those who ate a restricted-calorie diet on a daily basis. The difference in weight loss was so small that it wasn’t considered clinically significant, and 38 percent of people in the alternate-day fasting group had trouble sticking with the diet.
It’s also worth pointing out that weight loss due to intermittent fasting isn’t guaranteed to last. “Some people may experience weight loss in the short-term, but many people eventually gain that weight back,” Rumsey says.
And, most intermittent fasting research has only been done on people who are obese. “There is almost no credible research that shows intermittent fasting is good for people at a healthy weight,” says Keatley. If you’re curious about intermittent fasting but don’t have much weight to lose, it might not be the best diet for you.
Are there any long-term health benefits of intermittent fasting?
There is a growing body of research that suggests intermittent fasting does have health benefits outside of weight loss. One study published in Cell Metabolism in 2018 linked intermittent fasting and lower insulin levels and blood pressure. Researchers followed a small group of obese men with prediabetes—some were put on a 16:8 diet, while others ate over a period of 12 hours. Both groups didn’t gain or lose weight. But after five weeks, the men in the 16:8 group had much lower insulin levels and better insulin sensitivity. They also significantly lowered their blood pressure and said they had decreased appetites. They weren’t as hungry as they were before—even though they were fasting.
It might seem counterintuitive, but appetite control is a big benefit of fasting. A recent study in the journal Obesity showed that people who ate only during a six-hour window, compared to following a normal eating schedule, felt less hungry than the control group, even though both groups ate the same amount of calories. Intermittent fasting has also been linked to increased neuroplasticity, or the brain’s ability to form new synaptic connections and fight injury.
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Unfortunately, there hasn’t been a ton of research on humans that follows the effects of intermittent fasting long-term. While some studies have followed participants for a year, that’s about as long as most go. It’s a slightly different story when it comes to rats. “In rodents, intermittent fasting has been shown to prevent age-related diseases, including tumors, heart disease, diabetes, dementia, and even extend lifespan,” says Keatley. But, he adds, “there is a big difference between rats and humans, and the research does not show these benefits enough to actually recommend it as the fountain of youth.”
What foods can I eat and what do I have to avoid?
This is a huge perk of intermittent fasting: You technically don’t have to alter what you eat—you just have to eat within a certain window of time. “On an intermittent fasting diet you don’t need to avoid any foods,” Gans says. “However, one should focus on consuming foods that are high in fiber and healthy fats for satiety, such as quinoa, black beans, spinach, strawberries, avocado, and almonds, and of course other nutrient-dense foods for overall health.”
At the same time, it’s a good idea to limit sugary drinks, processed foods, and simple carbs like white bread, advises Cording. “They’re not providing you with much nourishment,” she says. “You really want to focus on foods that do you a favor.”
That said, plenty of people have paired intermittent fasting with another type of diet, like keto. Again, it’s not a requirement, but doing intermittent fasting while also altering your diet could help you get results faster.
Does intermittent fasting have any side effects?
There are a few things that can happen when you follow an intermittent fasting diet. One thing to note is that fasting can interfere with your hunger cues. “Since people are only allowing themselves to eat in a certain window of time, they are completely ignoring their internal cues of hunger,” Rumsey says. “It causes people to disregard hunger cues, which then means once they are ‘allowed’ to eat, they are starving and it can be hard to stop eating.” This could lead to an unhealthy obsession with food for some people, Rumsey says.
Intermittent Fasting Side Effects To Watch Out For
Some fasters find that the diet makes them feel sharper and more alert, but alternately, fasting for long periods can also lead to mental fogginess, disrupted sleep, and decreased alertness, Rumsey says. And, she adds, fasting for extended periods of time “takes a large toll on your blood sugar levels,” causing you to flip-flop between low blood sugar and a spike when you eat again.
Proposed Health Benefits of Intermittent Fasting
When it comes to weight loss, there are two thoughts behind why IF has the potential to work. The first: “Periods of fasting produce a net calorie deficit, so you lose weight,” explains Rekha Kumar, MD, a specialist in endocrinology, diabetes, and metabolism at the Comprehensive Weight Control Center at Weill Cornell Medicine and NewYork–Presbyterian in New York City.
The other concept is more complex: This approach may prevent what’s called the “plateau phenomenon” from happening, Dr. Kumar says.
You may remember the famous, so-called Biggest Loser study. The researchers followed up with participants from the TV show after six years, and despite their initial impressive weight loss, they had regained most of the weight, and their metabolic rates had slowed, such that they burned far fewer calories than would have been expected.
Though more research is needed on the safety and effectiveness of IF, one of the touted benefits of this approach is that it may prevent this metabolic sputtering. “Most people who try diet and exercise to lose weight tend to fall off the wagon and regain weight,” Kumar says. “Hormones that promote weight regain, like hunger hormones, are kicked into full gear, and the thought is that IF may be a way to prevent this metabolic adaptation from happening.” The idea is that the normal periods of eating in IF “trick” your body into losing weight before the plateau happens.
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So, does it actually lead to weight loss? Anecdotal evidence has led proponents of the plan to believe so. “For the people who can adhere to IF, it does work,” Kumar says. But fans of the approach claim there’s so much more to IF than just a lean body. Lori Shemek, PhD, a nutrition and weight loss expert in Dallas and author of How to Fight FATflammation, explains to clients that IF may improve their insulin sensitivity (lowering the risk of type 2 diabetes), reduce inflammation, and “boost longevity by bettering the health of your mitochondria (cell powerhouses),” she says.
Research has found that IF can lead to weight loss of 1 to 8 percent from the starting weight, which is comparable to the amount of weight loss expected while on a calorie-restrictive diet. IF may also improve other areas of cardiometabolic health, such as lowering blood pressure and reducing insulin resistance.
Also, separate research analyzed 11 IF trials that were at least eight weeks long and involved adults who were considered obese or overweight.
Nine of those studies showed that an IF program was as effective as traditional dieting (restricting calories every day) at helping participants lose weight and body fat. Finally, another study found that 12 weeks of IF didn’t affect cholesterol levels, but it did lead to weight loss and decreased systolic blood pressure.
That said, it’s important to note that studying human longevity is much more complicated than simply looking at weight loss. That’s why much of the research that suggests IF promotes a longer life span has been done in animals, including fruit flies.
Other research has suggested that the metabolic advantage of IF is that it shifts your body into a state of ketosis (the process involved in the keto diet), which burns fat, rather than carbohydrates, for fuel.
Beyond the weight loss effects, researchers have said, is the idea that ketones may trigger the body’s own repair system, ultimately protecting against disease and aging.
Still, it’s important to temper your expectations surrounding IF. Because a lot of research has been done with animals, it’s more difficult to apply the results to humans, who are certainly free-thinking and have to deal with the effects of lifestyle issues — work stress, crazy schedules, emotional eating, cravings, to name a few — that can affect one’s ability to stick with a specific diet. IF may be promising, but it’s “really no more effective than any other diet.”
Who Should Not Try Intermittent Fasting
Not everyone should (or needs to) try IF. A few groups who shouldn’t: women who are pregnant or trying to become pregnant (extended fasting periods may throw off your menstrual cycle), those taking diabetes medication (blood sugar can drop too far in the absence of food), or anyone who takes multiple medications (food, or lack of it, can affect absorption and dosage), Kumar says. Also, if you have a history of eating disorders, introducing periods where you’re “not allowed” to eat can put you on a dangerous path toward a relapse.
Know that IF has some side effects. You may be cranky — “hanger” is real — during fasting periods because low blood sugar can mess with your mood. You also still need to have a healthy diet when you do eat. “One thought is that it would be difficult to make up a calorie deficit if you fasted for two days, but in our society with access to calorie-dense items, you could probably do it,” Kumar says. Focus on balanced, nutrient-packed choices, like fruits, vegetables, lean meats, legumes, and whole grains (though some experts, like Dr. Shemek, also pair IF with low-carb or keto styles of eating). Expect that for the first couple of weeks you may deal with lower energy, bloating, and cravings until your body adjusts, Shemek says.
7 Types of Intermittent Fasting to Consider
There are so many different ways to do IF, and that’s a great thing. If this is something you’re interested in doing, you can find the approach that will work best with your lifestyle, which increases the chances of success. Here are seven:
1. 5:2 Fasting
This is one of the most popular IF methods. The bestselling book The FastDiet introduced it to the mainstream, and it outlines everything you need to know about this approach. The idea is to eat normally for five days (don’t count calories); then on the other two days eat 500 or 600 calories a day, for women and men, respectively. The fasting days are any days of your choosing.
The idea is that short bouts of fasting keep you compliant; should you be hungry on a fasting day, you just have to look forward to the next day, when you can “feast” again. “Some people say, ‘I can do anything for two days, but it’s too much to cut back on what I eat all seven days,’” Kumar says. For those people, a 5:2 approach may work better than cutting calories for the entire week.
That said, the authors of The FastDiet advise against fasting on days that you may be doing a lot of endurance exercise. If you’re prepping for a bike or running race (or run high-mileage weeks), evaluate whether this type of fasting will work with your training plan. Or speak with a sports nutritionist.
2. Time-Restricted Fasting
With this type of IF, you choose an eating window every day, which should ideally leave a 14- to 16-hour fasting period. (Due to hormonal concerns, Shemek recommends that women fast for no more than 14 hours daily.) “Fasting promotes autophagy, the natural ‘cellular housekeeping’ process where the body clears debris and other things that stand in the way of the health of mitochondria, which begins when liver glycogen is depleted,” Shemek says. Doing this may help maximize fat cell metabolism and optimize insulin function, she says.
With this approach, you set your eating window from 9 a.m. to 5 p.m., for instance. It can work especially well for someone with a family who eats an early dinner anyway, Kumar says. Then much of the time spent fasting is time spent sleeping anyway. (You also don’t technically have to “miss” any meals, depending on when you set your window.) But this is dependent on how consistent you can be. If your schedule is frequently changing, or you need or want the freedom to go out to breakfast occasionally, head out for a late date night, or go to happy hour, daily periods of fasting may not be for you.
3. Overnight Fasting
This approach is the simplest of the bunch, and it involves fasting for a 12-hour period every day. For example: Choose to stop eating after dinner by 7 p.m. and then resume eating at 7 a.m. with breakfast the next morning. Autophagy does still happen at the 12-hour mark, though you’ll get more mild cellular benefits, Shemek says. This is the minimum number of fasting hours she recommends.
A benefit of this method is that it’s easy to implement. Also, you don’t have to skip meals; if anything, all you’re doing is eliminating a bedtime snack (if you ate one to begin with). But this method doesn’t maximize the advantages of fasting. If you’re using fasting for weight loss, a smaller fasting window means more time to eat, and it may not help you decrease the number of calories you consume.
4. Eat Stop Eat
This approach was developed by author Brad Pilon in his book Eat Stop Eat: The Shocking Truth That Makes Weight Loss Simple Again. His approach differs from other ones in that he stresses flexibility. Simply put, he emphasizes the idea that fasting is just taking a break from food for a time. You complete one or two 24-hour fasts per week and commit to a resistance training program. “When your fast is over, I want you to pretend that it never happened and eat responsibly. That’s it. Nothing else,” he says on his website.
Eating responsibly refers to going back to a normal way of eating, where you don’t binge because you just fasted, but you also don’t restrict yourself with an extreme diet or eat less than you need. Occasional fasting combined with regular weight training is best for fat loss, Pilon says. By going on one or two 24-hour fasts during the week, you allow yourself to eat a slightly higher number of calories on the other five or six nonfasting days. That, he says, makes it easier and more enjoyable to end the week with a calorie deficit but without feeling as if you had to be on an extreme diet.
5. Whole-Day Fasting
Here, you eat once a day. Some people choose to eat dinner and then not eat again until the next day’s dinner, Shemek explains. With whole-day fasting, the fasting periods are essentially 24 hours (dinner to dinner or lunch to lunch), whereas with 5:2 the fasting period is actually 36 hours. (For example, you eat dinner on Sunday, then “fast” on Monday by eating 500 or 600 calories, and break it with breakfast on Tuesday.)
The advantage of whole-day fasting, if done for weight loss, is that it’s really tough (though not impossible) to eat an entire day’s worth of calories in one sitting. The disadvantage of this approach is that it’s hard to get all the nutrients your body needs to function optimally with just one meal. Not to mention, this approach is tough to stick to. You might get really hungry by the time dinner rolls around, and that can lead you to consume not-so-great, calorie-dense choices. Think about it: When you’re ravenous, you’re not exactly craving broccoli. Many people also drink coffee in excess to get through their hunger, Shemek says, which can have negative effects on your ability to sleep. You may also notice brain fog throughout the day if you’re not eating.
6. Alternate-Day Fasting
This approach was popularized by Krista Varady, PhD, a nutrition professor at the University of Illinois in Chicago. People might fast every other day, with a “fast” consisting of 25 percent of their calorie needs (about 500 calories), and nonfasting days being normal eating days. This is a popular approach for weight loss. In fact, research found that, in overweight adults, alternate-day fasting significantly reduced body mass index, weight, fat mass, and total cholesterol.
You may be concerned about feeling hungry on fasting days. Previous research published by Dr. Varady and colleagues found that side effects of alternate-day fasting (like hunger) decreased by week two, and the participants started feeling more satisfied on the diet after week four. The downside was that during the eight weeks in the experiment, study participants said that they were never really “full,” which can make adhering to this approach challenging.
7. Choose-Your-Day Fasting
This is more of a choose-your-own-adventure approach to IF. You might do the time-restricted fasting (fast for 16 hours, eat for eight, for instance) every other day or once or twice a week, Shemek says. What that means is that Sunday might be a normal day of eating, where you stop eating by 8 p.m.; then you’d resume eating again on Monday at noon. Essentially, it’s like skipping breakfast a few days a week.
Something to keep in mind: The research on the effect skipping breakfast has on weight loss is mixed. There isn’t strong evidence to suggest that skipping breakfast affects weight.
But other research has shown that eating a morning meal can modestly impact weight loss.
And other research has linked breakfast skipping with an increased risk of mortality from cardiovascular disease.
This approach may be easily adaptable to your lifestyle and is more go with the flow, meaning you can make it work even with a schedule that changes from week to week. But a looser approach may mean milder benefits