Mayo Clinic Minute: Is intermittent fasting healthful or harmful?
Before changing the way you eat and altering your diet in any significant way, please speak with a health professional to make sure it's the best decision for you.
You’ve read about people’s shocking transformations on Facebook, seen the hashtags trending on Instagram, and know all about the weight-loss buzz from your friends at the gym. But what is intermittent fasting? And can you actually pull it off when you barely can make it to 10 a.m. without ripping someone’s head off in a hangry rage?
What Is Intermittent Fasting?
Fasting is nothing new. Our ancestors did it (because they didn’t have constant access to food) before anyone ever muttered the words “skinny jeans,” “thigh gap,” or “cleanse.” Countless religious ceremonies revolve around some form of dietary fast—from the 25-hour fast of Yom Kippur in Judaism to the month-long daytime fast of Ramadan in Islam.
Today, it’s not so much used by the masses as an enlightening or spiritual experience, but rather a simple way to restrict calories and lose weight. Intermittent fasting (IF) involves alternating cycles of abstaining from food and eating it. Sounds simple enough, right? I mean, we basically do that every day anyway when we go to bed and wake up. While this popular weight-loss diet doesn’t dictate which foods to eat or even how much to consume, it’s definitely a little more structured than just deciding to eat and deciding to stop.
During the fasted period, whatever the length may be, no food should grace your lips. You are, however, allowed to drink water, tea, coffee, and other non-caloric beverages (so no, a Grande Frappuccino doesn’t fly). Today, IF has been adopted by health and wellness gurus all over the world, each whom advocate a specific IF regimen.
Types of Intermittent Fasting
This is a beginner's IF regimen and probably something a lot of people are doing without even thinking about it. It means you’re fasting for 12 hours a day and eating within a 12-hour window. If you eat your last meal at 7 p.m. and have breakfast the next morning at 7 a.m., congratulations, you’re already an IF pro.
In this IF method, you fast for 16 hours each day and restrict your eating window to just eight hours, which likely means you can squeeze in two, maybe three meals. Most people following the 16/8 method simply skip breakfast, have their first meal midday, and refrain from eating too late at night.
Here’s where it’s getting a bit more intense. Now, you’re fasting for a full 20 hours and allowing yourself one four-hour window to eat. It’s likely you might get just one, maybe two meals in each day, depending on what you find works for you.
In this more extreme “alternate-day fasting" (ADF) method, you’re severely restricting calories two days of the week (to approximately 500 calories each day) and eating whatever you want the other five days.
Like the 5:2, this ADF method doesn’t provide a consistent daily fast schedule, but rather allows you to choose one or two days in the week to have a full 24-hour fast. TBH, this one sounds like pure hell.
What Are Some of the Benefits of Intermittent Fasting?
1. You’ll eat less.
If this is the only benefit 99 percent of you care about because you're interested in IF as a weight-loss aid, we get it. But does it actually work? Well, it probably will. Since the purpose of IF is to restrict your window of eating, it’s likely you’ll eat fewer meals, and assuming you don’t binge during your “feasting” window, you could lose weight.
One study compared a high-protein, reduced-calorie IF regimen with a traditional heart-healthy diet plan and found that while they lost comparable amounts of weight, the IF had a bit of an edge for minimizing weight regain after one year. Another study found that IF was as effective as daily caloric restriction for weight loss, but the IF group had less lean muscle mass loss during dieting. Bonus!
2. It keeps your brain sharp.
We all want to keep our brain sharp, and IF may help. Animal research suggests that fasting may help increase the production of brain-derived neurotrophic factor, which may play a role in brain dysfunction and degeneration.
Other studies suggest that IF helps prevent short-term and special memory loss, can reduce brain damage and functional impairment, and even mediate the severity of Alzheimer’s in rats. While we’re still waiting on the credible studies on humans to draw any solid conclusions, it’s definitely some good food for thought.
3. It helps reduce the risk of diabetes.
Reducing the risk of diabetes has always been top of mind for most public health professionals, and it’s looking like IF may play a role. One preliminary study on mice discovered that a fasting cycle diet could restore insulin secretion and promote new insulin-producing pancreatic cells in mice with both type 1 and type 2 diabetes.
from human trials suggests that ADF does not alter fasting concentrations of glucose but may play a role in improving insulin sensitivity, an important risk factor for type 2 diabetes. Most recently, a review of the literature found that IF helped reduce visceral belly fat, fasting insulin, and insulin resistance as effectively as traditional caloric restriction. So, if a fasting protocol is more sustainable for you than consistent calorie cutting, then it looks like IF may be a valid option.
4. It might help maintain a healthy heart.
Early research on heart health has suggested that IF may help improve a number of risk factors for heart disease, like blood pressure and cholesterol. One very small human study found that a month of fasting resulted in declines in total cholesterol, triacylglycerol, LDL cholesterol, and systolic blood pressure.
Another found that after three weeks, good HDL cholesterol increased in women and triacylglycerol decreased in men, but neither systolic nor diastolic blood pressure budged. Considering the inconsistencies in the findings, experts have therefore concluded that we definitely need larger randomized control trials to see how IF can improve heart health.
5. It might help fight cancer.
While research on cancer is dicey and currently limited to rat models, one study found that after 16 weeks of IF, the incidence of lymphoma in rats was 0 percent compared to that in the control group whose incidence was 33 percent. Another found that 50 percent of the rats who were put on a fasting regimen lived 10 days after being inoculated with a tumor, compared with only 12.5 percent of those that were fed as much as they wanted. While interesting, we clearly need some human trials to make any sound inferences about the link between IF and cancer.
What are Some of the Concerns of Intermittent Fasting?
We know that adequate caloric and nutrient intake is essential for reproductive health, particularly because (loss of menstrual cycle) is directly linked to under eating and low body weight. While we don’t have any large human trials specific to IF, it’s quite possible that the restrictive nature of the diet can interfere with nature doing its thing. In fact, one found that an IF regimen interfered with the fertility of rats.
2. It could impair athletic performance.
Getting the most out of your workout really comes down to carefully timed fuel, and restricting calories for long periods of the day can definitely get in the way. Not only can it leave you too sluggish to really push the pedal to the medal, but if your workout isn’t timed perfectly during your “feasting” phase, you could be missing out on a really important window for muscle growth and glycogen replenishment. The result is that you might actually start breaking down metabolism-boosting muscle, not building it.
3. It's not realistic in the long run.
Like most diets, IF can be really hard to stick with in the long run. actually compared IF to daily caloric restriction and found that the dropout rate was significantly higher among fasters than calorie-cutters. Interestingly, this study also suggested that the individuals assigned to the fasting group gradually just ended up cutting calories over time, suggesting that this may just be a more natural way to eat.
4. It could cause disordered eating.
While it’s not officially documented, as a dietitian, I can attest that it’s very easy for people in the “diet mindset” to overdo their “feast” phase. In other words, if you’re given free rein to eat whatever you want for a short few hours and you’re hangry when you get there, it’s not unreasonable to assume you’re going to face plant into the first thing you see. Hello, office donuts! Not only will this negate any weight-loss benefits, but it’s also possible it can lead to some dangerous disorderly binging behaviors over time.
5. There's a lack of solid evidence.
So the preliminary research we do have sounds promising, but there’s still a really long way to go. Most of the studies we do have are on animals, and our human trials are generally very small. Also, the long-term effects of IF have not been studied, so we don’t know for sure if there are any dangers or major benefits for dieters eating this way in the long run.
Who Is Intermittent Fasting Good For?
As you can see, there are a plethora of different methods of varying difficulty when it comes to intermittent fasting, so it’s not necessarily a one-size-fits-all diet. Considering the benefits and risks, IF may be best for people who enjoy a wide range of foods and who struggle with diets that restrict certain types of food groups or macronutrients. With IF, the focus isn’t on the quality or even the quantity of food you’re eating, rather just the time frame that you’re eating in. So, if you want to keep pasta, chocolate, and wine on the table while staying “on your diet,” it may be a palatable option.
IF may also be helpful for those who tend to have digestive problems at night time as bumping up the last meal of the day before bed may help prevent heartburn, acid reflux, and other digestive woes.
Who Should Avoid Intermittent Fasting?
While early research suggests that fasting may have some benefits on blood sugar control, individuals with insulin-dependent diabetes should likely avoid radical fasting regimens. Enjoying regular, healthy meals can help prevent blood sugar spikes and dips that can be life-threatening for diabetics if not controlled. It may also not be ideal for amateur or professional athletes who depend on perfectly timed fuel before and after activity for athletic performance and recovery.
We're still really just scratching the surface of trying to understand the role of IF in overall health and weight management. Like all diets, it might work for one person but fail miserably for another. If you’re looking to get started in IF, I highly suggest speaking to a registered dietitian and your doctor, who can walk you safely through the best regimen for you and make sure you’re getting all of the necessary nutrients you need.
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